R E S O N A N C E R A M A N STUDIES OF T T F- T C N Q By Tyronne Christopher Sebastian Mayadunne B . Sc. (Hon.), University of Toronto, 1995 A THESIS T H E S U B M I T T E D IN P A R T I A L R E Q U I R E M E N T S M A S T E R F U L F I L L M E N T F O R T H E D E G R E E O F O F O F S C I E N C E in T H E F A C U L T Y D E P A R T M E N T O F O F G R A D U A T E PHYSICS A N D STUDIES A S T R O N O M Y We accept this thesis as conforming to the required standard T H E U N I V E R S I T Y O F BRITISH C O L U M B I A August 1997 © Tyronne Christopher Sebastian Mayadunne, 1997 In presenting this thesis i n partial fulfilment of the requirements for an advanced degree at the University of B r i t i s h C o l u m b i a , I agree that the L i b r a r y shall make it freely available for reference and study. I further agree that permission for extensive copying of this thesis for scholarly purposes may be granted by the head of my department or by his or her representatives. It is understood that copying or publication of this thesis for financial gain shall not be allowed without my written permission. Department of Physics and A s t r o n o m y T h e University of B r i t i s h C o l u m b i a 6224 A g r i c u l t u r a l R o a d Vancouver, B . C . , C a n a d a V 6 T 1Z1 Date: Abstract T h e resonant R a m a n spectra of powder samples of protonated, partially and fully deuterated T T F - T C N Q and protonated T S e F - T C N Q have been measured as a function of temperature ( 1 0 K - 2 9 5 K ) using a Brucker R F S 100 spectrometer w i t h an infrared laser. A number of previously unobserved lines appear i n the spectra of T T F - T C N Q below approximately 200 K . T h e intensity of these lines increases w i t h decreasing temperature. However they show no discontinuity at the three phase transtions, which establish the three-dimensional order of the fluctuating charge - density - wave ( C D W ) in T T F - T C N Q . These new lines are assigned to the normally R a m a n - inactive (and infrared - active) B modes on the basis of the measured isotopic shifts. 3 u The appearance of these B3„ o u t - of-plane intramolecular distortion modes of the T C N Q molecule are i n agreement w i t h the results of a recent x - r a y study which found that such o u t - o f plane distortions of the T C N Q molecule occur w i t h the appearance of the C D W on the T C N Q chain. O n l y the intramolecular modes of the T C N Q molecule are resonant and thus observed. A few of the new lines also appear i n the low - temperature spectra of T S e F - T C N Q but are generally much weaker i n intensity. ii Table of Contents Abstract ii List of Tables v List of Figures vi Acknowledgements 1 2 3 viii T T F - T C N Q and T S e F - T C N Q 1 1.1 Introduction 1 1.2 Organic Conductors 2 1.3 Molecular structure and General properties 4 1.4 R a m a n and Infrared Spectra 8 1.5 Thesis Outline 12 Group Theory 13 2.1 Introduction 13 2.2 Number of N o r m a l Modes 2.3 N o r m a l modes of the T T F and T C N Q molecules 16 2.4 Selection Rules 23 \ . 14 E x p e r i m e n t a l Techniques and Measurements 26 3.1 Introduction 26 3.2 Fourier Transform Spectroscopy 28 iii 4 5 3.2.1 T h e Michelson Interferometer 28 3.2.2 Fourier Transformation 31 3.2.3 Phase Correction 32 3.2.4 A p o d i z a t i o n and Leakage 33 3.2.5 Aliasing 35 3.2.6 T h e Picket Fence Effect 38 3.2.7 Advantages of Fourier Transform Spectroscopy 39 3.3 Fourier Transform R a m a n Spectroscopy 40 3.4 Brucker R F S 100 Spectrometer 41 3.5 Cryogenic Equipment 44 3.6 Sample Preparation and M o u n t i n g 47 3.7 Experimental measurements 49 E x p e r i m e n t a l Measurements and Results 50 4.1 R o o m Temperature spectra 50 4.2 L o w Temperature Spectra 57 4.3 Check Measurements 73 Conclusions 76 Bibliography 78. iv List of Tables 2.1 T h e Character Table of D 18 4.1 T h e observed frequencies of the totally - symmetric A 2h TTF-(/i /d )-TCNQ-(Vd ) 4 4.2 4 4 a n d (u - v§) modes i n 9 2 T S e F - T C N Q at 295 K T h e observed frequencies of the o u t - o f - p l a n e B 3 u (u 50 T T F - ( / i / d ) - T C N Q - ( / i / d ) and T S e F - T C N Q at 10K 4 4 4 4 v 56 - z/ ) modes in 54 60 List of Figures 1.1 T h e crystal structure of T T F - T C N Q 4 1.2 T h e overlap of the 7r-orbitals i n a molecular stack of T C N Q 5 1.3 T h e Peierls distortion resulting i n the formation of a charge-density wave . . 6 2.1 T h e T T F and T C N Q molecules 2.2 T h e totally symmetric A 2.3 T h e totally symmetric A 2.4 T h e totally symmetric A 2.5 Examples of normal modes belonging to the eight vibrational symmetry 17 [y\ and v ) modes of the T C N Q molecule g 2 9 g (U^-UQ) (v -v ) 7 w modes of the T C N Q molecule modes of the T C N Q molecule 19 20 21 species of a T T F molecule 22 3.1 T h e optical setup of a Michelson Interferometer 29 3.2 Several apodization functions and thier corresponding Fourier Transforms. 3.3 T h e overlapping of negative frequencies and A l i a s i n g 37 3.4 T h e optical layout of the R F S 100 F T - R a m a n spectrometer 43 3.5 T h e platform used to support the H e l i - tran refrigerator 45 3.6 A typical LT-3-110 Heli-tran system flow diagram 46 3.7 T h e sample holder used for the measurements 48 4.1 T h e room temperature (295 K ) spectra of T T F - / i - T C N Q - / i 4 51 4.2 T h e room temperature (295 K ) spectra of T T F - d - T C N Q - / i 4 52 4.3 T h e room temperature (295 K ) spectra of T S e F - T C N Q 4.4 T h e room temperature (295 K ) spectra of T T F - / 1 4 - T C N Q - d 4 4 vi . 34 53 4 54 4.5 T h e room temperature (295 K ) spectra of T T F - d - T C N Q - d 4.6 T h e 10 K spectra of 4.7 T h e 10 K spectra of T T F - / i - T C N Q - d 4.8 T h e temperature dependence of the intensity of the ^ 4 TTF-/14 - TCNQ-d -TCNQ-/i 58 4 59 4 mode i n T T F - / i - T C N Q - / i 4 52 4 TTF-/14 -TCNQ-d 4 4 -TCNQ-/i 4 65 5 1 mode i n T T F - / i - T C N Q - / i 4 4 66 4 T h e temperature dependence of the intensity of the and T T F - / i - T C N Q - d mode i n T T F - / i 4 4.11 T h e temperature dependence of the intensity of the i / and 4 64 4 T h e temperature dependence of the intensity of the v and T T F - / i - T C N Q - d 4 63 4 4 4 4.12 5 4 T h e temperature dependence of the intensity of the z/53 mode i n T T F - / i - T C N Q - / i and T T F - h - T C N Q - d 4.10 4 4 and 4.9 T T F - / J 55 4 1/50 mode i n T T F - / 1 4 - T C N Q - / i 4 67 4 4.13 Temperature dependence of the frequency of the f 5 4 mode i n T T F - f o - T C N Q - d 4 68 53 mode i n T T F - / t - T C N Q - d 4 69 mode i n T T F - / i - T C N Q - d 4 70 4.16 Temperature dependence of the frequency of the f i mode i n T T F - / i - T C N Q - d 4 71 4.17 Temperature dependence of the frequency of the v 4 72 4.14 Temperature dependence of the frequency of the v 4.15 Temperature dependence of the frequency of the u 52 5 50 4.18 T h e 10 K spectra of T S e F - T C N Q 4 4 4 4 mode i n T T F - / i - T C N Q - d 4 74 vii Acknowledgements I wish to thank my supervisor D r . J . E . Eldridge for his patient, helpful and knowledgeable supervision of this thesis project, as well as for useful discussions of the experimental results. I wish to also thank Y u a n k u n L i n for a great deal of help and advice offered during the experiments. I especially wish to acknowledge a number of experiments performed by h i m , to confirm the experimental results obtained i n this work. F i n a l l y I wish to thank my parents, Dennis and M a r i s , and my brother Tony, for a l l the emotional support provided to me throughout m y academic career. viii Chapter 1 T T F - T C N Q and T S e F - T C N Q 1.1 Introduction Organic molecular solids come i n an amazing diversity of forms and have a variety of interesting properties. However one of the physical properties very seldom displayed by them is electrical conductivity. T h i s is m a i n l y due to the fact that a l l of the valence electrons present, have taken part i n the covalent bonding inherently present i n organic solids. However a certain group of organic materials, known as the organic semiconductors, have been found to possess high carrier mobilities [1]. In such materials electrons created for example, by optical excitation, should be able to play a role similar to that of free electrons i n metals, thus contributing to a significant electrical conductivity. A n o t h e r way of creating such "free" electrons i n these semiconductors, is to introduce them from other materials. Such resulting organic complexes, i n which there has been a transfer of charge from the atoms or molecules of the electron donors (which may be organic or inorganic) to those of the organic semiconductors are termed "charge transfer salts" and are expected to have high electrical conductivities. Section 1.2 briefly describes the progress made i n the studies of such organic charge transfer salts, especially that of tetrathiafulavalene tetracyanoquinodimethane ( T T F - T C N Q ) which was found to possess the highest electrical conductivity among organic materials, prior to the discovery of the organic superconductors i n the early 1980's. Section 1.3 describes the molecular structure and physical properties of T T F - T C N Q and that of it's 1 Chapter 1. TTF-TCNQ and 2 TSeF-TCNQ selenium analogue T S e F - T C N Q , in which the four sulphur atoms i n the T T F molecule are replaced by selenium. F i n a l l y section 1.4 gives a summary of the most important infrared and R a m a n vibrational results obtained on T T F - T C N Q and T S e F - T C N Q up to now. 1.2 Organic Conductors Tetracyanoethylene ( T C N E ) was the first organic material to be investigated i n detail as a potential electron acceptor. However studies of T C N E complexes failed to indicate that any of them had a significant electrical conductivity [1]. In 1960 Acker et al. [2] at E . I . du Pont de Nemours & Company synthesized tetracyanoquinodimethane ( T C N Q ) by modifying TCNE. Complexes of T C N Q such as q u i n i o l i n i u m ( Q ) - ( T C N Q ) 2 and N-methylquinolium ( N M Q ) - T C N Q were found to possess higher electrical conductivities, generally of the order of 1 0 r 2 c m . T h i s was however, still orders of magnitude lower 2 _ 1 _ 1 than the conductivities of typical metals. Meanwhile little effort was made to produce any new electrical donors until 1970 when tetrathiafulvalene ( T T F ) was synthesized by W u d l et al. [3]. T T F was found to form good electron conducting complexes w i t h the halogens such as T T F - C 1 , T T F - B r etc. In 1972, the charge transfer salt T T F - T C N Q was synthesized by Ferraris et al. [4]. I n i t i a l measurements by them on four samples of T T F - T C N Q showed it had a room temperature dc conductivity of (1.92 - 6.52 ) x l O 2 fi cm _1 _1 which was considerably higher than that of any other T C N Q salt. T h e i r studies also showed that the dc conductivity of T T F - T C N Q increased w i t h decreasing temperature, like a metal, until about 66 K , after which it decreased. T h e maxi m u m value of the dc conductivity observed among the 4 samples at 66 K , was i n the range of (0.3 - 1.5 ) x l 0 4 ft-^cm- . 1 T h i s data indicated that T T F - T C N Q underwent a m e t a l - t o - semiconductor transition around 66 K . T h e y also found that the conductivity was highly 1 1 Later studies showed that the metal - to - semiconductor transition actually took place around 54 K . Chapter 1. TTF-TCNQ and 3 TSeF-TCNQ anistropic w i t h a conductivity of only 1 0 _ 1 cm - 1 measured i n a direction perpendicular to the direction of m a x i m u m conductivity. Later i n 1973 C o l e m a n et al. [5] reported that a few 'anomalous' crystals of T T F - T C N Q exhibited an even higher conductivity w i t h a k > 1 0 58 6 fi cm _1 _1 and a slope (dcr/dT) which was apparently divergent. T h e y interpreted this data as arising from fluctuations towards a B C S superconducting state. T h i s possibility of observing a (relatively) high temperature superconductivity i n T T F - T C N Q caused a large flurry of activity, which resulted i n the publication of hundreds of papers on T T F - T C N Q . Unfortunately these studies have since eliminated any possibility of a high-temperature superconductivity i n T T F - T C N Q [6, 7, 8]. Instead a clearer picture of T T F - T C N Q has been drawn i n terms of a Peierls-Frohlich charge-density wave conductor having structural phase transitions at 54, 49 and 3 8 K . Chapter 1.3 1. TTF-TCNQ and 4 TSeF-TCNQ Molecular structure and General properties B o t h T T F and T C N Q are large planar molecules as shown i n F i g . 2.1a. In T T F - T C N Q crystals they separately stack, one on top of another, like pancakes to form segregated columnar stacks of T T F and T C N Q molecules. The axis along which the stacks are formed is traditionally labelled the fe-axis of the crystal. Figure 1.1: (a) Structure of T T F and T C N Q molecules ; (b) C r y s t a l Structure (c) Stacking arrangement i n T T F - T C N Q ; D-donor ( T T F ) , A-acceptor ( T C N Q ) and X - r a y diffraction studies by Kistenmacher et al. [9] give the interplanar spacings i n the T T F and T C N Q molecular stacks at room temperature to be 3.47 A and 3.17 A respectively. T h e T T F and T C N Q molecular planes tilt at an angle of 2 4 . 5 and 3 4 . 