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The effects of pressure on the after-glow of nitrogen Clayton, Henry Hubert 1937

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THE  EFFECTS OK  AFTER-GLOW  OF PRESSURE THE OF NITROGEN.  by Henry  Hubert  Clayton  A Thesis submitted, f o r the Degree of Master of Arts In the Department of Physics.  THE UNIVERSITY OF BRISISH COLUMBIA September -  1937.  TABLE OE CONTENTS. I.  PURPOSE.  I I . DISCOVERY OE THE AFTER-GLOW.  Page 1 . Page 1.  I I I . PREVIOUS EXPERIMENTAL RESEARCHES. A. The Spectrum of the After-glow. B. Spectra excited by Active Nitrogen.  Page 1 , Page 3.  C. E f f e c t s of Temperature^ on the After-glow.  Page 4.  D. Effects of Pressure on the After-glow.  Page 5.  E. E f f e c t s of an E l e c t r i c F i e l d on the After-glow. F. Rate of Decay of the After-glow.  Page 6. Page 7.  IV. THE NATURE OF ACTIVE NITROGEN. A. Metastable Molecules.  Page 8.  B. Metastable Atoms.  Page 9.  C. D i s s o c i a t i o n .  Page 10.  V. THE PROCESSES OF EXCITATION AND DECAY OF THE AFTER-GLOW. A. E x c i t a t i o n .  Page 12.  B. Decay (1) The sharp s e l e c t i o n of the bands and deactivation of the metastable atoms. Page 14. (2) Deactivation of the metastable molecules. . Page 15. VI. DEPENDENCE OF INTENSITY AND RATE OF DECAY ON CONCENTRATION.  Page 17.  TABLE OF CONTENTS, (Contd.) VII. APPARATUS. A. Source of Nitrogen.  Page 24.  B. The Discharge Tube.  Page 24.  C. The Compression Tube.  Page 25.  D. E l e c t r i c a l Arrangements.  Page 26,  BIBLIOGRAPHY.  ILLUSTRATIONS. Figure 1.  Energy l e v e l diagram of Nr>. Facing Page 2.  Figure 2.  Discharge and Compression Tubes.  Facing Page 24.  THE EFFECTS OF PRESSURE ON THE AFTER-GLOW OF NITROGEN, I.  PURPOSE. The purpose of the work described i n t h i s thesis was  to Investigate the e f f e c t of pressure on the luminosity of the afterglow of nitrogen.  I t was hoped thereby to c l a r i f y the  processes taking place when the afterglow i s emitted, and i n p a r t i c u l a r , to determine whether they involve c o l l i s i o n s of two, or more, bodies. I I . DISCOVERY OF THE AFTER-GLOW. The modification of nitrogen known as "active n i t (1) rogen" was discovered by E. P. Lewis  i n 1899.  Lewis found  that the passage of a condensed discharge through nitrogen i s followed by the emission of a yellow afterglow, which may p e r s i s t for some minutes after the termination of the e x c i t i n g discharge.  The glowing gas was found to have a considerable  chemical a c t i v i t y , whence- the name "active nitrogen". discovery i n i a t e d a long series of experimental  This  researches,  In the course of which the glowing gas was found to have many important properties. A b r i e f o u t l i n e of these researches be given III.  PREVIOUS EXPERIMENTAL RESEARCHES. A.  The Spectrum of the Afterglow.  will  To face Page 2  C  i3 ' £  «  2  C  -I  /a  A/,  hern  cTS  /6 J  C*7T V  \-h  /2  /s/V  6 -  4^ 4  A4  Energy  /eve/ aft a^r am of A/a  2«  The spectrum of the afterglow, which i s e n t i r e l y d i f f e r e n t from that of the e x c i t i n g discharge, consists  (2)  of nearly a l l the "bands of the F i r s t P o s i t i v e Group of These bands are ascribed to the t r a n s i t i o n B TT—»•  nitrogen. 3  A 22 (see figure 1.) of the n e u t r a l nitrogen molecule.  The  d i s t r i b u t i o n of i n t e n s i t y among the bands d i f f e r s markedly from that found i n ordinary discharge, c e r t a i n bands being selectively intensified. These are, at ordinary temperatures, (3)  the bands  o r i g i n a t i n g on the v i b r a t i o n a l l e v e l s IS to 10  of the e l e c t r o n i c state B and terminating on the l e v e l s 9 to 7 of the state  A,  and also the bands  and  Bg-^A^  In the  By—*A . 4  f i r s t of these groups the bands B ^ — * A g and B-Q—*-A are most intense. At lower temperatures the i n t e n s i t y maximum i s s h i f t e d (4) towards the v i o l e t , and the s e l e c t i o n i s narrower, the bands 7  (3)  s t a r t i n g on B  1 2  and B  &  being most intense.  These bands are  the ox-bands of the afterglow. Three other systems of bands, the yfland > groups, discovered by l e w i s ^ and the group by X h a u s s ^ , were (2)  shown by S t r u t t  to belong to the spectrum of NO, and to be  due to traces of oxygen,  (7)  In addition to the above bands Bay and S t e i n e r ' v  found l i n e s of N-I and N-II i n the afterglow,  K i c h l u and  to)  Achaiya glow.  investigated the i n f r a - r e d spectrum of the a f t e r The found an extension of the F i r s t P o s i t i v e system,  but no trace of atomic l i n e s i n t h i s region, The absorption spectrum of the afterglow has been investigated by Sponer  to)  and by Ruark, Foote, Rudnick and  3» Chenault^ ^. 10  transition  Sponer f a i l e d to obtain any trace of the  A Z>-»X 'Z> 3  1  i n that region of the u l t r a - v i o l e t i n  which i t was to be expected.  She also examined the u l t r a -  v i o l e t spectrum of nitrogen i n emission, but f a i l e d to f i n d bands corresponding to t h i s t r a n s i t i o n .  Ruarx, Foote et a l  investigated the region between 6,500A and 3,200A and found no absorption by the afterglow. B.  Spectra excited by Active Nitrogen.  I t was early found that active nitrogen i s able to excite other gases or vapours with which i t comes i n contact to the emission of t h e i r c h a r a c t e r i s t i c spectra.  