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A new method for obtaining long optical paths and its use in spectroscopy Marshall, James Kelso 1941

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•A. HEW  METHOD FOR  AND  OBTAIMIMGi- LOUS  ITS USE  OPTICAL  PATHS  IN SPECTROSCOPY  by-  James Kelso Marshall  A Thesis submitted i n P a r t i a l Fulfilment of The Requirements f o r the Degree of  MASTER OF ARTS  i n the Department  TABLE  OF  COmBJTS  Introduction Description of Apparatus (a) New device f o r obtaining long opt i c a l paths (b) Apparatus employed f o r gases at various pressures (c) Apparatus employed f o r lique-  Experiments and Results Bibliography  Appendix Reprint of paper published on part of t h i s work. Acknowledgements  A  NEW  METHOD FOR AND  1.  ITS  OBTAINING- LONG USE  IN  OPTICAL PATHS  SPECTROSCOPY  INTRODUCTION In order to obtain absorption spectra f o r certain substances, i t  i s necessary to employ very long path, lengths of the material being investigated.  For example, i n the case of hydrogen gas, i t can be calculated  that a path length of 1000 kilometers at atmospheric pressures would be required to enable predicted bands i n the near infra-red region due to quadrupole radiation of the hydrogen molecule to be observed, and, f o r the absorption, even i n l i q u i d hydrogen, to be detected, i t would be necessary to have nearly 13 meters of path.  The problem of radiation makes the use of  a straight tube of such a length impracticable.  It can be seen that, i f an  apparatus were designed to pass a beam of light many times through a short column of the l i q u i d , the problem of radiation would be greatly minimized. However, i f gases are being studied, i t i s possible i n certain cases to increase the working pressure, making possible a decrease i n path length.  corresponding  There are, however, two reasons why  i t i s prefer-  able to use long paths at r e l a t i v e l y low pressures rather than short paths at higher pressures. In the f i r s t place, at excessively high pressures, there may be a change i n position, r e l a t i v e intensity and width of the absorption bands. For example, i t was thought that work carried out using a 6-ft. path and a pressure of 1700 l b . per sq.in. and vork using 34 f t . path length and 45 l b . per sq.in. might y i e l d different absorption spectra. In the second place, i f only a small quantity of the gas were available, i t would be imperative to use the same column of gas repeatedly  to get s u f f i c i e n t path length. This long path may be used to advantage also i n studying the Faraday effect, which i s the rotation of the plane of polarization of l i g h t by a substance placed i n a magnetic f i e l d .  Here, a traversal of the tube by  the beam of l i g h t i n the direction opposite to that of the i n i t i a l beam w i l l give double the rotation, sinee the reflected beam i s now traveling i n the opposite direction i n the magnetic f i e l d , and n traversals of the tube produce, n times the angle of rotation of the plane of polarization of the l i g h t .  This, however, i s not the case i n natural rotation by sugar  solutions and quartz, where a r e f l e c t i o n of a beam of l i g h t back on i t s e l f cancels the rotation.  2.  DESCRIPTION OF APPARATUS (a)  New Device f o r Obtaining Long Optical Paths Attempts were made, therefore, to devise an apparatus that could  be used i n the-study of long path lengths of liquefied gases and gases at low and high pressures. for each r e f l e c t i o n .  The f i r s t t r i a l was made using a separate mirror  The i n i t i a l beam from the source, on striking the  spherical mirror M as shown i n F i g . I was reflected t o a small ^ i n . square of aluminized mirror M T , fastened to a universal swivel b a l l and socket joint, placed at approximately 45° to the beam. From t h i s , i t went to a similar small mirror Mg, then down to the spherical mirror M again. process was repeated f o r another set of small mirrors Mg and M  At  to a t h i r d set M5 and Mg. spectrograph.  This  and thence  The beam could f i n a l l y be reflected out t o the  This apparatus was too clumsy f o r any compact work, since  the space available i s governed by the inside dimensions of the tube or vacuum f l a s k containing the substance under investigation.  