0 respectively w i t h 0 0 respect to the /3-axis w i t h the T C N Q planes t i l t i n g i n a direction opposite to those of the T T F planes. The crystal structure of T S e F - T C N Q is essentially the same as that i n T T F T C N Q w i t h the interplanar spacings i n the T S e F and T C N Q stacks increasing to 3.52 A and 3.21 A respectively. The t i l t angles of the T S e F and T C N Q molecular planes w i t h the /3-axis also slightly increase to 2 4 . 8 ° and 3 4 . 1 ° respectively [10]. Chapter 1. TTF-TCNQ and TSeF-TCNQ 5 In the T C N Q molecules, the 7r-orbitals of the carbon atoms are elongated perpendicular to the molecular planes, causing them to overlap w i t h those of the neighbouring molecules, as depicted i n F i g . 1.2. Thus electrons donated to the neutral T C N Q molecules are expected 6-W31 Figure 1.2: T h e overlap of the 7r-orbitals i n a molecular stack of T C N Q to move freely v i a this orbital overlap, along the 6-axis, giving rise to a conduction band along this axis. A similar situation is realised in the T T F molecular stacks. A difference here however, is that, it is the vacancies in the 7r-orbitals of sulphur atoms caused by the electron donation to the T C N Q molecules, that contribute to the formation of the conduction band (hole band). There is very little overlap between adjacent stacks of the T T F and T C N Q molecules resulting i n very little inter-stack interaction and hence i n a very low conductivity along this axis, the a-axis of the crystal. Due to this large anisotropy i n electrical conductivity (the conductivity along the /3-axis being a factor of 500 or greater than that along the a-axis), T T F - T C N Q is often considered a quasi one-dimensional conductor. Chapter 1. TTF-TCNQ and 6 TSeF-TCNQ A s the temperature is lowered the conductivity of T T F - T C N Q increases i n a metallic fashion until at about 5 4 K , the crystal undergoes a Peierls distortion [11], where the uniform molecular spacings in the molecular stacks gives way to an energetically-more-favourable structure i n which the separation of adjacent molecules i n the stack varies. A consequence of this is a change in the electron density along the direction of the stacks. T h i s periodic concentration and rarefaction of the electron density is called a charge-density wave ( C D W ) [12]. MOMENTUM Figure 1.3: T h e Peierls distortion resulting i n the formation of a charge-density wave Structural studies of T T F - T C N Q [13] indicate that there are 3 successive phase transitions; the C D W transition occuring i n the T C N Q stacks at 5 4 K , the C D W transition i n the T T F stacks at 4 9 K and the establishment of three-dimensional order at 3 8 K , due to the pinning of the oppositely charged C D W ' s on adjacent stacks by coloumb attraction. Chapter 1. TTF-TCNQ and 7 TSeF-TCNQ T h e wavelength A of the C D W (i.e. the periodic lattice distortion associated w i t h the c Peierls transition) is incommensurate along the stacking direction, w i t h A equal to 3.46 c where b is the lattice period along the 6-axis. In terms of the reciprocal lattice vectors (a* = 2 ^ b*=^-, c*=^-), the Q component of the modulation wavevector Q, characterizes a the three phase transitions. Between 54 K and 49 K it is equal to \a*. Below 49 K it decreases approximately linearly w i t h temperature, until 39 K , where it jumps and locks at the commensurate value of \ a*. Qb is equal to 0.295 6* — 2kj, where kf is the fermi wave vector and Q is equal to zero. A second modulated structure having Qb = 0.59b = 4kf has c also being observed [14] which is qualitatively observed even at room temperature. T h e onset of the Peierls distortion i n quasi one-dimensional materials results i n the electron conduction band being split into two bands seperated by an energy gap 2 A ; the lower band being completely filled and the upper band being completely empty. Thus conduction may only occur i f electrons are promoted across the energy gap by thermal activation or photo-absorption. Thus, the material suddenly changes from a conductor to a semiconductor or an insulator. T h i s is what takes place i n T T F - T C N Q during the phase transitions. A l t h o u g h the valence electrons are now not available for conducting, an alternate mechanism for conduction is one where the C D W " slides". Here the atoms or molecules oscillate back and forth producing a travelling potential along w i t h which, the conduction electrons moves. It is this effect which is responsible for the enhanced conductivity i n T T F - T C N Q around 60 K . A s the temperature is lowered below 54 K for T T F - T C N Q , there is a decrease i n the conductivity due to the pinnning of the sliding CDW. In contrast i n T S e F - T C N Q only one phase transition at 29 K has been observed [15], which is assumed to be due to the C D W condensing on the T S e F stack. T h e possibility of a much weaker C D W occuring on the T C N Q stacks exist but has not been confirmed yet. Chapter 1. 1.4 TTF-TCNQ and 8 TSeF-TCNQ R a m a n and Infrared S p e c t r a 1 T h e infrared and R a m a n spectra of T T F - T C N Q and those of it's i n d i v i d u a l components, T T F and T C N Q , i n thier neutral and ionic forms, have been reported by several authors. A summary of the most important of these measurements is given here. A l l measurements were taken at room temperature, unless otherwise noted. Visible laser exciting sources were used i n a l l the R a m a n measurements. T h e R a m a n spectra of T T F and it's deuterated analogue T T F - d i n powder and solution 4 form were reported in Feb 1977 by Berlinsky et al. [16], who made a preliminary assignment of the A g fundamental modes, based on depolarisation studies and isotopic shifts. A few months later, Bozio TTF-d 4 2 et al. [17] reported the R a m a n and infrared spectra of T T F - / i 4 and i n powder and solution form. T h e y performed a vibrational analysis of the T T F - / i and T T F - d 4 4 molecules, including a n o r m a l mode calculation of the i n - p l a n e vibrational modes and used this to make an extensive assignment of the fundamental vibrational modes, w i t h agreement for the most part, w i t h the A fundamental assignments made by Berlinsky g et al. T h e polarised infrared spectra of T C N Q - / i form were reported i n M a r c h 1970 by L u n e l l i and T C N Q - d 4 4 i n powder and single crystal et al. [18] who made a tentative assignment 2 of the infrared - active modes. Soon afterwards Takenaka [19] also reported the polarised infrared spectra of T C N Q - / i 4 and T C N Q - d the R a m a n spectra of T C N Q - / i 4 4 i n powder and single crystal form as well as and T C N Q - d 4 i n powder form. He classified the observed bands into the three infrared - active species and made an assignment of most of the observed frequencies i n the infrared and R a m a n spectra to the fundamental v i b r a t i o n a l modes, w i t h T h i s section assumes knowledge of the distribution of the normal modes and their activity of the T T F and T C N Q molecules, which is discussed in Chapter 3. Reports by Bozio, Lunelli and Girlando were all reported from The Institute of Physical Chemistry, University of Padova, Padova, Italy. 1 2 Chapter 1. TTF-TCNQ and 9 TSeF-TCNQ the aid of the selection rules, the product rule and a normal coordinate analysis of the i n plane vibrations carried out using basic and modified U r e y - B r a d l e y force fields. However there was a considerable amount of disagreement between the infrared assignments made by Takenaka and those made by L u n e l l i et al. T h i s was clarified i n a report in 1973 by Girlando 2 et al. [20], which also included the first polarised R a m a n spectra of T C N Q - / i and T C N Q - d 4 crystals. T h e y evaluated both the R a m a n and infrared spectral predictions 4 i n terms of the oriented gas model and also calculated the frequencies of the i n - p l a n e normal modes using a 26 parameter Valence Force field. T h e y then used these results to assign 43 out of the 50 optically active fundamental vibrational modes, confirming nearly all of the infrared assignments made by Takenaka. T h e infrared spectra of the ( T C N Q - / i ) ~ and ( T C N Q - d ) ~ anions i n powder samples 4 4 of L i T C N Q and L i ( T C N Q - d ) respectively, were reported by G i r l a n d o et al. [21] in 1973. 4 T h e y suggested preliminary assignments to the infrared fundamentals, by correlation w i t h the assignments made for the neutral molecules and by modifying the Valence Force Fields developed for the neutral molecules [20] to provide the best fit w i t h the experimentally observed frequencies. In 1975, Bozio et al. [22] reported the infrared and R a m a n spectra of the ( T C N Q - f c ) and ( T C N Q - d ) ~ anions i n the light and deuteurated (1:1) T C N Q salts of - 4 4 L i (in powder and solution form) and R b (in powder form only). T h e y made assignments of the totally symmetric A g modes based on the intensities, isotopic shifts and depolarisation ratios of the R a m a n lines and by comparison w i t h the assignments made for the corresponding neutral molecules. T h e y also suggested assignments for the remaining R a m a n and infrared - active modes, w i t h the suggested assignments for the infrared fundamentals, the same as those previously made [21]. Reports by Bozio, Lunelli and Girlando were all reported from The Institute of Physical Chemistry, University of Padova, Padova, Italy. 2 Chapter 1. TTF-TCNQ and 10 TSeF-TCNQ The same year, C h i et al. [23] reported the room temperature and 77 K R a m a n spectra of the TCNQ-/14 and T C N Q - d 4 salts of R b and K , in the form of pressed K B r pellets, using exciting wavelengths of 4579A, 4880A, 5145A, 5682A and 6 4 7 l A . T h e y made an assignment of the observed features to the fudamental modes, based on the and T C N Q - d 4 TCNQ-/14 assignments made by G i r l a n d o et al. [20]. T h e y also found evidence of a strong resonant scattering dependent on the exciting wavelength. A more detailed study of the light and deuteurated T C N Q salts of L i and R b i n solution, by Bozio et al. [24] i n 1978, indicated that some of the observed infrared bands, which were previously assigned to infrared fundamentals [21, 22], were instead due to vibronic effects. T h i s resulted i n a reassignment of some of the infrared fundamentals, which were aided by comparison w i t h the assignments of the neutral molecule, the isotopic effect and an improved normal coordinate analysis. The R a m a n spectra of T T F - T C N Q crystals were first reported by K u z m a n y et al. [25] 4 in 1977. T h e y measured the unpolarized 77 K spectra of several samples of T T F - T C N Q crystals using exciting wavelengths between 4545A and 6764A. There was a considerable scattering in the position of the m a i n R a m a n lines among the various samples, and thus a representative distribution of these lines were obtained by averaging their frequencies and intensities over several samples. A n assignment of the observed frequencies i n the R a m a n spectra of T T F - T C N Q to the intramolecular modes of T T F and T C N Q were presented in a later paper by K u z m a n y et al. [26]. T h e y also reported observing a strong resonant scattering, on changing the exciting frequency from blue to red, which was not suprising considering the strong resonant scattering observed i n T C N Q - anions by C h i et al. [23]. The same year the polarized 300 K and 77 K R a m a n spectra of T T F - T C N Q was reported by T e m k i n et al. [27]. T h e y reported difficulty i n obtaining their R a m a n spectra due to R a m a n spectra of T T F - T C N Q were investigated prior to this, only on thin polycrystalline films by MacFarlane et al. 4 Chapter 1. TTF-TCNQ and 11 TSeF-TCNQ the fragility of the small crystals and the weak scattering observed. However thier results were i n good agreement w i t h those of K u z m a n y et al. [26], w i t h many of the observed frequencies lying between those of the neutral and completely charged T T F and T C N Q molecules, supporting the idea of an incomplete charge transfer occuring i n T T F - T C N Q . K u z m a n y et al. [25, 26, 28] has also reported of the instability of the R a m a n spectrum of T T F - T C N Q upon prolonged irradiation, w i t h the appearance of a set of new lines, which were either, observed next to (within 10 c m a doublet structure. - 1 of) a stable line, or which exhibited T h e y also observed this instability i n the R a m a n spectrum of T T F [28] and thus they attributed this instability to a photochemical process taking place i n some of the T T F molecules which probably caused the T T F molecule to break up at the center C = C bond. T h e room temperature spectra of T T F - T C N Q was also reported by M a t s u z a k i et al. [29] i n 1980. They were able to obtain reproducible spectra only under weak l a s e r - i r r a d i a t i o n . T h e y made an assignment of the observed R a m a n lines to the A g fundamental modes of the T T F and T C N Q molecules. Later i n 1985 M a t s u z a k i et al. [30] reported the low temperature (liquid nitrogen and helium temperatures) polarised spectra of powder samples of T T F - T C N Q . T h e y observed an increase i n the intensity of the ( A ) z / s mode of the T T F molecule w i t h decreasing temperature. 