This  e x c i t a t i o n i s often accompanied by chemical r e a c t i o n . Thep, yand£"bands mentioned above, which belong t o the spectrum of NO and are due to the presence of oxygen, are an example of this.  Other instances are the e x c i t a t i o n of m e t a l l i c l i n e  spectra, which i s u s u a l l y accompanied by the formation of nitrides.  S t r u t t and Fowler  examined the spectra of  several metals and compounds.  They found the spectra of Na,  K, Hg, Th to approximate to the arc spectra, while that of Mg contained no arc l i n e s , but was nearly the flame spectrum. The admixture of CuCl produced many l i n e s of copper, besides the bands of the c h l o r i d e .  S i m i l a r l y admixture of stannous  and stannic chlorides produced l i n e s of t i n . (12) Mullllcen  examined the spectra of the copper halides  when excited by active nitrogen.  He found, confirming Strutt  and Fowler, the entire arc spectrum of copper produced.  The  4 .  i o n i s a t i o n p o t e n t i a l of copper i s 7.7 v o l t s .  The bands of  Cul excited were those corresponding to e l e c t r o n i c l e v e l s with energy of from 2 . 4 4 to 2.96 v o l t s . Foote, Ruark and Chenault  ' found that l i n e s of  mercury r e q u i r i n g up to 9.52 v o l t s for their e x c i t a t i o n were strongly developed, while exposures of 150 hours f a i l e d to. . (6) bring out l i n e s r e q u i r i n g 9.66 v o l t s or higher.  Knauss  experimented with hydrogen, oxygen, n i t r i c oxide and carbon monoxide.  The f i r s t e l e c t r o n i c l e v e l of the hydrogen  molecule has energy of 11.1 v o l t s . molecules were obtained.  No bands due to hydrogen  In the case of carbon monoxide,  bands whose i n i t i a l l e v e l s have energyfrom 8.2 to 9.0 v o l t s were photographed. Those r e q u i r i n g higher e x c i t a t i o n were (14)  not observed.  Kaplan  x  "*' found that the auroral green l i n e  of the oxygen molecule can be excited by active nitrogen i n the presence of argon. its excitation.  This l i n e requires 4.17 v o l t s for (15)  Okuba and Hamada  spectra excited by active nitrogen.  studied many m e t a l l i c Their conclusion was  that the highest energy a v a i l a b l e i s about 9.5 v o l t s . For example, i n the case of mercury the highest l e v e l excited required 9.51 v o l t s .  Evidence of the e x c i t a t i o n of other  l e v e l s r e q u i r i n g s l i g h t l y higher energy was not obtained. C.  E f f e c t s of Temperature on the Afterglow.  S t r u t . t h e a t e d l o c a l l y a glass tube through which active nitrogen was flowing.  He found that the luminosity  of the afterglow was diminished or extinguished at the point of a p p l i c a t i o n of the heat, and that the luminosity and  5. attendant a c t i v i t y reappeared after the streaming gas passed on to a cooler part of the tube.  had  On the other hand he  observed that l o c a l cooling caused an increase i n luminosity, accompanied by a faster decay of the afterglow and l o s s of activity.  Heat however applied to a vessel containing the  glowing gas diminished the luminosity, and quickly destroyed the a c t i v i t y ;  while cooling caused an increase of luminosity  and s i m i l a r f a s t e r reversion to ordinary nitrogen.  The  a p p l i c a t i o n of heat i n the two cases apparently causes contradictory results.  S t r u t t pointed out that t h i s e x p l i c a b l e by A  the assumption of two processes i n the decay of the afterglow. F i r s t l y there i s a w a l l e f f e c t by which the active nitrogen reverts to the normal state without the emission of the a f t e r glow.  This e f f e c t i s increased by heating.  Secondly there  i s a process taking place i n the body of the gas, the reversion to the normal state being accompanied by r a d i a t i o n . process must be delayed by heating.  This  Conversely the body  process appears to be a c t u a l l y accelerated by cooling. D.  E f f e c t s of Pressure on the Afterglow.  S t r u t t p o i n t e d out that the e f f e c t of cooling on active nitrogen, e s p e c i a l l y i n the case of the flowing gas, " might be a t t r i b u t e d to a l o c a l condensation. possibility he^ ^ 1  To test t h i s  compressed the glowing gas i n a vessel by  means of a column of mercury.  The increase of pressure  was  accompanied by a large increase i n the i n t e n s i t y of the a f t e r glow, and by a decrease i n i t s duration.  This dual effect of.  pressure i s evidence that the emission of the afterglow i s not  a monomolecular e f f e c t .  S t r u t t was not able at that time to  make any measurements to determine whether the process involves c o l l i s i o n s of two or more bodies, E.  E f f e c t s of an E l e c t r i c F i e l d on the Afterglow.  In order to investigate whether there are ions present i n active nitrogen, S t r u t t ^ ) passed a stream of the glowing 1 8  gas through a tube containing two electrodes, there was an e l e c t r i c f i e l d .  between which  He found that, provided  the  electrodes were i n contact with the glowing gas, a current passed.  He was unable to obtain a saturation current, the  increase of p o t e n t i a l caused the current to increase t i l l a discharge passed.  