Fig. I  The,second method used was suggested when, during the work, two sets of reflections were obtained unexpectedly on the set of mirrors Mi and Mg.  This new system consisted of two pairs of 4 i n . by 1 i n . alumi-  nized mirrors with r e f l e c t i o n s occurring as i n F i g . 2, where the central rays of the incident and reflected beams are shown i n the v i c i n i t y of the upper plane mirrors, the arrows indicating the directions of the rays. The lower spherical mirror M i s not exhibited i n t h i s or succeeding diagrams.  F i g . 2.  optical  The-holder consisted of a' rectangular plate of brass PQRS with rectangular hole cut symmetrically out of the center as shown i n F i g . 3& The construction of one i s shown i n Figs. 3(b) and 3(c).  P i g . 3(b)  F i g . 3(c)  A strip of brass J i n . wide and 1/8 i n . thick was bent into the shape GHJKLM.  Two screws at B and B* and single screws at A, 0 and D held the  strip of brass i n place by pressing against the large rectangle.  At the  mid-point of section JK of each s t r i p a hole was bored and a c o l l a r E attached, through which passed an axle FN, to provide a surface to which the mirror could be glued.  By means of t h i s axle and the screws, three  translational and three rotational degrees of freedom f o r a mirror were obtainable.  However, t h i s apparatus was large and cumbersome, and i t was  found almost impossible to get a regular series of reflections at the mirrors. During t h i s experiment, as a possible alternative, two 90° t o t a l l y r e f l e c t i n g prisms were t r i e d instead of the two pairs of mirrors, but were found unsatisfactory f o r the following reasons: I.  When a prism i s used, not more than one beam traversing the  tube w i l l enter the long face of the prism at normal incidence.  A ray  s t r i k i n g t h i s surface obliquely from without w i l l give r i s e to a reflected ray at the glass surface which may t r a v e l back and f o r t h between the spheric a l mirror M and the prism.  Also, because of multiple r e f l e c t i o n s within  the prism, other extraneous beams w i l l originate there which cannot, without d i f f i c u l t y , be separated or distinguished from the desired beam. II.  For best results, the angle of the prism, or the equivalent  angle between the mirrors, i s dependent on the radius of curvature of the spherical mirror.  "When mirrors are employed, the required angle f o r a  given spherical mirror may be obtained readily.  Concerning the extent of  v a r i a t i o n of t h i s angle, i t was found that f o r a p a r t i c u l a r  experimental  arrangement where a series of mirrors of r a d i i of curvature ranging from 8 i n . to 6 f t . were used, the angle between the mirrors varied from 87°45' to 89°41».  I I I . Under certain conditions better results may be obtained by reducing the length of one of the plane r e f l e c t i n g surfaces.  When employ-  ing mirrors, t h i s may be done by overlapping one mirror with the other. TV. For a given spherical mirror, one may obtain quickly a wide range 'of path lengths, without moving spectrograph, tube, or source, by a rotation of the plane mirrors about t h e i r axes, accompanied by an adjustment of the spherical mirror, which i s not p a r t i c u l a r l y c r i t i c a l . (b) Apparatus Employed for Gases at Various Pressures The f i n a l o p t i c a l arrangement used proved highly successful. I t i s described f u l l y i n a paper e n t i t l e d "Method f o r Obtaining Long Optical Paths" published i n the Journal of the Optical Society.  A reprint of t h i s  paper i s attached to the present report. To get the greatest amount of l i g h t , the image of the hot crater of the horizontal p o s i t i v e carbon i n the carbon arc source must be focussed on the diaphragm D attached to the back of the mirror  (Fig. 4), with the  distance between the condensing lens and the diaphragm the same as that between the diaphragm and the spherical mirror, i f the same diameter beam at t h i s mirror as at the lens i s desirable.  This holds only i f the aper-  ture through which the l i g h t enters the absorption tube does not r e s t r i c t the size of the beam.  