3 A p a r t from this, they failed to notice any dependence of the spectra w i t h temperature. Far fewer experimental studies have been performed on T S e F - T C N Q than on T T F T C N Q . In fact, to the best of the author's knowledge, no R a m a n studies of T S e F - T C N Q have been previously reported and the only noteworthy infrared study of T S e F - T C N Q is a far-infrared study by Bates, Eldridge and Bryce [31]. T h e y used a bolometric technique to measure the polarised low - temperature infrared spectra of T S e F - T C N Q (as well as those of normal and partially and fully deuterated T T F - T C N Q ) . T h e intense features i n the spectra of T S e F - T C N Q were very similar to those of T T F - T C N Q but considerably Chapter 1. TTF-TCNQ and 12 TSeF-TCNQ reduced in wavenumber. T h i s should be expected of the vibrational modes of T T F ( T S e F ) as the molecular weight of T S e F (79g) is more than double that of T T F (32g). Tentative assignments of the observed spectral features were made, based on the predicted wavenumber shifts and the visual similarity of the T S e F - T C N Q and T T F - T C N Q spectra. Thus so far, only three R a m a n studies of T T F - T C N Q have been reported, a l l of which employed visible exciting sources. In a l l three cases difficulties w i t h the measurements were reported, especially under prolonged l a s e r - i r r a d i a t i o n , presumably due to the p h o t o dissociation of the T T F molecule under these conditions. In particular a detailed R a m a n study of T T F - T C N Q at low temperatures is absent. It is also believed that no R a m a n studies of T S e F - T C N Q has been reported hitherto. Thus under these conditions it seems a temperature - dependent R a m a n study of T T F - T C N Q , it's partially and fully deuterated analogues, and T S e F - T C N Q would be highly motivated and is the main reason why the present study was undertaken. Here the use of an infrared laser R a m a n spectrometer, which have become only recently available, is expected to eliminate the problems of thermal dissociation. 1.5 Thesis O u t l i n e Chapter 2 describes how the symmetry of a molecule may be used to determine the number of normal modes of a molecule and thier symmetry species. T h e T T F and T C N Q molecules are used as examples. T h e experimental techniques and equipment used i n the measurements, are described i n Chapter 3. T h e experimental results obtained i n the work is presented and discussed i n Chapter 4. F i n a l l y the conclusions obtained from this work are stated in Chapter 5 Chapter 2 Group Theory 2.1 Introduction M o s t molecules possess a certain degree of symmetry, such that under the action of certain symmetry operations eg. rotations, reflections, etc. the molecule is left i n a configuration unchanged or indistinguishable from that of it's original. T h e complete set of such covering operations which carry a molecule into itself, form a mathematical group known as the point group of the molecule. It has been shown that there are only 32 possible point groups. Knowlege of the symmetry or the point group of a molecule can greatly simplify the calculation of the normal modes of v i b r a t i o n of the molecule. T h e symmetry and geometry 1 of a molecular model can be used to determine the number of fundamental frequencies, their degeneracies, the selection rules for the infrared and R a m a n spectra, the polarization properties of the R a m a n lines, and other useful information. A detailed discussion on how this can be done shall not be provided here, as there are a number of very good texts which discuss the subject. (The texts by Herzberg [32] and W i l s o n [33] are especially recommended). Rather a cursory introduction as to how the symmetry of a molecule can be used to determine the number of normal modes, their symmetry species and infrared and R a m a n activities w i l l be given here. T h e T T F and T C N Q molecules w i l l be used as examples. A normal mode of vibration in a molecule is one in which all atoms in the molecule vibrate in phase with each other, at the same frequency. X 13 Chapter 2. 2.2 Group 14 Theory N u m b e r of N o r m a l M o d e s T h e whole method of using the symmetry of a molecule to study it's vibrational properties is based on the fact that normal modes of v i b r a t i o n of symmetrical molecules have certain special symmetry properties. Thus each normal mode of vibration transforms according to a certain symmetry, under the various symmetry operations of the molecule's point group. In non - degenerate normal modes, the vibrations always transform either symmetrically or anti - symmetrically w i t h respect to a given symmetry operation. For degenerate normal modes, each symmetry operation transforms any given member of the degenerate set of vibrations into a linear combination of the members of the degenerate set. W h e n calculating the normal modes of a molecule, it is often more convenient to use the normal Q k coordinates of the molecule. T h e normal coordinates Qk are defined in terms of the mass - weighted cartesian displacement coordinates by the linear equation 3N Qk = Y. ^i i=i l k = \,2,...,ZN (2.1) where the coefficients Iki are chosen so that, i n terms of the normal coordinates Qk, the kinetic and potential energies have the form T= 1 z 37V Eft i=i 1 3JV V = -J2hQl z i=i (2.2) T h i s is done to ensure that the potential energy involves no cross products, but only squares of the Qk's, while the kinetic energy retains it's original form. E a c h normal coordinate is associated uniquely w i t h a normal mode of v i b r a t i o n . Thus the symmetry operations of the point group of a molecule, transforms it's non - degenerate normal coordinates into symmetric or anti - symmetric multiples of themselves and sets of degenerate normal coordinates into linear combinations of the degenerate set. Thus each normal coordinate of a molecule forms a basis for a representation of the point group of Chapter 2. Group 15 Theory that molecule. It can easily be seen that the non - degenerate normal coordinates give rise to one-dimensional representations, while the degenerate normal coordinates give rise to n - dimensional representations (where n is the level of degeneracy). More importantly the representations formed using these basis can be shown to be irreducible. Thus each normal coordinate of a molecule corresponds to an irreducible representation of that molecule's point group, possessing the same symmetry characteristics as that representation. In general this correspondence is not one - to - one, w i t h the irreducible representations formed from the basis of several normal coordinates found to be equivalent . T h e number of nor2 m a l coordinates (or normal modes) associated w i t h a given irreducible representation can be found by considering the representation formed by the basis of the entire set of normal coordinates. T h i s representation would obviously be reducible, containing each irreducible representation a number of times equal to the number of normal coordinates associated w i t h that irreducible representation. T h e number of times each irreducible representation is contained i n a given reducible representation is given by n M =X £ ^x\ h (2-3) i=classes where h = order of the group hi = number of elements in class C ; Xi — character of class Ci in irreducible representation n x\ = character of class Ci i n the reducible representation V If the point group symmetry of the molecule is known the values of h, hi and xt c a n be looked up i n the appropriate character table. T h e calculation of the class characters x j i n the representation formed from the normal coordinates of the molecule is slightly Equivalent representations are representations transformation. 2 that are related to each other by a similarity Chapter 2. Group 16 Theory less straightforward especially i f the normal coordinates of the molecule are unknown to begin w i t h . However as the character of a transformation is independent of the choice of coordinate orientation, i t is not neccessary to use the normal coordinates here. Instead the most convenient set of coordinates may be used to calculate the class characters xFA convenient set of such coordinates are the 3 Cartesian coordinates of each atom i n the molecule. T h e 3 N - dimensional representation derived from these coordinates is known as the Cartesian representation (where N is the number of atoms i n the molecule). Thus once the symmetry structure of a molecule is known, the number of normal modes of the molecule and their respective symmetry species can be found by a straightforward application of E q u a t i o n 2.3. 2.3 N o r m a l modes of the T T F a n d T C N Q molecules T h e T T F and T C N Q molecules both possess symmetry belonging to the point group £>2/i which contains the following elements (each of which falls into a conjugate class by itself). • E -corresponding to the identity operation. • / - corresponding to inversion through the center. • C f - corresponding to a 180° rotation about the X - a x i s . • C% ~ corresponding to a 180° rotation about the Y - a x i s . • C% ~ corresponding to a 180° rotation about the Z - a x i s . • o xy - corresponding to reflection i n the XY plane. • o - corresponding to reflection i n the YZ plane. • o - corresponding to reflection i n the XZ yz xz plane. Chapter 2. Group 17 Theory Y N • \ \ c \ X H. ^ c H / c=c / ^ H i TTF N TT TJ c 7 c=c A \ I N / c=c \ TCNQ / c / c—c \ c \ N Figure 2.1: T h e T T F and T C N Q molecules The calculation of the class characters i n the 3 N - dimensional Cartesian representation can be quite tedious, especially when N is large. However this calculation can be greatly simplified if one notes that a non-zero contribution is made to the given class character only for those atoms that are left unshifted under the symmetry operations belonging to that class [34]. T h e contribution per atom, made to the character of the identity, reflection, inversion and rotation elements may be calculated by standard geometrical techniques to be (E) X = 3 x(a) = l x(/) = - 3 x(C ) 2 =-1 The characters of the group D h i n the Cartesian representation of the T C N Q molecule, 2 FTCNQ can be calculated by m u l t i p l y i n g the number of unshifted atoms i n each symmetry operation by the appropriate contribution made per atom, above. T h e characters i n the Cartesian representation TTCNQ X (E)=60 x(<?f) = 0 x(/)=0 x{o )=0 xy a r (C ) e then found to be = 0 x(Cf) = - 4 X ( 0 = 4 x ( ^ ) = 20 y X 2 Chapter 2. Group 18 Theory T h e characters of the various classes of D h in each irreducible representation, is sum2 marised in it's Character Table i n Table 2.1 E C\ I c\ 1 1 1 x ,y ,z 1 1 1 1 1 1 1 1 Big 1 1 -1 -1 1 1 -1 -1 R xy Bg 2 1 -1 1 -1 1 -1 1 -1 Ry xz Bg 1 -1 -1 1 1 -1 -1 1 R yz A u 1 1 1 1 -1 -1 -1 -1 B\ u 1 1 -1 -1 -1 -1 1 1 B 2u 1 -1 1 -1 -1 1 -1 1 Bzu 1 -1 -1 1 -1 1 1 -1 3 z z x T T x Table 2.1: T h e Character Table of D 2H Thus using the E q u a t i o n 2.3 we find that for the T C N Q molecule, n{A )=10, n(B )=4, n{B )=6, n{B )=10 n(B )=10, n( B )=10, n(5 )=6 g l s n(Aj=4, 2g ltt J 3g 2u 3 u Hence the number of normal modes of the T C N Q molecule and thier associated symmetry species is given by TTCNQ = lOA + AB g lg + 6B 2g + 10B 3g + 4A U + 10B lu + 10B 2u + 6B Zu A similar derivation for the T T F molecule gives T T T F = 7A g + 3B lg + 4B 2g + 7B 3g + 3A U + 7B lu + 7B 2u + 4B 3u Chapter 2. Group 19 Theory Now T T F and T C N Q being non-collinear molecules have 3 translational degrees of freedom, 3 rotational degrees of freedom and 54 vibrational degrees of freedom. Thus removing the normal modes of v i b r a t i o n due to pure translation and rotation gives (J^TCNQ^vib = lOA g {T T )vib T F = 7A g + 3B lg + 2B + 5B + 3B lg + 9B 2g 2g + AA 3g + 6B 3g U + 3A U + 9B lu + 6B lu + 9B 2u + 6B 2u + + 5B 3u 3B 3u as the number of vibrational normal modes belonging to each symmetry species for the T C N Q and T T F molecules respectively. T h e ten totally symmetric A 9 modes of the T C N Q molecule are shown i n F i g , 2 . 2 - F i g . 2.4 while examples of normal modes belonging to each of the eight symmetric species of the T T F molecule are shown i n F i g . 2.5. \ Figure 2.2: T h e totally symmetric A {y\ and u ) modes of the T C N Q molecule. arrows indicate the direction of the atomic displacement. p 2 / The Chapter 2. Group 20 Theory Figure 2.3: T h e totally symmetric A ( z ^ - z ^ ) modes of the T C N Q molecule. T h e arrows indicate the direction of the atomic displacement. g Chapter 2. Group Theory 21 Figure 2.4: T h e totally symmetric A (VT~V ) modes of the T C N Q molecule. T h e arrows indicate the direction of the atomic displacement. 