This subject was further investigated by  Constantinides^*  He employed two sets of electrodes, the  f i r s t set acting as an ion-trap to remove any stray ions s t i l l surviving from the a c t i v a t i n g discharge.  By t h i s means he  able to show that the large conductivity observed i s not to ions from the discharge.  was  due  Moreover, by varying the area of  the electrodes he showed that the conductivity i s due not to ions o r i g i n a t i n g i n the body of the gas, but to electrons l i b e r a t e d from the electrodes.  The observation of S t r u t t ,  already mentioned, that the electrodes must be immersed i n the glowing gas for a current to pass, shows that t h i s emission of electrons i s a w a l l e f f e c t , and not p h o t o - e l e c t r i c . S t r u t t , i n the research mentioned, had found a large increase i n conductivity to r e s u l t from the admixture of sodium vapour with the active nitrogen, while mercury vapour caused no such increase.  This research was extended by  Constantinides  to hydrogen and iodine. With hydrogen also no increase i n conductivity was found.  I n the case of Iodine however an  Increase was observed, and t h i s increase Constantinides showed to be due to the produetion of ions i n the body of the gas, that i s , to i o n i s a t i o n of the iodine and sodium. F.  Rate of Decay of the Afterglow.  The rate of decay of the afterglow has been studied by several workers, with a view to determining whether the deactivation i s caused by c o l l i s i o n s of two, or more, bodies. (PQ)  Rudy found the rate of decay to be increased by increased pressure, and to follow a bimolecular law for the f i r s t 180 (21) seconds of decay. Eonhoeffer and XCaminslcy also showed that the reaction which produces the afterglow i s bimolecular (22) and not trimoleeular.  Kneser  v  , on the other hand, came  to the conclusion that the deactivation i s due to a threebody c o l l i s i o n .  This question i s complicated by the dual (16) process of deactivation pointed out by Strut t • later (23) he  investigated the e f f e c t s of various coatings on the  w a l l s of a large glass vessel i n which active nitrogen was produced by the electrodeless discharge.  He found that by  using metaphosphoric a c i d an afterglow of very long duration can be obtained, showing that the w a l l e f f e c t has been very l a r g e l y eliminated.  He then investigated photometrically  the rate of decay and i t s dependence on pressure.  Under these  circumstances he found that the rate of decay i s consistent with the assumption of a bimolecular process, but the t r i molecular process, on account of the s l i g h t difference i n the  8 .  curves obtained, i s not excluded.  However, an i n v e s t i g a t i o n  of the dependence of the i n t e n s i t y on pressure gave r e s u l t s i n agreement with a bimolecular hypothesis, and quite incomp a t i b l e with a trimolecular hypothesis.  He concluded, also,  that the excess of n e u t r a l nitrogen molecules play no part i n the process of deactivation.  IT. THE NATURE OE ACTIVE NITROGEN, A.  Metastable Molecules.  Several theories as to the nature of active nitrogen have been proposed during the course of the experimental researches outlined above.  That which i s now most generally  accepted and which appears to account best for the experimental (3) facts was proposed by Carlo and Kaplan  .  According to them  active nitrogen consists e s s e n t i a l l y of metastable molecules and metastable atoms.  The f a i l u r e to obtain the A—X  trans-  i t i o n i n either emission or absorption shows that the A S> state of the molecule i s metastable.  Since t h i s l e v e l i s the  lower l e v e l of the <x-bands which are c h a r a c t e r i s t i c of the afterglow spectrum, metastable molecules must be present i n active nitrogen. by Sponer  The energy of the A S s t a t e was 3  determined  from the electron bombardment method, i n eon-  junction with spectroscopic data, as 8.0 v o l t s .  There i s some  question of the accuracy"; of t h i s value, but i n any case the r e a l value appears to be not more than a few tenths of a v o l t less.  The highest v i b r a t i o n a l l e v e l s of the sate A involved  i n the emission of the afterglow are 9 to 7.  These l e v e l s  9. have energy of about 9.5  v o l t s , which should then be the'energy  available i n active nitrogen for the e x c i t a t i o n of other spectra. This w a l u e i s i n good agreement with the data given above.' This explains also the findings of S t r u t t and Constaninides on the effect of foreign vapours on the conductivity of active nitrogen.  Iodine and sodium, which were found to increase  conductivity, have i o n i s a t i o n potentials of 9.4  and 5.10  r e s p e c t i v e l y , while mercury, which gave no e f f e c t , has I o n i s a t i o n p o t e n t i a l of 10.4 B.  the  volts an  volts.  Metastable Atoms.  Cario and K a p l a n ^ ) also assume the presence of metastable atoms i n active nitrogen. shown by some work of Wrede  v  u /  • The presence of atpms i s on d i f f u s i o n .  of the neutral nitrogen Datomo.lo Is a 2  D with 2.37  v o l t s and a ? with 3.56 2  4  The lowest l e v e l  S state, followed by a  volts  two states are known to be metastable.  ( 2 6  ^.  