To get a maximum path length, the second downwardly  directed beam must be as close as possible to the o r i g i n a l beam, since then a l l beams are closer together and therefore more traversals of the tube may be obtained.  The aperture H (Fig. 4) i n diaphragm D i s placed almost  v e r t i c a l l y above the bevelled edge AB of the mirror M^, and the f i r s t h o r i zontal beam from Mg strikes  as close as possible to A.  # Smith and Marshall; J.O.S.A., 30, 338, 1940.  The f i n a l beam,  Fig. 4  reflected to the spectrograph by the small mirror M , would be cut o f f s l i g h t l y by the edge of the mirror Mg and consequently have a cross-section that was not completely c i r c u l a r . In the apparatus used f o r gases at various pressures, the absorpt i o n tube consisted of a 4 f t . length of extra heavy steel pipe, 5 i n . inside diameter with -J i n . walls, and 10 i n . flanges welded on each end. Steel end plates were bolted to the flanges.  The top plate, as  shown i n F i g . 5, had three holes A, B and G d r i l l e d along a diameter.  The  l i g h t enters and leaves the system through A and C respectively, while B serves as an observation port during the lining-up of the various mirrors. The f i r s t aperture A was 3/4 i n , i n diameter, while the other two were -g- i n . i n diameter.. A l l were recessed on the under side and c i r c u l a r glass windows -J i n . i n thickness were waxed i n place.  The c i r c u l a r windows i n A and  C were placed with axes passing through the spherical mirror M, so that the l i g h t would enter and leave perpendicular to the glass windows to cut down r e f l e c t i o n losses. angle.  This was done by boring the recessions at the required  Attached to the under side of t h i s plate was the suspension f o r  - 9-  - l o -  ng.  6(a)  holding the plane mirrorsffi, M„ and 1L as shown i n F i g . 6(a). I S 3  Since  these mirrors had to be rotated about a horizontal axis p a r a l l e l to the l i n e of centers of the three windows, the actual block PQ holding the mirrors was joined to the suspension by a strip of spring brass at ES with two screws TU to get the required adjustment.  In order that these  screws could be adjusted from outside the apparatus, two holes were d r i l l e d i n the upper plate at points half way between A and B, and B and C, and 1§  inches away from the l i n e of centers of the windows.  - 12 When pressure i s applied to the tube, these holes are closed by 5/8" threaded machine plugs.  F i g . 6(b) i s a photograph of t h i s plate and sus-  pension. The lower plate supports a 4 inch stand A on which the spherical mirror rested.  In order to move the spherical mirror H, a strip of brass  B was bent i n the form of a c i r c l e around i t .  A nut D was soldered on one  side, and opposite t h i s , a hole was bored i n the pipe G i n a l i n e with the l i n e of centers of the three windows i n the top plate. a threaded rod E was passed, screwed into the nut D.  Through this hole, This hole i s closed  with a threaded plug when gases under pressure are being studied.  Fig. 7  - 13 -  Since the mirror H had to be t i l t e d s l i g h t l y while i n the pipe, three lugs C were fastened to the c o l l a r on which the mirror rested.  The c o l l a r also  had three nuts F soldered onto i t i n a horizontal plane through which pointed screws pressed on the bottom plate A.  By turning these screws,  the mirror could be t i l t e d . In the wall of the pipe at the level of M-j_ and Mg,(Fig. 6(a) , and d i r e c t l y below the center window, a 3/8 inch hole was bored through which Mj, Mg and Mg could be moved, and the gas introduced. The mirrors were fastened to a p a i r of f l a t brass plates A which were screwed to the axles B (Fig. 8(a)). p a i r had a threaded c o l l a r D projecting back.  The front washer C of each The rear washer E slipped  over t h i s and a nut F clamped i t tight on the block G supporting the mirrors.  G consists of a -5 inch brass plate with a rectangular hole cut  symmetrically i n i t . F i g . 8(b) i s a photograph of t h i s without the plate G.  A ri £  8  F i g . 