9 W Chapter 2. Group Theory / B 2u B% u Figure 2.5: Examples of normal modes belonging to the eight vibrational symmetry species of a T T F molecule. T h e arrows indicate the direction of the atomic displacement, + indicates a displacement out of the page and — a displacement into the page. Chapter 2.4 2. Group Theory 23 Selection Rules Experimentally, the frequencies of the normal modes of vibration of a molecule are measured by absorption or emmision techniques in infrared spectroscopy and by scattering mechanisms i n R a m a n spectroscopy. Molecules m a k i n g transitions between v i b r a t i o n a l states emit or absorb radiation w i t h frequencies proportional to the energy difference of these states (the proportionality constant being one over Planck's constant). T h e proba- bility of such a transition occuring, determines the intensity of the resulting spectral line. For induced emmision or absorption, the probability of a transition occuring between two states v and v' is proportional to the square of the transition moment integral fi > vv l / W I = I (fJ>x)vv> I + I (HY)w' 2 where(//i) 2 w = J ip* ^ ip , dr v v I + I (fJ-z)w' I 2 i = X,Y,Z (2.4) 2 (2.5) Here ip nd V v are the complete wavefunctions of states v and v' respectively, fix is the v (permanent or induced) electric dipole moment of the system, dr is the volume element of configuration space and the integral is over the entire configuration space of the system. If nx, V-Y and fj, Y are all independent of the vibration, they factor out of the respective integrals i n E q u a t i o n 2.5 leaving (A*iW = j V v dr i-X,Y,Z (2.6) which is zero due to the orthogonality of the wavefunctions ip and V v • Thus i n order for v the probability of the transition from state v —> v' to be non-zero, at least one component of the electric dipole moment has to change during the vibration (This is a neccessary, but not a sufficient condition, as shall be seen below). Transitions which have a zero probability of occuring are labelled as forbidden transitions while those which have a n o n - zero probabilty, as allowed transitions. T h e rules which Chapter 2. Group Theory 24 specify which transitions are allowed and which are forbidden are known as selection rules. Infrared - active transitions are transitions which cause a change i n the permanent electric dipole moment of the system while R a m a n - active transitions are those that cause a change in the polarizability a (and thus i n the induced dipole moment of the system) . 3 In each case it should be noted that the reverse is not neccessarily true. T h i s is due to the fact that a varying (permanent or induced) dipole moment does not neccessarily cause the components of the transition moment integral to be non - zero. To see this we note that the components of the transition moment integral are of the form / fafafadr where fa, fa and fa are some position - dependent functions. For a molecule belonging to a particluar point group, each of these functions fa, fa and fa w i l l form a basis for an irreducible representation r 1 ; T 2 and T representations, T i x T 2 of the group. 3 x T 3 Thus the direct product of these irreducible w i l l be an reducible representation of the group. Now if any given component of the transition moment integral is non-zero, it must have the same value for a l l indistinguishable configurations of the molecule. Thus it follows that the direct product T i x T x T must contain the totally symmetric irreducible representation of the 2 3 point group at least once. Thus a given transition v —> v' i n a molecule is infrared - active if and only i f T(i/; ) x T(//) x T(v') contains the totally symmetric irreducible representation v of the point group of the molecule. S i m i l a r l y such a transition is R a m a n - active i f and only if r ( ^ „ ) x T ( a ) x T(v') contains the totally symmetric irreducible representation of the point group. In general one is most interested i n the fundamental transitions i.e. those from the ground state to the first excited state. Thus it might be worthwhile here to consider this special case. It can easily be seen that the ground state wavefunction of a molecule w i l l be invariant under a l l the symmetry operations of the molecule and thus w i l l transform T h u s a infrared - active or Raman -active vibration is one where the corresponding transition from the vibrational ground state to the excited state is infrared - or Raman - active. 3 Chapter 2. Group Theory 25 according to the totally symmetric representation of the molecule's point group. T h e wave function of the first excited state transforms according to the symmetry of the normal coordinate corresponding to that vibration. T h e components of the electric dipole moment obviously transforms according to the symmetry of the respective Cartesian coordinates. Obviously the direct product of the totally symmetric representation w i t h any other representation is equal to that representation. Thus for the direct product of the i n d i v i d u a l representations of the ground state wavefunction, the excited wavefunction and the electric dipole moment (or polarizability), to transform as the totally symmetric representation, it is neccessary for the direct product of the latter two representations to transform as the totally symmetric representation. B y the orthogonality properties of representations, this is only possible, i f these two representations are identical. Thus the only infrared - active fundamental normal modes of vibration are ones which have the same symmetry as the three components of the electric dipole moment \i (which is the symmetry of the respective translation) while the only R a m a n - active ones are ones which have the symmetry of the six components of the polarizability a . 4 It is also useful to note here that for molecules w i t h a center of symmetry, the electric diple moment has ungerade always (u) symmetry while the polarizability a always has gerade (g) symmetry. Thus only u modes can be infrared - active while only g modes can be R a m a n - active. (Not all modes need to be active.) Thus i n T T F and T C N Q , both of which posses a center of symmetry, the B , B i u 2u and B 3u are infrared - active while the A , ig B\ , g B 2g R a m a n - active. The polarizability is a nine component tensor which has three pairs of identical terms. and B 3g are Chapter 3 E x p e r i m e n t a l Techniques and Measurements 3.1 Introduction R a m a n Spectroscopy is a research technique often used by physicists and analytical chemists to obtain information about the vibrational modes of molecules i n materials. T h i s is useful as the frequencies of these vibrational modes are unique and hence provide important constituent information about materials. Hence R a m a n spectra are often used for material characterization. In cases where the constituents of a material are already known, R a m a n spectroscopy can be used as a " p r o b e " to obtain structural information, and thus also to detect structural changes (such as those that take place i n a phase transition) i n materials. R a m a n spectroscopy is based on the " R a m a n effect", a very weak quantum effect which was theoretically predicted by A . Smekal [35] i n 1923 and first experimentally observed by C . V . R a m a n [36] i n 1928. Raman's discovery consisted of the observation that when a sample is irradiated by monochromatic radiation of frequency u , the spectrum of scattered 0 light includes (in addition to the incident frequency v Q ) a number of shifted frequencies. These shifted frequencies, known as the R a m a n spectrum, are independent of the incident frequency u and are found on either side of it. T h e lines of reduced frequency are known as Q " Stokes lines " and those of increased frequency as " A n t i - Stokes lines " . T h e positions of the A n t i - S t o k e s lines are a mirror image about the incident frequency, of the Stokes lines. Thus generally for every Stokes line there is a corresponding A n t i - Stokes line. However the intensities of the Stokes lines are generally much greater than those of the corresponding 26 Chapter 3. Experimental Techniques and Measurements 27 A n t i - S t o k e s lines w i t h the intensity of the latter dropping very rapidly as Au increases. Thus i n practice, A n t i - S t o k e s lines are very rarely observed. T h e R a m a n specta are characteristic of the scattering species and have been found to correspond to rotational and vibrational transitions of the molecules in the scattering species. In this work only the fundamental vibrational modes (i.e. direct transitions from the vibrational ground state to the first excited state) w i l l be considered. A theoretical quantum - mechanical treatment of the R a m a n effect is rather complex and shall not be given here. T h e interested reader is referred to the articles by K r a m e r s et al. [37], Dirac [38] and W o o d w a r d [39] for a discussion of the subject. For now it suffices to note from section 2.4 that R a m a n spectra arises from the change i n polarizability of the scattered molecules during vibrations where the quantum - mechanical transitions to the excited (rotational or vibrational) states are induced by the perturbation of the scattered molecule by the electric field i n the incident monochromatic light. Resonance R a m a n spectra which results i n the enhancement of the probability of certain transitions, arises when the exciting wavelength is close to those of the electronic transitions. Fourier Transform ( F T ) R a m a n Spectroscopy is a relatively new and advanced form of R a m a n spectroscopy which makes use of a near - infrared laser exciting source and the interferometric techniques of an F T - infrared spectrometer to measure the scattered R a m a n radiation. Section 3.2 and section 3.3 provide a brief introduction to Fourier Transform spectroscopy 1 and F T - R a m a n Spectroscopy respectively. Sections 3.4 gives some general information about the Brucker R F S 100 R a m a n spectrometer used while 3.5 describes the cryogenic equipment used for the the low temperature measurements. Section 3.6 describes the sample preparation and mounting and section 3.7 provides information about the experimental measurements. 1 T h e interested reader is referred to the text by Bell [40] for a more detailed discussion on the subject Chapter 3. Experimental 3.2 Techniques and Measurements 28 Fourier Transform Spectroscopy Fourier Transform ( F T ) Spectroscopy is an advanced form of spectroscopy which is based on an application of the interferometer, invented by M . Michelson i n 1888, and the mathematical concept of a Fourier Transform. F T - Spectroscopy has several advantages over conventional spectroscopy. However w i t h these advantages comes an increased degree of complexity due to it's intrinsically mathematical nature and due to the practical limitations of using an idealized concept such as a Fourier .Transform. T h e Michelson interferometer and Fourier Transformation is described i n sections 3.2.1 and 3.2.2 respectively. Some of the practical limitations of using the Fourier Transform and the solutions designed to circumvent these problems are discussed i n sections 3.2.4 to 3.2.6. F i n a l l y the advantages of F T - spectroscopy over conventional spectroscopy is listed in section 3.2.7. 3.2.1 T h e Michelson Interferometer One of the most important optical components i n a F T - spectrometer is an interferometer. T h e optical setup of an idealized Michelson interferometer is shown i n F i g . 3.1A. A M i c h e l son interferometer consists of a source S, two mirrors Mi and M , a beamsplitter B S , which 2 is so - named beacuse ideally it reflects half the incident radiation on it and transmits the other half, and a detector D . T h e radiation from the source is focused on the beamsplitter which splits the radiation into two beams, and sends .them off i n perpendicular directions towards mirrors Mi and M . 2 Ideally these two mirrors are positioned such that the the beams from the beamsplitter are incident normally on them. T h e two beams are then reflected from the two mirrors and are returned to the beamsplitter. A g a i n half of each returned beam is reflected by the beamsplitter and the other half transmitted. T h i s results, for an ideal beamsplitter, i n half the incident radiation from the source being returned to it and the Figure 3.1: (A) T h e optical setup of a Michelson Interferometer. (B) Signal measured by detector, for a polychromatic source. (C) Interference pattern of the laser source. T h e zero crossings of this pattern are used to define the sampling positions of the interferogram. Chapter 3. Experimental Techniques and Measurements 30 other half sent off i n a perpendicular direction towards the detector D . T h e interferometer is constructed such that the mirror M is fixed a distance L from the beamsplitter while the 2 mirror Mi can be moved, so that it's distance from the beamsplitter can be varied precisely through L±x. Thus the two beams that were reflected from the mirrors Mi and M travel 2 through different path lengths when they recombine at the beamsplitter. Since half of each of these beams is focused on the detector the detector measures the time - averaged intensity of the sum of these two incoherent beams. T h i s intensity which obviously is a function of the optical path difference 2x between the two beams, is known as the interferogram. For a monochromatic source, the interferogram is given by I{x) = I(v)[cos(2irux) + 1] (3.1) where v is the wavenumber, equal to the reciprocal of the wavelength A a n d the intensity of the monochromatic source at wavenumber v. In the case where the source emits polychromatic radiation, the interferogram is given by I(U)[COS{2TTUX) + l]du (3.2) min where u m i n and u m a x are the m i n i m u m and m a x i m u m wavenumbers respectively of the radiation emitted by the polychromatic source. E q u a t i o n 3.2 can rearranged as I(x) - - 1 ( 0 ) = / I(v) cos{2nvx)dv (3.3) where 1(0) is the intensity at zero path difference. T h e limits i n the integral i n E q u a t i o n 3.3 may be extended to ± o o subject to the restriction that I(u) = 0 for u < u m i n and v > v . max In this case we have I'(x) = I(x) - - 1 ( 0 ) = / I(u) cos(2nvx)dv 2 J — OO (3.4) Chapter 3. 