These l a s t  I f atoms i n these  metastable states are present i n active nitrogen they should be e f f e c t i v e i n e x c i t i n g by c o l l i s i o n l i n e s of other spectra requiring suitable energy.  The most important fact that  can  be explained by t h i s supposition Is the e x c i t a t i o n of theocbands of active nitrogen i t s e l f .  The highest l e v e l occurring  i n the emission of these bands i s the B , 12  v o l t s for i t s e x c i t a t i o n from the A  Q  which requires  metastable state.  3.47 The  second prominent Intensity maximum Is at bands s t a r t i n g from the l e v e l Bg. excitation.  This l e v e l requires about 2.5 v o l t s for i t s The e x c i t a t i o n of these two l e v e l s can then be  explained as due to c o l l i s i o n s of metastable molecules i n the  10. A  0  state with %  and  2  D metastable atoms respectively.  As a  farther instance we may r e f e r to the observations of Mulliken (12) on the e x c i t a t i o n of the band spectrum of Cul. C.  Dissociation. 2  I t seems probable that the  2 D and  originate i n the e x c i t i n g discharge.  P metastable atoms  Considerable l i g h t i s  thrown on t h i s question by a paper of Kaplan  ' on the prod-  K  ucts of d i s s o c i a t i o n from the various electronic l e v e l s of the neutral nitrogen molecule.  Kaplan f i n d s , i n the known energies  of the v i b r a t i o n a l l e v e l s of the molecule at which predissociation occurs,  strong i n t e r n a l evidence to show that there i s  d i s s o c i a t i o n at about the 13th. v i b r a t i o n a l l e v e l of the B TF state, the i n i t i a l electronic l e v e l of the oc-bands, into a 4 o normal S atom and a metastable atom; at the 20th. v i b r a t 3  i o n a l of the same state into a normal  atom and a metastable  2p atom; and at the 4th. v i b r a t i o n a l l e v e l of the C ^ T T state ( i n i t i a l electronic l e v e l of the second p o s i t i v e bands occurring i n the spectrum of the a c t i v a t i n g discharge) into two metastable D atoms. 2  A Z?metastable  The v i b r a t i o n a l l e v e l s of the  3  state have been followed to 2.1 v o l t s above the 0th. l e v e l . Kaplan, i n the same paper, gives the most probable value of the d i s s o c i a t i o n p o t e n t i a l of the nitrogen molecule as about 9.0 (24) v o l t s , so that taking account of the f a c t that Sponer's value of 8.0 v o l t s for the energy of the A Q state i s probably somewhat too high, i t appears that d i s s o c i a t i o n should take place from the upper v i b r a t i o n a l l e v e l s of the two normal nitrogen atoms.  A Z5 3  state into  ZLZL« In view of these f a c t s , mild electronic bombardment of active nitrogen should be s u f f i c i e n t to produce d i s s o c i a t i o n . This i s a reasonable explanation of the observations of Strutt on the effects of l o c a l heating.  Heating r e a d i l y causes  d i s s o c i a t i o n of the metastable molecules and hence destroys the afterglow.  Subsequent withdrawal of the heat, however,  permits r e a s s o c i a t i o n of the atoms into metastable and renewed emission of the afterglow.  molecules  Since the metastable  atoms remain when the afterglow has been destroyed by heating, the gas should be s t i l l a c t i v e , though to a' less extent.  The  energy available for e x c i t a t i o n will-how be that of the metastable atoms above t h e i r ground state, 2.47 and 3.56 Cario and K a p l a n ^  volts.  were able to show the a c t i v i t y of t h i s  "dark modification".  They found i t to be capable of e x c i t i n g  the D l i n e s of sodium (2.09 v o l t s ) , though the l i n e s 5683A and 5686A (4.3 v o l t s ) were not observed.  This however i s not  conclusive evidence of the existence of metastable atoms i n (oo\  active nitrogen.  In f a c t Okubo and Hamada  ' question t h e i r  presence at a l l .  They claim to have observed the emission of  the oc-bands and the e x c i t a t i o n of other spectra up to 9.51 volts-i-even i n active nitrogen heated up to 600-650 degrees. Cario and K a p l a n I n v e s t i g a t e d also the effect of a mild, uncondensed discharge on active nitrogen.  Strutt had  already noted that t h i s destroys the afterglow.  Carlo and  Kaplan found that the fourth positive bands of nitrogen were strongly developed.  These bands, which originate on the D S  l e v e l , at 14.8 v o l t s above the normal l e v e l of the molecule,  1 2 .  appear i n the spectrum of the condensed discharge, but cannot be excited i n ordinary nitrogen by an uncondensed discharge. This i s e a s i l y explicable by the hypothesis of metastable molecules, since the D Z7 state i s at only about 6 v o l t s above 3  the A ^ Z ? s t a t e .  Further, they found that the passage of an  uncondensed discharge through the "dark modification" produced by heating does not excite the fourth p o s i t i v e bands, c l e a r l y because of the d i s s o c i a t i o n by heat of the metastable molecules. The evidence i n favour of the presence of metastable molecules and of the d i s s o c i a t i o n thus appears to be strong.  As already  (31)  mentioned, Olcubo and Hamada  question the existence of the  metastable atoms. V.  THE PROCESSES OE EXCITATION AND DECAY OF THE AFTERGLOW. A.  Excitation.  