8(a)  Fig.  8(b)  - 14 -  (c) Apparatus Employed f o r Liquefied Gases In the ease of l i q u e f i e d gases contained i n glass vacuum f l a s k s , the apparatus used resembled that employed i n the study of gases at various pressures i n many respects, but a l l adjustments must be made from above. The top consisted of a disc of balsa wood, completely enclosed i n thin sheet s t e e l , with c i r c u l a r apertures provided f o r the entrance and exit of the l i g h t beams.  The apparatus f o r holding the plane mirrors was similar  to that used f o r the pressure apparatus.  The spherical mirror was sup-  ported from the top by three adjustable rods, and was moved i n the l i n e of centers of the holes by the device shown i n F i g . 9(a).  F i g . 9(a)  This consists  F i g . 9(b)  - 15 -  of the rod A: stipported from above with a thick washer clamped at B.  The  rod A i s free to move i n the sleeve C which i s held a fixed distance above plate H by means of rods D. lever arm E.  The lower end of A i s threaded onto a  This i s pivoted at F to a fixed clamp K.  Rigidly fastened  to E i s a rod having a peg G which extends through a slot i n the plate H. G projects into a hole i n the attachment at the rear of the mirror, and so, when A i s turned, the mirror moves across the plate H.  F i g . 9(b) i s  a photograph of the device. In order to cut out reflections at the surface of the liquefied gas, the plane mirrors M  1  and Mg must be completely immersed, and so the  supports f o r these mirrors, as well as f o r the spherical mirror, must be made of some material of low conductivity, such as stainless steel or German s i l v e r .  3.  EXPERIMENTS AND  RESULTS  For t h i s work on pressure oxygen, a set-up was obtained using twelve passages of l i g h t between the upper mirrors mirror M, the equivalent of a 34 f t . path length.  and Mg and the lower The light from the sys-  tem was reflected from Mg to the s l i t of a Hilger constant deviation spectrograph. For the region between 7000 A  0  and 8600 A°, the most satisfac-  tory plate was the Eastman I - R spectrographic plate, sensitized with a solution of three parts 28% ammonia, two parts of Ethyl alcohol, and seven parts of water.  The plate was placed i n t h i s f o r one minute and then  dried with a fan.  - 16 -  Using such a plate, the absorption spectrum of oxygen under a pressure of 45 l b . per sq.in. was investigated. A print of the spectrogram obtained i s shown i n F i g . 10. 7660 A  I t w i l l be noted that the band at  i s very narrow and actually appears double on the plate.  J i g . 10  For comparison with t h i s plate, another plate of the same sensit i v i t y was taken using a 6 f t . path length with a pressure of 1700 l b . per sq.in.  A reproduction of this plate i s shown i n F i g . 11. I t w i l l be noted  that, although the exposure i s longer, shown by a more complete spectrum, which would tend to make the l i n e s narrower, the band at 7660 A° i s actually broader, and only with considerable d i f f i c u l t y can i t be seen to be a double band. o  F i g . 11  In order to get a complete, general picture of the absorption bands of oxygen, a spectrogram was taken using the 6 f t . tube and 1500 l b s . per sq.in. pressure. to 9000 A°, was used. 0  0  An Eastman I - L spectrographic plate, sensitive up 0  Well defined bands are shown at 7660 A , 6920 A°, 0  6360 A , 5830 A , 5360 A , 4980 A° and 4800 A°.  A print of t h i s plate i s  - 17 shown as F i g . IE.  F i g . 12 As a f i n a l exposure, an Eastman I - Z spectrographic plate, sens i t i v e i n the extreme infra-red up to 12,000 A° was used, and the bands at o 9100 A  ° and 10,400 A  f i r s t observed, using absorption of light by l i q u i d  oxygen, by McLennan, Smith and Wilhelm,^ and, as f a r as i s known, never before observed with gaseous oxygen, were clearly v i s i b l e .  A print of thii  i s included as F i g . 13.  J F i g . 13  5.  BIBLIOGRAPHY McLellan, Smith and Wilhelm; Trans. Roy. Soc. Can. I l l , 24, 1, 1930. Smith and Marshall; J.0.S.A,, 30, 338, 1940.  # McLennan, Smith and Wilhelm; Trans. Roy. Soc. Can. I l l , 24, 1, 1930.  Reprinted from J O U R N A L OF T H E O P T I C A L SOCIETY OF A M E R I C A , Vol. Printed in U . S. A.  30,  No.  8, 338-342, August,  1940  Method for Obtaining Long Optical Paths H.  D . , SMITH AND J. K .  