3.2.2 Experimental Techniques and Measurements 31 Fourier Transformation Mathematically the Fourier Transform T of a function F[x) +oo / is defined as F(x) exp(i2irvx)dx (3.5) -00 while the inverse Fourier Transform T~ is given by Y f + OO F{v)exp(i2irvx)dv T~ (F{u))= l (3.6) J —oo In the event that F(x) is strictly an even or odd function, the Fourier Transform i n E q u a t i o n 3.5 reduces to the corresponding Cosine and Sine Transforms respectively. same applies for the inverse Fourier Transform. Thus when F(x) The is an even function, we have +oo / F(x) cos(2nux)dx (3.7) -oo and r+oo F(u) cos(2irvx)dv J — oo T- (F(v))= 1 while i f F(x) (3.8) is an odd function, +oo / x) sm(2^vx)dx (3.9) r+oo F(v)sm(2irux)dv (3.10) -oo and T-\F(u)) = -i J — oo Thus comparing E q u a t i o n 3.8 w i t h Equation 3.4 we see when I'(x) is an even function, that the interferogram I'{x) Conversely is the inverse Fourier Transform of I(v). intensity as a function of wavenumber can be expressed as the Fourier Transform of the intensity as a function of the position of the moving mirror +oo I(x) exp(i2-Kux)dx /. -oo the I'(x), M\. r+oo = / I(x) cos(2nvx)dx J—oo (3.11) Chapter 3. Experimental Techniques and Measurements 32 One should note that E q u a t i o n 3.11 holds true only when I'(x) of x. is an even function In practice this is rarely true, but this shall be discussed later (See Section 3.2.3). E q u a t i o n 3.11 is the basic integral at the heart of F T - spectroscopy and is known as the Fourier Transform Integral. 3.2.3 Phase C o r r e c t i o n A l t h o u g h i n Section 3.2.2 we assumed that I'(x) was an even function of x, in practice this is rarely so. T h i s asymmetry i n I'(x) is caused by the fact that none of the sampling positions may exactly coincide w i t h the position of zero path difference, leading to an asymmetric sampling of the interferogram, as well as possible misaligned interferometric optics which would cause a wavenumber dependent phase error. B o t h of these factors which remove the symmetry about x = 0 may be accounted for by the inclusion of a phase factor ip(v) i n E q u a t i o n 3.4. 1 = I{x) - -1(0) 2 I'(x) /-+oo = / I(u) cos(2irvx - ip(v))dv J — OO (3.12) T h e Fourier Transform T of this phase - corrected interferogram is given by T{V(x)) = = = J^ r(x)exp(i2T7ux)dx oo JZ r(x)cos(i2Trux)dx o + iJ^ sm(i2Trux)dx ll(u)[cos(ip(v))+ism(ip(v))] Thus, in this case the intensity as a function of wavenumber, I(y) / ( i / ) = 2 \T(I'(x))\ where C(v) and S[y) (3.13) 00 is given by = 2 [C\u) + < S ( ^ 2 denote the Cosine and Sine Transforms of I'(x) (3.14) respectively. T h u s we see, for an asymmetric interferogram, it is neccessary to calculate the complex Fourier Transform. A n alternative to this is to symmetrize the interferogram prior to Fourier Transformation, by i n i t i a l l y calculating the phase error ip(v). Chapter 3. 3.2.4 Experimental Techniques and Measurements 33 A p o d i z a t i o n and Leakage In order to evaluate E q u a t i o n 3.11 one has to know I'(x), the intensity as a function of the position of the moving mirror Ml, contdnously for all values of x between —oo a n d +oo. T h i s of course, is impossible as the moving mirror can only travel through a finite distance. Thus I'(x) is only known over a limited range of values, from — x m a x to +x . max Mathematically, truncating the l i m i t s of the integral i n E q u a t i o n 3.11 is equivalent to m u l t i p l y i n g the integral w i t h infinite limits, by the rectangular function rect(x), defined as rect(x) = < 1 IX I ^ Xfnnx 0 | ^ | ^ (3.15) Xfyidx A c c o r d i n g to the convolution theorem , the F T of this truncated interferogram is equal 2 to the convolution of the F T of the infinite interferogram together w i t h the F T of the rectangular function, known as the Instrumental Line Shape (ILS) function. T(rect(x)) = ILS{y) = 2x max sinc(2niyx ) = 2x max s max m ^ 7 r u X m a x ^ (3.16) Z*KVX max T h e effects of this is best illustrated by considering the spectra of a monochromatic source of wavelength u . T h e spectra of such a monochromatic source is given by 0 +oo / I(v ) cos(2nu x) 0 0 = 1 S(v - u ) 0 (3.17) 2 -oo T h e convolution of this spectra w i t h the ILS i n Equation 3.16 gives I(y) ® ILS{y) = I(u ) x 0 m a x sinc[27r(^ - v )x ] 0 max (3.18) T h i s function is depicted i n Figure 3 . 2 A . T h e Convolution Theorem states that the Fourier Transform of the product of two functions is the convolution of thier individual transforms, where convolution is denned as 2 r+oo -t-oo / G(y)F(x -oo - y)dy Chapter 3. Experimental Techniques and Measurements 34 Figure 3.2: Several apodization functions and thier corresponding Fourier Transforms. Chapter 3. Experimental Techniques and Measurements 35 It's is clear by truncating the limits of the Fourier Transform integral, one has given a w i d t h to the monochromatic line, which ideally should be a delta function (See E q u a t i o n 3.17). A l s o i n addition to the m a i n peak there now appear several side peaks or ' feet', which cause a ' leakage' of the spectral intensity. Since these feet, the largest of which is 22% of the m a i n peak amplitude, do not correspond to any spectral intensity but merely are an artifact due to the abrupt truncation of the inteferogram, it is desirable to reduce thier amplitude. T h i s attenuation of the spurious feet is known as apodization and can be achieved by truncating the interferogram less abruptly, which is done by m u l t i p l y i n g the interferogram by a "gentler"apodization function, prior to Fourier Transformation. Figure 3.2 B - E shows several such apodization functions and thier corresponding Fourier Transforms. In each of these cases, one can see that the negative sidepeaks are absent and the size of the sidepeaks are smaller. However the w i d t h of each of these m a i n peaks is broader than that of the sine function, which results i n a decrease i n resolution. 3.2.5 Aliasing A n o t h e r l i m i t a t i o n of E q u a t i o n 3.11 is that it is not possible to know I'(x) continously as this would require an infinite number of data points. Rather, i n practice, I'(x) only at discrete intervals of x, a distance Ax apart. is recorded M a t h e m a t i c a l l y such a "discrete" interferogram can be expressed as the product of the continous interferogram w i t h the shah function W ( ^ ) , where the shah function W (x) is defined as +oo W(x) = Y, ( 5 x - ) n (- ) 3 19 n=—oo Thus by the convolution theorem, the F T of the "discrete" interferogram is given by the convolution of the F T of the continous interferogram w i t h the F T of the shah function Chapter 3. Experimental Techniques and Measurements 36 W ( ^ ) which is ? ( W ( " r 4 ) = A x >V (i/Arr) (3.20) Thus the F T of the discrete interferogram, Id(v) is given by I (v) = ( A x W ( W \ x ) ) ® J (i/) d where I (v) c (3.21) c represents the F T of the continous interferogram. Therefore / W=[ oo E \ ^ - T " ) \n=—oo 3 oo ® W = / E />-nAi/) (3.22) n=—oo Thus the result of Fourier Transforming the "discrete " interferogram is an infinte number of complete spectra, which are periodically repeated after nAv. (n — 0, ± 1 , ± 2 , . . . ) . E a c h complete spectra consists of positive and negative frequencies as illustrated in Figure 3.3B. However only the postive frequencies have a real physical meaning. T h e negative frequencies are just a mirror image of the positive frequencies. Thus i n the repeat spectra obtained from the F T of the "discrete"interferogram, it is important that the negative frequency range of the (n + l)th spectra do not overlap w i t h the positive frequency range of the nth spectra. T h i s overlapping of the positive and negative frequency components is known as aliasing and can by avoided by requiring that Av > 2u max or Ax < — ! — = (3.23) In other words, one must sample at least twice i n each cycle of the smallest wavelength i n the interferogram. 3 The following property of the delta function was made use of, twice 1 ' Ax n v Ax J Chapter 3. -Av Experimental Techniques O and Measurements Av 37 2Av -v Figure 3.3: (A) Interferogram. (B) C o m p u t e d t w o - s i d e d spectrum from a continuous complete scan. (C) T h e result of aliasing. Chapter 3. Experimental 3.2.6 Techniques and Measurements 38 T h e Picket Fence Effect A l t h o u g h E q u a t i o n 3.2.5 is useful to show the effects of aliasing, it is of little practical use, as for experimentally determined data, I {v) is not known anyway. Instead i n practice, one c uses the Discrete Fourier Transform ( D F T ) to Fourier Transform the discrete interferogram. The D F T is defined as I (kAu) d = £ I'(nAx) e x p ( ^ p ) n=0 (3.24) ^ Here the continuous variables x and v have been replaced by the discrete variables and kAu, nAx where Au is defined as A u = NAx Similarly the inverse Discrete Fourier Transform is given by I\nAx) = ^ I { k A v ) e M - 1 ^ ) (3-25) v 1 (3.26) fc=0 A l t h o u g h the D F T is a very good approximation to the continuous Fourier Transform, it has a l i m i t a t i o n which arises when the spectrum contains features that do not coincide with the frequency sampled points kAu. If, i n the worst case, one of these features lies exactly halfway between two sampled points, an erroneous signal reduction up to 36% can occur. In effect it appears as if one is viewing the actual spectrum through a picket fence, with the result that the features lying behind the pickets are clipped. In practice this problem is less extreme as the spectral features are generally broad enough to be spread over several sampling positions. The solution to this problem is to decrease the spectrum spacing Au by increasing the number of points, iV i n the D F T . T h i s is achieved by adding zeros to the end of the interferogram before performing the D F T . Such "zero filling "has the effect of interpolating the spectrum and reducing errors as well as the added advantage of smooting out sharp discontinuities i n the spectrum. In general a zero filling factor ( Z F F ) of at least 2 should be used i.e. one should double the original interferogram size by zero filling. Chapter 3. 3.2.7 Experimental Techniques and Measurements 39 Advantages of Fourier Transform Spectroscopy F T - spectrometers has several advantages over conventional grating spectrometers. The so-called Jacquinot or throughput advantage arises from the fact that a F T - spectrometer can have a large circular source at the input or entrance aperture of the instrument w i t h no strong l i m i t a t i o n on the resolution. It also can be operated w i t h large solid angles at b o t h the source and the detector. T h e resolution of a conventional grating spectrometer, on the other hand, depends linearly on the w i d t h of the input and output slits. Also, for high resolution, a grating spectrometer requires large radii for the collimation mirror, and this condition i n turn necessitates small solid angles. Thus, for the same resolution, a F T - spectrometer can collect much a larger signal than a conventional grating spectrometer. T h e Fellget or mutiplex advantage arises from the fact that in a F T - s p e c t r o m e t e r the entire frequency range is simultaneously observed whereas i n a conventional grating spectrometer, only a narrow band of frequencies is observed at any one time. Thus assuming that the signal - to - noise ratio depends linearly on x/T,where T is the time taken for observe the entire frequency range , one can see that for the same signal - to - noise ratio, it takes 4 a shorter time to observe a given frequency range using a F T - s p e c t r o m e t e r compared to a grating spectrometer. In fact, the time is reduced by a factor of \[M where M is the number of bands in the entire frequency range, that were individually observed using the grating spectrometer. T h e Connes advantage is a statement of the high wavenumber accuracy present i n a F T spectrometer. T h i s is a consequence of the high precision used i n the tracking of the moving mirror and the determination of the sampling intervals, both of which are controlled by the interference pattern of the monochromatic light of a H e - N e laser. (See E q u a t i o n 3.1). T h e sampling positions i n the interferogram are determined by the adjacent zero crossings i n 4 T h i s is true when the signal is proportional to T and the noise is random and thus proportional to y/T Chapter 3. Experimental Techniques and Measurements 40 this monochromatic interference pattern, as shown i n F i g . 3.1 B and C . Since the spectrum sample spacing Au is inversely proportional to the sampling interval Ax, the error i n Au is of the same order as i n A x and thus is of the order of 10~ c m 2 - 1 . A d d i t i o n a l advantages are described i n the literature, such as i n the text by B e l l [40]. 