On the assumption that active nitrogen consists e s s e n t i a l l y of metastable A S molecules and metastable 2  D and  P atoms, some explanation of the processes of e x c i t a t i o n and  decay of the afterglow van be given.  No v i b r a t i o n a l l e v e l s  above the Oth. have been observed f o r the D E state of the 3  molecule, upper l e v e l of the fourth p o s i t i v e bands which are strongly developed i n the a c t i v a t i n g discharge.  According to  Kaplan(32) ^ the most probable explanation of t h i s i s that a molecule at a higher v i b r a t i o n a l l e v e l would dissociate into a 2  2  D and a P metastable atom.  I t seems probable'then, i n view  of the prominence of the fourth p o s i t i v e bands i n the exciting discharge, that metastable atoms i n these two states are  Ie3 ©  produced i n large numbers by the discharge.  Okubo and  Hamada(33) made a determination of the T e l o c i t y required by electrons f o r the e x c i t a t i o n of active nitrogen.  They found  that a large increase i n the production of active nitrogen takes place when the accelerating p o t e n t i a l reaches 16 v o l t s . This they a t t r i b u t e d to the production of ionised molecules, low the i o n i s a t i o n p o t e n t i a l of the nitrogen molecule, according to Jevons^ ^, i s from 16.3 to 16.9 v o l t s by d i r e c t electronic 34  bombardment, and from 15.85 to 16.45 v o l t s as calculated, spectroscopically from the e x c i t a t i o n p o t e n t i a l of the N negative bands.  +  The value 16.45 Tolts seems most probable, as  i t y i e l d s a value f o r the d i s s o c i a t i o n p o t e n t i a l best i n agreement with other methods.  Also the energy of the Oth.  l e v e l of the D S state i s about 15 v o l t s . 3  In view of the  uncertainty of these values i t seems equally reasonable to a t t r i b u t e the e f f i c i e n c y of 16 v o l t electrons as exciters of active nitrogen to the production by them of metastable atoms by d i s s o c i a t i o n from the -p^ZJ state, as suggested by Kaplan, as to suppose i t due to the production of ionised molecules, the suggestion of Okubo and Hamada. The production of metastable A^E. molecules i s e a s i l y accounted f o r .  Apart from the p o s s i b i l i t y of their d i r e c t  e x c i t a t i o n i n the discharge, they would be the end product of the association of metastable atoms to form molecules i n the upper states associated with the emission of the oc-bands. '(35) t h i s connection some work of Hamada  In  suggests that the  metastable molecules may not be, to a large extent, a d i r e c t  14. product of the discharge.  He found that the negative "bands  and the second p o s i t i v e bands are emitted after an i n t e r v a l —8 of .4 X 10  seconds from the beginning of the discharge,  while the f i r s t p o s i t i v e bands of the afterglow are emitted mainly a f t e r an i n t e r v a l of 7 X 10  seconds.  I t i s also  possible that the metastable molecules may r e s u l t from the association of atoms i n the normal  S state.  Ho observations  of p r e d i s s o c i a t i o n from the A^SD state seem to have been made, but the energy of d i s s o c i a t i o n of the nitrogen molecule into (27) normal atoms i s known  to be about one v o l t more than the  energy of the 0th. l e v e l of t h i s state.  I t i s then possible  that normal atoms produced i n the discharge combine to form (36)  metastable molecules. the  I t was pointed out by lewis  that  assumption of a three-molecular c o l l i s i o n i n t h i s process,  made by some e a r l i e r Investigators, i s , unlike the case of hydrogen, unnecessary.  Normal nitrogen atoms may combine  d i r e c t l y without the intervention of a t h i r d body, owing to the  coincidence of the energy of d i s s o c i a t i o n with the energy  of the upper v i b r a t i o n a l l e v e l s of the A ID state. B.  Decay. (1) The sharp selection of the bands and deactivation of the metastable atoms.  As mentioned above, the spectrum of the afterglow shows very marked s e l e c t i v e emission of c e r t a i n bands of the (3)  f i r s t p o s i t i v e group, which i s e s p e c i a l l y evident be expected, at low temperatures.  .  ^  , as i s oo  The above assumptions as  to the e n t i t i e s occurring i n active nitrogen, together with  15 A the Prancle-Condon p r i n c i p l e , enable a s a t i s f a c t o r y explanation of t h i s s e l e c t i o n to be given.  Molecules i n the Oth.  v i b r a t i o n a l l e v e l of the A Z? state are excited by c o l l i s i o n s p 2 3  with  !' or P metastable atoms to the 6-7 or 10-12 v i b r a t i o n a l  l e v e l s of the B^TT state.  Prom here t r a n s i t i o n s are made, i n  accordance with the Prancle-Condon p r i n c i p l e , to the A state (31 wi th emission of the a-bands.  According to Olcubo -and Hamada  the most probable f i n a l l e v e l s i n the A state w i l l be the 3-4 and 7-9 r e s p e c t i v e l y . a c t u a l l y observed.  This accounts f o r the strong selection I t appears that the v i s i b l e afterglow  should then be accompanied by the emission of the v i b r a t i o n a l bands due to t r a n s i t i o n s from these l a s t l e v e l s to lower levels of the A.state. This explanation of the afterglow accounts for the deactivation of the metastable atoms.  