MARSHALL  University of British Columbia, Vancouver, British Columbia (Received June 2, 1940)  W  H I L E studying the absorption Spectra of certain liquids, and gases at high pressures, it became necessary to go to very thick layers of the substances under investigation. Many of the methods previously described for obtaining long optical paths could not be employed due to the fact that the substances being studied were available only in columns of restricted length. A simple apparatus was devised finally by means of which an absorption spec-' trum, for a path length of several hundred feet, of a substance confined in a limited space, could be obtained with a relatively short exposure time. A brief description of the optical system evolved is given here, accompanied by a series of photographs of actual light paths illustrating several variations of the system which were used under special experimental conditions. The general experimental arrangement is indicated in Fig. 1, which is a schematic diagram of one modification of the optical system. The positions of the various optical parts were determined roughly in the following manner. The spherical mirror M% was placed at one end of the tube containing the absorbing medium in such a way that two plane mirrors Mi and Mi, situated at the other end of the tube, were at the center of curvature of Ms. A horizontal beam of light  from the source S was condensed by the lens L and reflected by a small plane mirror M in a direction designated by beam A in Fig. 1. Successive reflections of the beam at Ms, Mi, Mi and again at Ms, gave rise to the beams B, C,D as shown in the diagram. The light emerged in a horizontal direction after reflection at the upper part of Mi and was focused on the slit of a spectrograph. All mirrors used were frontsurfaced with evaporated films of aluminum or silver and were completely immersed in the absorbing medium. Regarding the finer adjustment of the apparatus, it was found that when the longest possible path lengths were desired, best results could be obtained by rotating either Mi or Mi about a horizontal axis until the portions of the beams between the two mirrors Mi and M% were parallel to one another as shown in Fig- 7a. Under these conditions it is obvious that the angle between Mi and Mi is slightly less than a right angle. An abstract of some recent work of Kratz and Mack gives a method for obtaining a long optical path in a limited space by the use of a truncated totally reflecting prism and a spherical mirror. A somewhat similar arrangement, involving the use of a 90° totally reflecting prism, was tried out during the present work. However, it was considered advantageous to replace the prism by the mirrors Mi and Mi, for the following reasons— 1  I. When a prism is used, not more than one beam of the group traversing the tube will enter the long face of the prism at normal incidence. A ray striking this surface obliquely from without will give rise to a reflected ray at the glass surface which may travel back and forth between Ms arid the prism. Also, because of multiple reflections within the prism, other extraneous beams will originate there which cannot, without difficulty, be separated or distinguished from the desired beam. F I G . 1. Schematic diagram of one modification of the optical system.  1  338  Kratz and Mack, Phys. Rev. 57, 1059A (1940).  M E T H O D  F O R  O B T A I N I N G  L O N G  O P T I C A L  b  a F I G . 3.  P A T H S  H .  D .  S M I T H  A N D  J .  K .  M A R S H A L L  340  b  a F I G . 7.  H.  D.  S M I T H  A N D  J.  K.  M A R S H A L L  342  II. For best results, the angle of the prism, or which the small mirror M is eliminated, the the equivalent angle between the mirrors M\ and initial beam striking Mi. It may be noted that, M%, is dependent on the radius of curvature offor the first two methods, the reflections at the the spherical mirror. When mirrors are employed plane mirrors M\ and Mi after successive trathe required angle for a given spherical mirror versals of the tube form a series approaching the may be obtained readily. Concerning the extent junction of Mi and Mi and emerging there. of the variation of this angle, it was found that Figures 4a and 5a show variations of a system in for a particular experimental arrangement where which the