3.3 Fourier Transform R a m a n Spectroscopy F T - R a m a n Spectroscopy which had it's b i r t h i n the mid-80's differs from conventional R a m a n Spectroscopy i n two m a i n ways. T h e first is that F T - R a m a n spectroscopy employs a near - infrared exciting frequency while conventional spectroscopy traditionally uses visible sources. T h e second is the means used i n measuring the scattered radiation. F T R a m a n spectroscopy uses interferometric techniques to measure scattered radiation of a l l frequencies simultaneously while i n conventional spectroscopy, grating machines are used to observe each band of frequencies individually. There are advantages as well as disadvantages of using F T - R a m a n over conventional spectroscopy although w i t h increasing technological developments, many of the disadvantages of using F T - R a m a n are gradually been reduced. One of the main advantages of using F T - R a m a n , is the large reduction of flouresence obtained in most samples. T h i s is due to the fact that, i n most samples, the photon energy from the near - infrared exciting source is usually not sufficient to cause the transitions between electronic states, that give rise to flourescence. A further advantage of this is the possibility of obtaining R a m a n spectra i n the absence of resonance enhancement, which arises from electronic transitions. T h i s can be useful as the band intensities of non - resonant R a m a n spectra are more representative of the chemical species present. A l s o w i t h these low photon energies, the possibility of sample heating and any subsequent photochemical sample disintergration is reduced. T h e other advantages of F T - R a m a n come from the use Chapter 3. Experimental Techniques and Measurements 41 of Fourier Transform spectroscopy in measuring the scattered radiation. These include the M u t i p l e x and Throughput advantages, discussed i n Section 3.2.7 A m o n g the disadvantages of using F T - R a m a n is the anticipated decrease i n sensitivity of the R a m a n lines. T h i s is due to the fact that the intensity of R a m a n lines are proportional to the fourth power of the exciting frequency. Thus comparing the N d : Y A G laser, which has an exciting frequency of 9394 c m - 1 , to a conventional A r g o n - i o n laser operating at 20492 c m , one sees that there is an anticipated decrease i n sensitivity, by at least a factor of - 1 22.6. Secondly the noise equivalent power ( N E P ) of near-infrared detectors is usually several orders of magnitude higher than that of photomultiplier tubes ( P M T ' s ) commonly used i n conventional spectroscopy. T h i s is however, becoming less of a problem w i t h the recent development of high sensitivity N I R detectors. F i n a l l y there is a " multiplex disadvantage " in using F T - spectroscopy as noise present i n the exciting radiation, that is scattered from the sample, is transformed into noise at a l l wavenumbers i n the transformed spectrum. However i n most cases this problem can be largely eliminated by carefully filtering the Rayleigh line out of the scatterred radiation. 3.4 B r u c k e r R F S 100 Spectrometer A l l measurements were made using a Brucker R F S 100 Fourier Transform interferometric R a m a n Spectrometer. T h e R F S 100 spectrometer contains an on-board aquisition processor ( A Q P ) which was used to perform the data aquisition and Fast - Fourier Transformations as well as to control a l l m o t o r - d r i v e n optical components i n the spectrometer. T h e A Q P is interfaced to a P C - based computer system running O P U S spectroscopic software designed by Brucker. T h i s enables the operator to remotely control variable parameters such as the incident laser power, resolution, polarisation etc. during measurements. T h e software also contains a variety of facilities for the display and manipulation of the measured spectra. Chapter 3. Experimental Techniques and Measurements 42 T h e exciting source used was a infrared diode - pumped N d : Y A G laser operating at a frequency of 9398 c m - 1 w i t h a power output of (0 — 350) m W . In a l l measurements a 180° back - scattering geometry was used. T h e optical layout of an R F S 100 spectrometer is shown i n Figure 3.4. T h e laser beam enters the sample compartment v i a one of a series of apertures (which permits the selection of 90° or 180° scattering geometries) located i n the b o t t o m of the sample compartment. T h e beam is then deflected horizontally by a small prism contained i n the objective lens assembly towards the sample, which is located at the focus of the objective lens. The scattered R a m a n radiation from the sample is then collected by the objective lens and passed through the interferomteric assembly. After exiting the interferometer the radiation is passed through a filter module, which removes the unshifted Rayleigh frequency and is finally focused on the detector. Chapter 3. Experimental Techniques and Measurements Laser 0 Sample Compartment Figure 3.4: T h e optical layout of the R F S 100 F T - R a m a n spectrometer Chapter 3. Experimental 3.5 Techniques and Measurements 44 Cryogenic E q u i p m e n t T h e low temperature measurements were made using a A i r Products A P D LT-3-110 Heli-tran liquid refrigerator together w i t h a M o d e l A P D - K cryogenic microprocessor temperature controller w i t h dual sensors. In order to facilitate the use of the H e l i - t r a n refrigerator w i t h the R F S 100 spectrometer, a sample platform and modified vacuum shroud designed by J . E . Eldridge, were used. T h e sample platform shown i n Figure 3.5, could be adjusted i n three perpendicular directions and about one vertical axis for sample alignment and signal optimization. T h e vacuum shroud contained a single window i n the front, made of 2 m i l polypropylene. T h e vaccum shroud was evacuated by a Precision Scientific C o . M o d e l 150 diffusion p u m p w i t h a liquid - nitrogen cold trap, to a pressure of below < 1 0 - 4 Torr, prior to cooling. A typical Heli-tran flow system is shown i n F i g 3.6. C o o l i n g was accomplished i n the refrigerator by the continous controlled transfer of liquid H e l i u m (Nitrogen) through a high efficiency evacuated transfer line to a heat exchanger which served as the mount or "cold finger" for the sample holder. T h i s provided lowest temperatures of approximately 77K and 5 K for liquid Nitrogen and H e l i u m respectively. Heat leak to the cryogenic flow stream w i t h i n the transfer line was minimised through the use of an internal suspension system and the interception of incoming heat by the shield flow circuit which surrounds the central flow. T h e transfer line bayonnet tube was placed into a pressurized H e l i u m dewar ( « 5 psi) while the other end of the bayonnet tube was placed into the Heli-tran refrigerator. T h e flow rate of liquid H e l i u m to the cold finger could be precisely regulated by means of a needle valve at the t i p of the refrigerator cold end bayonet. T h i s valve was engaged by means of an adjustment knob at the refrigerator end transfer line. A radiation shield made from thick O F H C copper surrounded the sample inside and served to m i m i m i z e the radiant Chapter 3. Experimental Techniques and Measurements 45 VALVE ADJUSTMENT KNOB IHTERFACE EXHAUST OAS HEATER SHIELD SUPPORTS HELIUM EXHAUST PORT VACUUM SHROUD PUMP OUT PORT TO TEMPERATURE CONTROLLER HORIZONTAL ADJUSTMENT MX CROMETER \ Figure 3.5: T h e platform used to support the H e l i - t r a n refrigerator Chapter 3. Experimental Techniques and Measurements Figure 3.6: A typical LT-3-110 Heli-tran system flow diagram 46 Chapter 3. Experimental Techniques and Measurements 47 heat load on the cold stage. T h e optical port i n the radiation shield was made as small as possible to avoid any excess heating from room-temperature radiation. A n activatedcharcoal getter was attached to the inside of the radiation shield. T h e radiation shield was cooled from helium exhausting from the cold finger. T h e temperature controller has two Scientific Instruments Inc. silicon diodes which act as temperature sensors, one located just below the cold finger and the second one mounted on the sample holder about 0.75 c m away from the sample. T h e automatic stability of the controller is ± 0 . 0 1 K from 5 K to 300K. T h e temperature between 5 K and 6 0 K is controlled by regulating the flow of liquid helium to the cold finger. Above 60 K the temperature is varied by adding heat from a small resistive heater wrapped around the neck of the cold finger. 3.6 Sample Preparation and M o u n t i n g T h e T T F - T C N Q crystals used i n this work were prepared in 1979 at the University of B r i t i s h C o l u m b i a , and have been carefully stored i n a dessicator since. E x a m i n a t i o n of the crystals under a microscope, showed that they looked visually unchanged, w i t h shiny black faces. T h e T S e F - T C N Q crystals were prepared i n 1994 by L . K . Montgomery . 5 T h e sample holder, shown i n F i g . 3.7, consisted of a copper mounting block which was screwed on the heat exchanger of the H e l i - t r a n refrigerator using an i n d i u m washer and grease to ensure good thermal contact. T h e mounting block contained two hemispherical copper disks about 1/16" i n thickness and 1" i n diameter. One of these disks had the silicon diode temperature sensor attached to it and was permanently screwed onto the mounting block. T h e second disk was removable, and had a small conical hole, 5/32" in diameter and 1/32" i n depth, into which the powder samples were pressed. T h i s made it possible 5 L . K . Montgomery, Department of Chemistry, Indiana University, Bloomington, I N 4705, U.S.A Chapter 3. Experimental Techniques and Measurements 48 Heli-tran Refrigerator Screw Copper cover Silicon Diode Sample Hemispherical Copper disks Figure 3.7: T h e sample holder used for the measurements. to change samples easily, without having to disconnect the temperature sensor. T h e two disks were firmly pressed against the mounting block by a circular copper cover which was screwed onto the mounting block. T h i s whole arangement served as a cold finger for the sample. P r i o r to insertion into the sample holder, the crystals were ground w i t h a quantity of Potassium B r o m i d e ( K B r ) powder i n an approximately 1:2 ratio, i n order to reduce laser heating (which raises the actual temperature of the sample) and to provide good thermal contact w i t h the copper sample holder. Chapter 3. Experimental 3.7 Techniques and Measurements 49 E x p e r i m e n t a l measurements T h e R a m a n spectra of each sample of each sample was measured as a function of temperature between 10 K and 295 K . Between these two values measurements generally were taken at 10 K , 27 K , 42 K , 51.5 K , 65 K , 100 K , 130 K , 170 K , 200 K , 235 K and 270 K . In a l l measurements, a constant laser power of 60 m W was used. T h e resolution of the measurements was 4 c m - 1 . In addition a number of measurements were performed by Y u a n k u n L i n 6 as a check on possible experimental effects. These measurements and results are described i n Section 4.3. Yuankun L i n is a P.hD student working in the same laboratory as the author. Chapter 4 E x p e r i m e n t a l Measurements and Results 4.1 R o o m T e m p e r a t u r e spectra The room temperature (295 K ) spectra of the protonated, partially and fully deuterated T T F - T C N Q compounds and T S e F - T C N Q are shown i n F i g . 4.1 - F i g . 4.5. C o m p a r i s o n of these spectra show a strong resemblance between the spectra of the protonated T C N Q compounds ( T T F - / i - T C N Q - / i , T T F - d - T C N Q - / i 4 4 4 4 and T S e F - T C N Q ) and between those of the deuterated T C N Q compounds ( T T F - / i - T C N Q - d 4 4 and T T F - d - T C N Q - d ) , b o t h i n 4 4 terms of the intensity and wavenumbers of the observed R a m a n lines. T h i s resemblance indicate that the observed R a m a n lines, the wavenumbers of which are listed i n Table 4.1, are due to the T C N Q molecule. T h i s indicates that the infrared laser is resonant w i t h an electronic transition in the T C N Q molecule and not w i t h the T T F molecule. A l s o listed are the observed isotopic shifts i n each line, upon deuteration of the T C N Q molecule. Comparison of the wavenumbers and the corresponding isotopic shifts of these lines w i t h the (calculated and observed) wavenumbers and isotopic shifts of the optically - active modes in the comprehensive vibrational analysis of the ( T C N Q - / i ) ~ and ( T C N Q - < i ) ~ anions by 4 4 Bozio et al. [24] permits an assignment of the observed lines i n these compounds to eight (1/2-^9) of the totally symmetric A modes of the T C N Q molecule. T h e observed values of s these modes as reported by Bozio et al. [24] are listed in Table 4.1. T h e excellent agreement between the values observed i n this work and those observed by Bozio, make the assignment of the observed R a m a n lines i n the r o o m temperature spectra of the compounds definite. 50 Chapter 4. Experimental Measurements and Results 51 14 1417.2(v ) 4 12 1604.1 (v ) 3 10 1202.2(vJ a> 6 599.3 (v.) 339.8(v ) 2209.7(v,) Q 0 0 250 500 750 1000 1250 1500 1750 2000 2250 2500 Wavenumber (cm ) 1 Figure 4.1: T h e room temperature (295 K ) spectra of T T F - / i - T C N Q - / i . 4 Number of scans: 2000 Resolution: 4 cm Incident Laser Power: 60 m W - 1 4 Chapter 4. Experimental Measurements and Results 52 16 1416.5(v ) 14 § 12 < 10 •ri 8 4 1603.4(v ) 3 1201.7(v.) 599.1 ( v j 339.4(v ) g 0 250 500 2208.9(v,) 750 1000 1250 1500 1750 2000 2250 Wavenumber (cm ) 1 Figure 4.2: T h e r o o m temperature (295 K ) spectra of T T F - d - T C N Q - / i . 4 Number of scans: 1600 Resolution: 4 cm Incident Laser Power: 60 m W - 1 4 2500 Chapter 4. Experimental Measurements and Results 53 10 1412.5(v ) 9 4 1603.4(v ) 3 8 7 6 1202.4(v.) 5 4 3 599.1 (v.) 339.9(v ) 2 2207.6(v,) 964.3 (v ) Q 6 717.0(v )' 7 1 0 0 250 500 750 1000 1250 1500 1750 2000 2250 Wavenumber (cm ) 1 Figure 4.3: T h e r o o m temperature (295 K ) spectra of T S e F - T C N Q Number of scans: 6500 Resolution: 4 cm Incident Laser Power: 60 m W - 1 2500 Chapter 4. Experimental Measurements and Results 22 i i i i i i 1 1 1 54 1 i 1 1 1 1 i 1413.2(vJ 20 ~ C 18 5 3 16 +-> C D a c 2 e * 1569.9(vJ 12 598.1 (v.) 10 693.7 (v ) s 7 870.0(v ) 5 6 4 338.5(v ) 0 2 00 250 500 750 1000 1250 1500 1750 2000 2250 Wavenumber (cm") 1 Figure 4.4: T h e room temperature (295 K ) spectra of T T F - / i N u m b e r of scans: 1600 Resolution: 4 cm Incident Laser Power: 60 m W - 1 4 -TCNQ-d . 