I t affords, so f a r ,  no process f o r the deactivation of the metastable molecules. I t cannot then be complete, f o r , disregarding the w a l l effect (23) which Rayl eight  has shown may be l a r g e l y eliminated, the  e x t i n c t i o n of the afterglow, owing to exhaustion of the metastable atoms, would then leave the nitrogen s t i l l with very considerable excess energy, that of the metastable molecules s t i l l present. made.  Ho observations suggesting this have been  The above explanation also cannot account for the very  long l i f e of the afterglow. (2j);j Deactivation of the metastable molecules. Various processes for the deactivation of the meta-  16, stable molecules may be proposed.  The most probable process  appears to be that of bimolecular c o l l i s i o n s of the second kind between metastable and normal molecules.  Assuming t h i s  to be the case, an increase at.constant volume, i n the concentration of normal nitrogen molecules should cause an increased rate of decay of the afterglow.  The e f f e c t of  increasing the concentration of normal nitrogen was t r i e d by f^7l (21) (22) {20) Strutt , Eonhoeffer and Kaminsky , Kneser and Rudy . 1  ;  The f i r s t two observed no change i n i n t e n s i t y , while Kneser and Rudy, on the other hand observed an increase i n i n t e n s i t y . However, as the normal nitrogen molecules are-not concerned i n the  emission of the afterglow, no instantaneous change i n  i n t e n s i t y i s to be expected.  Rudy, however, also found an  increased rate of decay of the i n t e n s i t y .  Rayleigh concluded (23)  also from h i s experiments on the e f f e c t s of pressure  that  neutral nitrogen molecules are not concerned In the deactivation. Nevertheless the experiment might be worth repeating, using the  eleetrodeless discharge and Rayleigh's precautions for  eliminating the w a l l e f f e c t .  I t should be observed that the  increase i n luminosity found by two of the above may have been due to a l o c a l condensation occurring when the normal nitrogen was added. One other reaction causing a decrease i n the number of metastable molecules suggests i t s e l f . the  We have already noted  e f f i c i e n c y of 16 v o l t electrons i n e x c i t i n g the afterglow,  found by Okubo and Hamada.  Now the probable energy of the  0>th, v i b r a t i o n a l l e v e l of the A S  state being somewhat less  17. than 8 v o l t s , i t appears that a t r i p l e c o l l i s i o n between two metastable and one normal moleaile may be considered.  The  process would cause the destruction of two metastable molecules and the regeneration of two metastable 2 k  0  atoms, thus  + normal molecule —> 2 normal molecules + P + D. 2  2  Accordingly, i f i t takes place, i t might be also of importance i n accounting f o r the long l i f e of the afterglow. In connection with the long l i f e , a suggestion of (32) Kaplan's i s of i n t e r e s t .  Owing to the regions of predissoe-  i a t i o n i n the upper v i b r a t i o n a l l e v e l s of the B^TT s t a t e , i t i s quite possible that many of the molecules excited to t h i s state by c o l l i s i o n s with metastable atoms do not return •7.  d i r e c t l y to the A S state  with emission of the afterglow.  Instead they d i s s o c i a t e .  I t i s , as Kaplan puts i t , as  though "we allow the molecule to waste i t s time during the process of decay of the afterglow".  At lower temperatures,  of course, the p o s s i b i l i t y of d i s s o c i a t i o n i s l e s s , so that the i n t e n s i t y i s greater and the duration of the afterglow i s  VI.  DEPENDENCE OF INTENSITY AND RATE OF DECAY ON CONCENTRATION. The purpose o f the present work was to measure the  change i n the i n t e n s i t y of the afterglow due to an instantaneous change of pressure.  We w i l l assume that the meta-  stable molecules are deactivated by c o l l i s i o n s of the second  18, kind with normal molecules.  The r e a c t i o n which causes  emission of the Of-lands i s then 2A + 2P~^2B + 2S —*2A + 2S -s- 2h —»3A + 2h where A denotes an  (1)  molecule  P denotes a P atom 2  B denotes a B^TT molecule and S denotes a  atom.  This assumes the  atoms t o combine at once i n pairs to give  an A^Z)molecule.  This, of course, i s not so, but the two  reactions have been combined as one i n order that a s o l u t i o n of the equations which f o l l o w may be obtained. The r e a c t i o n i n v o l v i n g  atoms i s p r e c i s e l y s i m i l a r .  For s i m p l i c i t y only one r e a c t i o n i s considered. The r e a c t i o n of the metastable and normal molecules i s A +H  21T + K.E.  2  2  (2)  l e t n]_ = concentration of normal molecules ng = concentration of A^S molecules. ng = concentration of P atoms, Then 2n-^ + 2ng + n^ = n  a constant.  (3)  so that dt  dt  dt  3-  0  (4)  Let f-^ and f g be the c o l l i s i o n frequencies of the two reactions.  In ( l ) two c o l l i s i o n s increase the number of A S  19. molecules by one and decrease the number of P atoms by two. 2  In (2) each c o l l i s i o n decreases the number of A^S molecules and Increases the number of normal molecules by one.  Thus we  have .  - fo , "  dt  fedt  £L- f 2  ,  2  553  dt  -  - %  .  1  low f  l = ^ a i n n 7 c | + c| 2  f  2  =lr  3  °-a2 l 2^ l n  n  C  +  °1  = kgn^g. W  h  e  r  e  ^al  ° L 1 ^  =  '^a2  2  =  °Llf2 2  the fir's being the K i n e t i c Theory diameters and the c's the average v e l o c i t i e s .  