series of reflections at the plane mirrors a series of mirrors of radii of curvature ranging approaches the junction, crosses over and then from 8 in. to 6 ft. were used, the angle between recedes from the junction. In Fig. 4a a small Mi and ikf varied from 87° 45' to 89° 41'. mirror at L is used to deflect the final beam comIII. Under certain conditions better results ing from the outer edge of Mi, while in Fig. 5a may be obtained by reducing the length of one this mirror is eliminated by moving Mi in its of the plane reflecting surfaces. When employing plane so that it overlaps Mi at the junction, and mirrors, this may be done as illustrated in Fig. the emergent ray is reflected from Mi. In Fig. 6a the initial beam enters between the two plane 5a by overlapping one mirror with the other. mirrors and after a series of reflections which IV. For a given spherical mirror one may obapproach the edges, the emergent beam is retain quickly, a wide range of path lengths, withflected from the spherical mirror to the spectroout moving spectrograph, tube or source, by the graph. Fig. 7a shows the pattern obtained when rotation of either Mi or Mi about its axis, accompanied by an adjustment of M% which is not the light traversed the tube 15 times. The final beam could be reflected out from the center of particularly critical. V. Some of the modifications of the' optical the group by means of a small mirror. As may be system exhibited in the photographs could not expected, it is difficult to obtain a photograph of be obtained with a prisrrl without considerable a larger number of traversals due to the fact that the intensity of the light along the path didifficulty. minishes rapidly as it is scattered by the smoke Figures 2a-7a show the actual paths of rays particles. in the vicinity of the plane mirrors for a number In general one sees that for every two traof variations of the optical system. These plates were obtained by photographing the light versals of the tube there are three reflections at scattered by smoke particles introduced in the the mirrored surfaces, one being at almost normal paths of the beams. The central rays of the beams incidence, and two, at approximately 45° inciare shown in the diagrams accompanying the dence. In one case with a mirror of 85 cm radius plates, the arrows indicating the directions of the of curvature 37 traversals were obtained without rays. If we suppose "that the oblique rays con- undue difficulty. With a mirror of longer radius verge at a point 0 on the spherical mirror, then of curvature an even greater number of traversals the path in each case may be written in terms of could be obtained. It was found that the above methods could be employed equally well with the letters in the diagrams. Thus, in Fig.-4b, the the absorption tube in either a horizontal or light follows the path ABOCDOEFOGHOJKLM. vertical position. The system may be used to For the photographs reproduced here, the spheradvantage also in light scattering experiments ical mirror had a radius of curvature of 5 ft., and in the determination of the Verdet constants and the plane mirrors, Mi and Mi were 2 in. of gases and liquids. long and 1 in. wide. A carbon arc, drawing 12 The latter part of this work was carried out amp. in a 110-volt, d.c. circuit, served as a source at the University of California and the authors of light. are indebted to the Physics Department of that Figure 2a shows a system, in which there are institution for the use of its facilities. Thanks are 10 traversals of the tube and where the absorp- due particularly to Professors Birge, Brode, tion path is approximately 50 ft. in length. Jenkins and White for their kindness in providing Figure 3a gives a modification of this method in special equipment. 2  ;  AOEITOILBDQ-EMENTS  Thanks are due to Dr. H. D. Smith, who suggested t h i s problem and assisted i n the work. Part of t h i s work was carried out at the University of C a l i f o r n i a , and the writer i s indebted to the Physics Department of that i n s t i tution f o r the use of i t s f a c i l i t i e s .  Thanks are  due p a r t i c u l a r l y to Professors Birge, Brode, Jenkins and White f o r t h e i r kindness i n providing special equipment.  

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