4 2500 Chapter 4. Experimental Measurements and Results 55 14 14 2.6(v ) 4 1569.1 (v ) 12 3 10 597.8(v ) g 693.4(v ) 7 870.1 (v ) 5 2208.7(v.) 338.2(v ) q 0 0 250 500 750 1000 1250 1500 1750 2000 2250 Wavenumber (cm ) 1 Figure 4.5: T h e room temperature (295 K ) spectra of T T F - d - T C N Q - d . 4 Number of scans: 1600 Resolution: 4 cm Incident Laser Power: 60 m W - 1 4 2500 Chapter 4. Experimental Measurements Assignment TCNQ TTF-/i - TTF-d - TCNQ-/I4 TCNQ-/i ^2 2210 ^3 and Results TSeFTCNQ TCNQ-/14 2209 1604 U 56 anion TTF-/i TCNQ-dj TTF-oLt TCNQ-d 2208 2206 2209(1) 2209(0) 2206(0) 1603 1603 1615 1570(34) 1570(33) 1581(34) 1417 1417 1413 1391 1413(4) 1413(4) 1389(2) "5 1202 1202 1202 1196 870(332) 870(332) 871(325) ^6 964 964 964 978 - - 970 IV 718 718 717 725 693(25) 693(25) 701(24) ^8 599 599 599 613 598(1) 598(1) 612(1) "9 340 339 340 337 339(1) 338(1) 337(0) A 4 4 4 4 TCNQ-d anion 4 4 Table 4.1: Comparison of the frequencies of the totally symmetric A [y - ^ 9 ) modes observed i n this work w i t h those observed i n the ( T C N Q - / 7 . ) ~ and ( T C N Q - d ) ~ anions, as reported by Bozio et al. T h e values i n parentheses i n columns six, seven and eight indicate the observed isotopic shifts i n each line, as compared to the protonated T C N Q molecule. g 4 2 4 1 The atomic displacements of the ten totally symmetric A g modes of the T C N Q molecule have been previously illustrated i n F i g . 2.3. Comparison of the frequencies in columns two and six (or three and seven) clearly show that none of the observed features are due to the T T F molecule and that only the T C N Q molecule is being observed. ° T h e frequencies reported by Bozio et al. were obtained from T C N Q - ions in a solution of L i T C N Q . Chapter 4. 4.2 Experimental Measurements Low Temperature S p e c t r a and Results 57 1 T h e 10 K spectra of T T F - / i - T C N Q - / i , T T F - / i - T C N Q - d 4 4 4 4 and T S e F - T C N Q are shown in F i g . 4.6 - F i g . 4.18. A s the temperature of the samples was lowered, the intensity of most of the A g lines decreased, accompanied by a possible splitting of these lines into doublets (^3, u , i / , and u ) and a triplet or a quadruplet (z/ ). T h e lowering of the temperature 5 6 g 4 below approximately 200 K also resulted i n the appearance of a number of new lines, which were not present i n the room - temperature spectra. A l t h o u g h thousands of scans were performed at each temperature, there was still a non - negligible amount of noise present in the spectra which made it hard to determine the exact temperature at which the new lines started to appear. However the intensity of these lines started to become significant enough to be distinguished from noise - related features at temperatures generally between 170 K and 200 K . A l t h o u g h the number of these new spectral features could be as many as twelve, only five of these features could definitely be classified as R a m a n lines, due to the afore - mentioned problem of noise. A comparison of the intensity and wavenumbers of the new R a m a n lines i n the spectra of the isotopically - substituted compounds indicates that these lines are again due to only the T C N Q molecule. T h e wavenumbers, relative intensities and the corresponding isotopic shifts upon deuteration of the T C N Q molecule, of each of the new lines is listed i n Table 4.2. A comparison of the wavenumbers and isotopic shifts of these new lines w i t h those of the optically - active modes of the ( T C N Q - / i ) ~ and ( T C N Q - r f ) ~ anions, as reported by Bozio 4 4 et al. [24] strongly suggest that the five new observed lines are due to the B 3 u out-of-plane distortional modes of the T C N Q molecule. The discussion of the experimental results presented here, are due, in large, to a report by J . E . Eldridge [42]. 1 Chapter 4. Experimental Measurements and Results 58 22 20 125.3(v ) 54 10 K 295 K 18 16 14 12 1419.7(v ) 4 ' 1607.8(v ) 235.9(v ) 3 53 10 488.7(v ) 1204.0(vJ 52 8 6 352.9 (v ) 9 588.6(v ) 2211.5(v ) 51 7 725.6(v ) 4 7 839.7(v ) sn 500 750 1000 1250 1500 1750 2000 Wavenumber (cm") 1 Figure 4.6: T h e 10 K spectra of T T F - / i - T C N Q - / i . 4 Number of scans: Resolution: Incident Laser Power: 2400 4 cm 60 m W - 1 4 2250 2500 Chapter 4. Experimental 24 Measurements and Results 59 122.1(v ) 54 1415.5(vJ 10 K 295 K 20 1574.2(vJ 16 232.4(v ) 53 •ri 12 426.2(v ) 52 347.2/ 734.2(v ) 50 871.3(v ) ( 3>/571.8 (v ) v fi 2212.2(v.) 51 250 500 750 1000 1250 1500 1750 2000 Wavenumber (cm ) 1 Figure 4.7: T h e 10 K spectra of T T F - / \ - T C N Q - d . 4 Number of scans: Resolution: Incident Laser Power: 4000 4 cm 60 m W - 1 4 2250 2500 Chapter 4. Experimental Measurements Assignment TCNQ TTF-/l TCNQ-/\ TTF-d TCNQ-/i TSeFTCNQ ^54 125vvs 125vvs 133 ^53 236vs 236vs ^52 489s 490s ^51 589m ^50 840w 4 4 2 and Results TCNQ-/i anion 60 TTF-/i TCNQ-dt TTF-d TCNQ-d 103 122(3)vvs 121(4)ws 105(2) 237 225 232(4)vs 232(4)vs 220(5) - 483 426(63)s 426(64s) 423(60) 590m - 585 572(17)m 571(19)m 565(20) 840w - 836 734(106)w 730(110)w 732(104) 4 4 4 4 4 4 TCNQ-d anion Table 4.2: Comparison of the frequencies of the new lines observed at 10 K i n this work w i t h those of the B modes observed i n the ( T C N Q - / \ ) ~ and ( T C N Q - d ) anions, as reported by Bozio et al. T h e values i n parentheses i n columns six, seven and eight indicate the observed isotopic shifts i n each line, as compared to the protonated T C N Q molecule. _ 3 u 4 4 1 T h e structure of T T F - T C N Q both above and below it's phase transitions, have been studied by the use of satellites i n inelastic x - r a y diffuse scattering [14]. These studies 3 have shown the existence of a 2kp modulated phase whose fluctuations are visible up to 150 K , as well as a Akp modulated phase which is qualitatively evident even at room temperature. These two phases have been found to have comparable intensities at 54 K while the intensity of the 2kp phase decreases rapidly w i t h increasing temperature above 54 K . However the small size of T T F - T C N Q samples generally available, and the weakness T h e frequencies reported by Bozio et al. were obtained from T C N Q ions in a solution of L i T C N Q . i n t e n s i t y of Raman lines as follows: v v s - v e r y very strong; v s - v e r y strong; s-strong; m - m e d i u m ; w - weak. Satellites are reflections off lattice planes which are formed by the periodic lattice distortion that occurs during a Peierls transition. The intensity of satellite reflections are usually several orders of magnitude lower (typically < 1 0 ) than those which arise from the original periodic structure, prior to the Peierls transition. x - 3 - 4 4 Chapter 4. Experimental Measurements and Results 61 of the satellite reflections, i n i t i a l l y prevented a detailed knowledge of the nature of the precursor effects above 54 K and the low temperature modulated phase from being found. T h i s lead to a work more than ten years later by Coppens et al. [43] who used intense synchrotron radiation as a probe to study the modulated phase at 15 K . They found the largest modulation to be a slip of the T T F molecules along thier mean molecular plane. In the T C N Q molecules, they found that a smaller slip along the mean molecular plane as well as a small but significant displacement perpendicular to the mean molecular plane. T h e i r analysis also showed that the m a i n molecular distortion was qualitatively conserved from the precursor effects above 54 K down to the lowest temperature modulated phase. T h e i r d a t a also raised the possibility that the (longitudinal) 4kp distortion might be localised m a i n l y on the T C N Q chain. In an attempt to improve the structural determination of the lowest temperature (T < 38 K ) 2kp modulated phase by including the possibility of l o w frequency intramolecular distortions, Bouveret et al. [44] used a conventional r o t a t i n g anode x - r a y source to measure a large number (621) of 2kp satellite reflections. These reflections showed that the T T F molecule d i d undergo a slip along it's mean molecular plane, i n agreement w i t h the study by Coppens et al. In the T C N Q molecules, however they found i n addition to the displacement of the molecule itself, a distortion involving only the quinoid ring of the T C N Q molecule. The quinoid ring behaviour consisted of a large translational component perpendicular to the molecular plane together w i t h a significant translational component along the a - a x i s . T h i s large o u t - o f - p l a n e distortion of the T C N Q molecule in the lowest temperature phase is confirmed by the results obtained i n this work using the technique of R a m a n scattering at 10 K . ^ 5 4 ( B ) , the lowest frequency B3„ mode is the most intense feature 3 u observed in the 10 K R a m a n spectra (see F i g . 4.6 and F i g . 4.7) and corresponds to the o u t - o f - p l a n e bend of the quinoid ring, reported by Bouveret et al. Chapter 4. The B Experimental 3 u Measurements and Results 62 distortional modes of the T T F - T C N Q molecule are usually infrared - active and R a m a n - inactive. In fact the presence of the C D W which is incommensurate along the b-axis (or the x - a x i s as i n F i g . 2.1) clearly removes the translational symmetry of the crystal, thereby removing this selection rule. However this alone would not be enough to produce such intense features. A s the amplitude mode of the C D W is R a m a n active [45] it is possible for oscillations in the amplitude of the C D W to appear in the R a m a n spectrum. Such oscillations in the amplitude of the C D W could arise if the C D W involved o u t - o f plane distortions of the quinoid ring, and if the two T C N Q molecules i n adjacent cells vibrate 180° out of phase. These B 3 u lines were not noticed i n the R a m a n spectra at 40 K and 77 K by K u z m a n y et al. [25, 26]. Thus the appearance of these lines i n the Resonant R a m a n spectra suggests that the electronic transition producing the R a m a n resonance involved the same carriers that formed the C D W , which led to the strong scattering. The integrated intensities of the observed B 3 t l R a m a n lines i n T T F - / i - T C N Q - / j 4 4 and T T F - / z - T C N Q - d t as a function of temperature are shown i n F i g . 4 . 8 - F i g . 4.12 while the 4 temperature dependence of the frequencies of these lines are shown i n F i g . 4 . 1 3 - F i g . 4.17. In each case the error bars are the standard deviation of the mean value among the total runs performed at each temperature. W i t h i n the accuracy of the data, there seems to be no indication of the phase transitions at 54 K , 49 K and 38 K . However due to the temperature range i n which these lines are observed, it seems very likely that they are caused by the fluctuating C D W (above 54 K ) as well as the three - dimensionally ordered C D W (below 54 K ) . In this case, one would assume that it is the 2k F 4k F modulated phase, rather than the phase that is responsible for these lines, as the latter is visible i n the x - r a y scattering even at 220 K . Chapter 4. Experimental Measurements and Results 40 I i i 0 25 i i . 50 i i i 75 100 i i 125 i i 150 i i , 175 i 200 i I 225 Temperature (K) Figure 4.8: T h e temperature dependence T T F - / i - T C N Q - / i and T T F - / i - T C N Q - d . 4 4 4 4 of the intensity of the f 5 4 mode Chapter 4. Experimental 16 i 0 Measurements and Results 64 , , 1 0 20 40 . , i , , , , 60 i 80 t 100 120 140 , , , ,i 160 180 Temperature (K) Figure 4.9: The TTF-/i -TCNQ-/i 4 4 temperature dependence and T T F - / i - T C N Q - d 4 4 of the intensity of the f 5 3 mode i n Chapter 4. Experimental Measurements and Figure 4.10: T h e temperature dependence T T F - / i - T C N Q - / i and T T F - / i - T C N Q - d . 4 4 4 4 Results of the 65 intensity of the u2 5 mode i n Chapter 4. Experimental Figure 4.11: Measurements T h e temperature and dependence T T F - ^ - T C N Q - ^ 4 and T T F - / i - T C N Q - d . 4 4 Results of the 66 intensity of the v$\ mode i n Chapter 4. Experimental Measurements and Figure 4.12: T h e temperature dependence T T F - / i - T C N Q - / i and T T F - / * - T C N Q - d . 4 4 4 4 Results of the intensity of the f 5 0 mode Chapter 4. Figure TTF-/14 Experimental Measurements 4.13: Temperature - TCNQ-<2 4 and dependence Results of the frequency of the u 5i mode Chapter 4. Experimental Measurements and Results 234 233 o <5 c 232 o > CB ^ 231 23 O 75 100 125 Temperature (K) Figure 4.14: Temperature TTF-/z -TCNQ-d 4 4 dependence of the frequency 150 of the 175 u 53 200 mode Chapter 4. Experimental Measurements and Results 428 427 o <5 426 r- co 425 h 424 75 100 125 T e m p e r a t u r e (K) Figure 4.15: Temperature TTF-/i -TCNQ-d 4 4 dependence of the frequency 150 of the 175 u 52 200 mode Chapter 4. Experimental Measurements and Results 71 574 573 572 o 571 - o> Q E 570 a> > 569 3 568 567 566 75 100 125 T e m p e r a t u r e (K) Figure 4.16: Temperature TTF-/i -TCNQ-d 4 4 dependence of the frequency 150 of the 175 v$i 200 mode in Chapter 4. Experimental Measurements and Results 72 740 7 3 8 f- V 5 0 (B 3 u ) 736 <5 734 732 a> > co 730 728 726 h 724 O 10 20 30 40 50 60 70 80 90 1 OO H O 120 Temperature (K) Figure 4.17: Temperature TTF-/i4-TCNQ-d 4 dependence of the frequency of the u 50 mode in Chapter 4. Experimental Measurements and Results 73 F i g . 