Assuming thermal equilibrium between the  various e n t i t i e s , we have 1^/kg = 1/1.09, so that we may take i t as approximately u n i t y . Hence  dn„  .,  , >  —2  = -1 n n„  c  9  - fc n n ?  D i v i s i o n of (5) by (6) gives dn.g n  3  and integration gives  _  k-^ dn-j k  2  n  l  T  P  (7)  so.  n  «•0  = 30[^ r' n  U  (8)  2  where n-^ and n^Q are the i n i t i a l concentrations. Then s u b s t i t u t i o n i n (3) gives 2n  + 2n  ±  + n pi.oj 2  (9)  = n  k  2  3 Q  and i n (4) 2 ^ 1 + 2^2 dt dt  - £l a 0 j f 1 0 ) 2 kg n-Lol ! / n  4nx  £  3  11  d t  = 0  whence *1 1 +  ^ 2 = 1 fSlS30/3aof dt 2 ) kgn-j^Q^^ / ^ f  1  n  3  - ) M j dt  2  2  " Vl) 2  (10)  x  by (7)  n  S u b s t i t u t i o n i n the l a s t l i n e f o r n  and n^ from (9) and (8)  g  r e s p e c t i v e l y gives 1)^30^10^2 2 Jlc n / 2  -  2(^1 j dt  1 0  ^ l f e l ^ o f e l o V ^  2 dn-, _ dt  (^2 10 n  T_  1 /  -2? j " n -  n  i  J C  2  ~ 3c.  2>£lj§ i ;  ~  n  In t h i s put k-^/kg = 1.  l - f' (f ) 2 3 0  Then  1 0  k  - Bgo/£i(f •c2  22 \ \] _n// J n  x  I f we assume "both (Strutt  and n^ small i n comparison with n-j_  , for example determined the percentage of active  nitrogen as 2,46%) then SHi *  ^ 1 0 ^ >  equation may be w r i t t e n dn-,  =  fso^io  s o  t i i a t  t h e  -kodt  »i( i-|) n  and i n t e g r a t i o n gives 1 log/  ) =  1  KB. - n /  n  n  2  For t=0 n  =n  ±  C  =  1 0  ,  »lc<>t +  •  so that  - f l o j ^ ) \ n10 " + 20 n?n"*g—30"-1n-, 0 n ' x  Xi  i n  2 l-,o g 2n-LU n ^i • n nn  - —  2 0  where  b  =  2 n  O U  3 0  - - l o g b, n 2n 10 20 30 +  n  Then log  b(n ~ 2n ] v 1J ————— 2n-, W  x  _nkr = - f f0 2  =  t  n  l a 3 t  22 © and s o l u t i o n for n-, gives bn 7/ * -ax\ ^^aft 2(b + e J  1 where  (11)  a = nk ~2~  g  Prom (10), putting l^/kg = 1 as before, we have an  n  . i ( f ^ 3 0 ..  2  2  .  2  -|(f3^2  +  8  n  j  a  i  )  n  i  +  c  so that n  and  20  C  ~|( 30  =  n  = n  4  2 n  lo)  +n 10 2 0  + 30 n  T~  n  n,2  " f  C  +  _  £  ~  Z  " l " 10 50 (12) 2*1 The i n t e n s i t y of the afterglow i s proportional to the n  n  n  c o l l i s i o n frequency of the f i r s t reaction, so that I  = KhgUg = i d - - n-, - 1 0 3 0 j 10 50 n  =  2 2 Tr/J 10 30 n n  n  n  2  +  n  Q  ^10*30 _ 4 4 4 ) e"^ V b ~ ^ ~ ) 2n n L ~2at |° f (13) b n 2  8 0  2  e  In the present work the i n t e n t i o n was to Investigate the e f f e c t of change of pressure on the i n t e n s i t y . of decay was not considered.  The rate  In t h i s case we put t=0, and  have simply, 1  =  20 30  m  >  n  Thus i f the concentrations  (l4  of a l l reacting e n t i t i e s are  changed i n the same r a t i o by a change of volume, the i n t e n s i t y should be inversely proportional t o the square of the volume, or, assuming an isothermal change of volume, d i r e c t l y prop o r t i o n a l to the square of the pressure.  The change of  volume should be c a r r i e d out as quickly as possible, since i t affects also the rate of decajr.  For, on d i f f e r e n t i a t i n g  (13) with respect to t and putting t=0, we have ell —  =  K  f-a-n n + j D^-10^30'  4 a n  C  10 3p' —Z-Z n  bn  *  _  I O c  i  2  n n  3o(  2 n  20  4  n  3o)  '  n ( 1 5 )  Thus the rate of decay, under an instantaneous change of volume, i s inversely proportional to the cube of the volume. I t may be remarked here that according to (14) the instantaneous value of the i n t e n s i t y i s independent of the concertration of normal nitrogen molecules, while the rate of decay, by (15), i s approximately proportional t o the concentr a t i o n of normal molecules.  Thus, i f the reactions are as  24.  (37)(21)  assumed, the observations mentioned above  to the effect  that the luminosity i s not changed by an increase i n the concentration of normal nitrogen do not prove that normal nitrogen i s not concerned i n the decay. This applies also to (23) Rayleigh's  conclusion from h i s experiments on the effect of  pressure that normal nitrogen molecules are not concerned. Rudy, as already noted, found an increased rate of decay under these conditions. VII.  APPARATUS. A. Source of Nitrogen. Commercial nitrogen was p u r i f i e d by standing i n tubes  containing yellow phosphorus, and dried by passing i t over phosphorus pentoxide.  The cleaned and dried nitrogen was  stored at about atmospheric pressure i n a 500 cc. bulb„ Thence any desired amount could be admitted to the discharge tube through a c a p i l l a r y tube. 33. The Discliarge Tube. The arrangements f o r generating and compressing the active nitrogen are shown i n Figure 2.  A i s the discharge  tube, about 45 cms. long and of 2 cms. i n t e r n a l diameter, with aluminium electrodes.  This type of discharge was chosen i n  preference t o the eleetrodeless, as i t permits generation of active nitrogen over a much larger range of pressure.  The  discharge tube was connected to the compression tube 33 by means of a short tube 33, at r i g h t angles to both A and 33.  25 e C.  The Compression Tube.  The compression tube was selected so as to be, as far as possible, s t r a i g h t and of even bore.  The front end, E, of  the plunger for compressing the active nitrogen was chosen to f i t as c l o s e l y as possible i n the compression tube, consistent with smooth running.  The back end of the plunger contained  a core, E, of soft i r o n wire, IS cms. long.  