4.18 shows the 10 K spectra of T S e F - T C N Q . In contrast to the T T F - T C N Q compounds, there was no dramatic changes observed upon cooling down. Nevertherless the relatively weak features at 133 c m a n d 237 c m c o u l d probably be assigned to the _ 1 ^54 ( B ) and f 3 u 5 3 _ 1 ( B ) modes respectively. In this case, the weakness of the features would 3 u suggest that the C D W on the T C N Q chain is very small or that the C D W d i d not involve o u t - o f - p l a n e distortions. A small contribution from the T C N Q chains to the C D W is i n agreement w i t h previous works which have suggested that the C D W is mostly confined to the T S e F chain leading to the one strong observed transition at 29 K . Diffuse x - r a y scattering by Y a m a j i et al. [46] have shown that the C D W involved a shift of the T S e F molecules i n a direction nearly parellal to their mean molecular planes. T h e large scattering factor of the massive selenium atoms made it hard to observe any scattering contribution from the T C N Q molecules. A n infrared analysis of T S e F - T C N Q by Bates, Eldridge and Bryce [31] has also indicated that the predominant contribution to the C D W came from the T S e F molecules. 4.3 Check Measurements A s a check on possible experimental effects, a number of additional measurements were performed by Y u a n k u n L i n . F i r s t i n order to ensure that the sample was cooling properly 4 at the end of the cold finger of the H e l i - t r a n refrigerator, the 2 K R a m a n spectra of TTF-/&4 - T C N Q - / 1 4 was measured using a Janis " supervaritemp" O p t i c a l Research D e w a r . 5 Here the sample was immersed i n superfluid liquid helium which ensured that the sample temperature was about 2 K . T h e new lines assigned to the B 3u modes were also observed in these spectra. 4 5 Y u a n k u n L i n is a P.hD student working in the same laboratory as the author. T h e Janis "supervaritemp" Optical Dewar has recently being described in [41]. Chapter 4. Experimental Measurements and Results 74 10 9 1415.1 (v ) 10 K 295 K 8 4 1607.8 (v ) 3 6 1204.8 (v ) s 5 133.1(v ) 54 4 |237.2(v ) 53 3 598.0(v ) 8 342.2(v )' 2211.4(v,). 966.4(v ) 6 9 721.6(v )' 2 7 750 1000 1250 1500 1750 Wavenumber (cm") Figure .4.18: T h e 10 K spectra of T S e F - T C N Q N u m b e r of scans: Resolution: Incident Laser Power: 4000 4 cm 60 m W - 1 2000 2250 2500 Chapter 4. Experimental Measurements and Results 75 Secondly to avoid the possible contamination of the original samples used, the 2 K R a m a n spectra of a new sample of T T F - / i - T C N Q - / i 4 4 prepared by L . K . Montgomery i n 1994, was also measured using the Janis Dewar. A g a i n the new B 3 u lines were observed. T h e intensity of the new lines measured using the Janis Dewar were lower than those obtained using the H e l i - t r a n refrigerator. T h i s might possibly be due to the reduction in the amount of scattered radiation being collected from the sample due to the liquid helium surrounding the sample. It should also be noted that the intensity of the new lines depended inversely on the cooling rate of the samples. T h i r d l y to avoid a similar contaminationarising from the use of K B r i n the samples, the R a m a n spectra of T T F - / i - T C N Q - / i 4 4 using only crushed and compacted crystals was measured using the H e l i - t r a n refrigerator. T h e absence of K B r i n this case caused the sample to heat, raising the it's temperature to about 120 K . T h i s approximate value for the temperature of the sample was calculated by measuring the frequency shift of the f ( A ) 3 3 line compared to it's 295 K and 10 K values and assuming an approximate linear variation of frequency w i t h temperature. A g a i n the new lines were observed, w i t h their intensity comparable to those measured at 130 K , using the H e l i - t r a n refrigerator. F i n a l l y the 2 K R a m a n of a single - crystal sample of T T F - / z - T C N Q - / t 4 4 prepared by L . K . Montgomery was measured using the Janis Dewar. In this case only the A 9 modes present i n the room temperature spectra, and possibly some other new lines were observed. T h e new B 3 u lines described were not observed. Chapter 5 Conclusions T h e resonant R a m a n spectra of powder samples of protonated, deuterated T T F - T C N Q and protonated T S e F - T C N Q partially and fully were measured as a function of temperature ( 1 0 K - 2 9 5 K ) . T h e observed lines at room temperature (295 K ) were assigned to eight of the totally symmetric A g modes of the T C N Q molecule, on the basis of the observed isotopic shifts. N o lines from the T T F molecule were observed due to the laser resonance w i t h only the T C N Q molecule. A s the temperature was lowered below approximately 200 K a number of new lines began to appear i n the T T F - T C N Q samples. T h e intensity of these lines grew as the temperature was decreased. These lines were assigned to the B 3 u o u t - o f - p l a n e distortional modes of the T C N Q molecule on the basis of the observed isotopic shifts. T h i s was i n agreement w i t h a recent x - r a y study which found that such o u t - o f - p l a n e distortions of the T C N Q molecule occur in the lowest temperature 2kp modulated phase. normally R a m a n - inactive B 3 u T h e observance of the modes was attributed to oscillations i n the amplitude of the R a m a n - a c t i v e amplitude mode of the C D W , due to the vibrations of the B 3u normal modes. It is assumed the carriers that formed the C D W were the same ones involved in the electronic transition that produced the resonant R a m a n spectra. T h e 10 K R a m a n spectra of T S e F - T C N Q was virtually identical to that at 295 K . However two of the features present at 10 K might be due to the v 54 ( B ) and ^ 3 u 5 3 (B ) 3 u modes. T h i s would indicate that the C D W on the T C N Q chain was quite weak or that the 76 Chapter 5. Conclusions C D W d i d not involve any T C N Q o u t - o f - p l a n e distortions. Bibliography [1] S. K a g o s h i m a , H . Nagasawa and T . Sambongi, (Springer-Verlag, B e r l i n Heidelberg), 48 (1988). One-Dimensional Conductors, [2] D . S. Acker, L . R . Melby, R . J .Harder, W . R . Hertler, W . Mahler, R . E . Benson and W . E . Mochel, J. Am. Chem. Soc., 82, 6408 (1960) ; D . S. Acker and D . C . B l o m s t r o m , U . S . Patent N o . 3162641, 1962. [3] F . W u d l , G . M . S m i t h , E . F . Hufnagel, J. Chem. Soc. Chem. Commun., 1453 (1970). [4] J . Ferraris, D . O . Cowan, V . V . W a l a t k a , Jr. and J . H . Perlstein, J. Am. Chem. 95, 948 (1973). Soc., [5] L . B . Coleman, M . J . Cohen, D . J . Sandman, F . G . Yamagishi, A . F . G a r i t o and A . F . Heeger, Solid State Commun., 12, 1125 (1973). [6] J . Bardeen, Solid State Commun., 13, 357 (1973). [7] P. A . Lee, T . M . Rice, P. W . Anderson, Phys. Rev. Lett., 31, 462 (1973). [8] J . T . Teidje, J . F . Carolan, A . J . Berlinsky and L . Weiler, Can J. Phys., (1975). [9] T . J . Kistenmacher, T . E . P h i l l i p s , D . O . Cowan, Acta. Cryst., [10] P. W . R . Corfield and S. J . L a P l a c a , Acta Cryst., [11] R . E . Peierls, Quantum (1955). [12] R . E . Thorne, Physics theory of Solids, 53, 1593 B30, 763 (1974). B52, 384 (1996). (Oxford University Press, London), 108 Today, V o l . 49, N o . 5, 42 (1996). [13] R . Comes and G . Shirane, Highly Conducting One-Dimensional J . T . Devreese (Plenum, New Y o r k ) . 44 (1978). Solids, E d i t e d by [14] J . P . Pouget, S. K . K h a n n a , F . Denoyer, R . Comes, A . F . G a r i t o and A . J . Heeger, Phys. Rev. Lett., 37, 437 (1976) and references contained therein. [15] T . D . Schultz and R . A . Craven, Highly Conducting One-Dimensional by J . T . Devreese (Plenum, New Y o r k ) . 147 (1978). 78 Solids, E d i t e d Bibliography 79 [16] A . J . Berlinsky, Y . Hoyano and L . Weiler, Chem. Phys. Letters, " (1977). V o l . 45, N o . 3, 419 R . Bozio, A . G i r l a n d o and C . Pecile, Chem. Phys. Letters, V o l . 52, N o . 3, 503 (1977). B . L u n e l l i and C . Pecile, J. Chem. Phys., T . Takenaka, Spectrochim. Acta., V o l . 52, N o . 5, 2375 (1970). A 2 7 , 1735 (1971). A . G i r l a n d o and C . Pecile, Spectrochim. Acta., A 2 9 , 1859 (1973). A . Girlando, L . M o r e l l i and C . Pecile, Chem. Phys. Letters, V o l . 22, N o . 3, 553 (1973). R . Bozio, A . Girlando and C . Pecile, J. Chem. Soc. Faraday II, 71, 1237 (1975). C . K . C h i and E . R . N i x o n , Spectrochim. Acta., A 3 1 , 1739 (1975). R . Bozio, I. Zanon, A . G i r l a n d o and C . Pecile, J. Chem. Soc, II, V o l 78, 235 (1978). H . K u z m a n y and H . J . Stolz, J. Phys. C : Solid State Phys., Faraday Transactions 10, 2241 (1977). H . K u z m a n y , B . K u n d u and H . J . Stolz, Proc. Int. Conf. Lattice Dynamics, 1977, E d i t e d by M . Balkanski, ( F l a m m a r i o n Sciences, Paris), 584 (1978). H . T e m k i n and D . B . Fitchen, Proc. Int. Conf. Lattice Dynamics, by M . Balkanski, ( F l a m m a r i o n Sciences, Paris), 587 (1978). Paris Paris 1977, E d i t e d H . Kuzmany, Physica Status Solidi B, 89, K 1 3 9 (1978). S. Matsuzaki, T . M o r i y a m a and K . Toyoda, Solid State Commun., 34, p.857 (1980). S. M a t s u z a k i , M . Onomichi, H . Tomura, S. Yoshida and K . Toyoda, Mol. Cryst. Cryst., 120, 93 (1985). Liq. F . E . Bates, J . E . Eldridge and M . R . Bryce, Can. J. Phys. V o l . 59, N o . 3, 339 (1981). G . Herzberg, Infrared and Raman spectra, (Van Nostrand Reinhold L t d , New Y o r k ) . 61 (1945). E . B . W i l s o n , Molecular Vibrations - The theory of infrared and Raman spectra, ( M c G r a w - H i l l ) , 77 (1955). Vibrational J . E . Eldridge, Fourier Transform Spectroscopy ( P H Y S 512) - a graduate course given in the Department of Physics at U B C . Bibliography 80 A . Smekal, Die Naturwiss., 11, 875 (1923). C . V . R a m a n and K . S. K r i s h n a n , N a t u r e , 1 2 1 , 501 (1928). H . A . K r a m e r s and W . Heisenberg, Z. Physik., 3 1 , 681 (1925). P. A . M . Dirac, Proc. Roy. Soc., 114, 710 (1927). L . A . W o o d w a r d , Raman Spectroscopy : Theory and Practice, manski (Plenum Press, New Y o r k ) , 1 (1967). R . J . B e l l , Introductory 1 (1972). C . C . Homes, M.Sc. Fourier Transform Thesis, University Spectroscopy, of British E d i t e d by H . A . Szy- (Academic Press, New Y o r k ) , Columbia, 32 (1985). J . E . Eldridge, Y . L i n , T . C . Mayadunne and L . K . Montgomery, submitted to Phys. Rev. Lett., ( 0 8 - 0 7 - 9 7 ) . P. Coppens, V . Petricek, D . Levendis, F . K . Larsen, A . Paturle, G . Y a n and A . D . L e G r a n d , Phys. Rev. Lett., 59, 1695 (1987). Y . Bouveret and S. Megtert, J. Phys. France, 50, 1649 (1989). G . Gruner, Density Waves in Solids. ( A d d i s o n - W e s l e y ) , 127 (1994). Frontiers in Physics, E d i t e d by D a v i d Pines, [46] K . Y a m a j i , S. Megtert and R . Comes, J. Physique, 4 2 , 1327 (1981).
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Resonance Raman studies of TTF-TCNQ Mayadunne, Tyronne Christopher Sebastian 1997
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Title | Resonance Raman studies of TTF-TCNQ |
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Mayadunne, Tyronne Christopher Sebastian |
Date Issued | 1997 |
Description | The resonant Raman spectra of powder samples of protonated, partially and fully deuterated TTF - TCNQ and protonated TSeF - TCNQ have been measured as a function of temperature (10K-295K) using a Brucker RFS 100 spectrometer with an infrared laser. A number of previously unobserved lines appear in the spectra of TTF - TCNQ below approximately 200 K . The intensity of these lines increases with decreasing temperature. However they show no discontinuity at the three phase transtions, which establish the three-dimensional order of the fluctuating charge - density - wave (CDW) in TTF - TCNQ . These new lines are assigned to the normally Raman - inactive (and infrared - active) B3u modes on the basis of the measured isotopic shifts. The appearance of these B3u out-of- plane intramolecular distortion modes of the TCNQ molecule are in agreement with the results of a recent x-ray study which found that such out-of plane distortions of the TCNQ molecule occur with the appearance of the CDW on the TCNQ chain. Only the intramolecular modes of the TCNQ molecule are resonant and thus observed. A few of the new lines also appear in the low - temperature spectra of TSeF - TCNQ but are generally much weaker in intensity. |
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Language | eng |
Date Available | 2009-03-26 |
Provider | Vancouver : University of British Columbia Library |
Rights | For non-commercial purposes only, such as research, private study and education. Additional conditions apply, see Terms of Use https://open.library.ubc.ca/terms_of_use. |
DOI | 10.14288/1.0087979 |
URI | http://hdl.handle.net/2429/6564 |
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Master of Science - MSc |
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Physics |
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Science, Faculty of Physics and Astronomy, Department of |
Degree Grantor | University of British Columbia |
Graduation Date | 1997-11 |
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