This end of the  plunger was somewhat smaller i n diameter than the front end. A short glass r i n g about 2 cms. long, attached with wax,  was  used as a bushing to hold t h i s end of the plunger centered i n the compression tube.  The two ends of the plunger were joined  by a tube of smaller diameter.  This arrangement permitted  the use of a f a i r l y close f i t f o r the front end of the plunger while g i v i n g s u f f i c i e n t play to allow for I r r e g u l a r i t i e s i n the compression tube.  The plunger was evacuated and sealed  o f f , to prevent contamination of the nitrogen.  The front  end, D, of the compression tube was closed by a pyrex window sealed i n ; the other end was closed by a window sealed on with wax, so that I t could be removed to give access to the plunger. The plunger was moved i n the compression tube by means of a solenoid C, coaxial with the compression tube.  The  solenoid was given a r e c i p r o c a l motion by means of a crank and connecting rod, driven by a l/8 H.P. a.c. motor.  The  stroke was 13 cms. and the speed about two strokes per second. The length of the plunger was arranged so that i t s front end just cleared the opening at C connecting the two tubes at one  26. end of the stroke, and came within 5 cms. of the window D at the other end of the stroke.  To prevent scoring of the glass  with formation of glass dust and large increase i n the w a l l e f f e c t , the compression tube and plunger were l i g h t l y coated (23) with Apeizon o i l B, which Rayleigh  found to have a some-  what l e s s w a l l e f f e c t than has glass.  The connection to the  pumps was from the under side of the compression tube, d i r e c t l y below the opening at C. D. E l e c t r i c a l Arrangements. To generate the discharge a transformer giving about 20,000 v o l t s i n the secondary for 110 v o l t s i n the primary was used.  The primary current was about 25 amperes. A  condenser was connected across the secondary, and the spark gap and discharge tube i n series were i n p a r a l l e l with the condenser,  A stream of a i r kept blowing on the spark gap  l a r g e l y increases the production of active nitrogen. The discharge used was intermittent, l a s t i n g for about 1/16 second when the plunger was at the extreme back end of i t s stroke, so that the opening at C was c l e a r .  This  was arranged by means of a commutator on the crank shaft, which broke the primary c i r c u i t . of f i b r e having a brass s e c t o r carbon brushes.  9  The commutator was a disc which made contact with two  This arrangement i s not very s a t i s f a c t o r y ,  owing to the large primary current.  The glowing gas diffused  i n s t a n t l y into the compression tube, so as to f i l l i t s front end, and was there compressed as the plunger moved forward. The discharge must kept cool, owing to the destructive e f f e c t .  27 . of heat on active nitrogen.  This was clone by wrapping round  the whole length of the discharge tube with t h i n rubber tubing carrying a stream of water.  With the intermittent discharge  the tube was only s l i g h t l y warm after an hour's run. As uniform conditions of e x c i t a t i o n were d e s i r a b l e , the streaming method was not used.  The discharge tube was  outgassed by pumping f o r several hours.  Heating should not  be used i n outgassing, as t h i s appears to increase the w a l l effect.  Nitrogen was then admitted to the desired pressure.  The pressure that gave the best production of active nitrogen was found to be about 1.5 mm Hg., but active nitrogen could be generated at pressures from 7.5 mm to .5mm.  At the higher  pressures however i t does not d i f f u s e f a r .  Under the best  conditions i t was found possible t o operate the tube for an hour with very l i t t l e change i n the production of active n i t r o gen. I t was intended to compare the i n t e n s i t i e s with and without the plunger i n operation by means of a Leeds and Northrup MacBeth Illuminometer.  The i l l u m i n a t o r telescope  was mounted i n l i n e with and close to the front end of the compression tube.  A sectored disc on a shaft p a r a l l e l to the  compression tube was turned by a bevel gear mounted on the crank shaft, so that i t made one r e v o l u t i o n to each stroke of the plunger.  The sector was arranged to allow l i g h t to pass  for about l/8 second when the plunger was at the extreme forward end of i t s stroke,  so as to measure the i n t e n s i t y at  the maximum compression when the plunger was i n operation. So that the two i n t e n s i t i e s to be compared should both be i n t e r m i t t e n t , the sectored disc was i n front of the eye-piece of the illuminometer telescope.  I t was found impossible  however to malce any comparison of i n t e n s i t i e s i n t h i s way, owing to the short duration of the l i g h t , and. i t s low Intensity I n conclusion I wish t o thank Dr. G. M. 3brum for suggesting the problem, and f o r advice given during the progres of the work.  I wish also to thank I J? . W. Eraser for design-  ing and constructing the mechanical parts of the apparatus.  BIBLIOGRAPHY. Lewis E,p.  Astrophysical Journal,  S t r u t t R.J,  Proc. Roy, Society.  Carlo G, & Kaplan J . • Zeits,f.Phys.  A93, 1917 p.254, 58, 1929 p.769,  Z e i t s . f . Phys.  49, 1918 p.512.  Lewis E.P,  P h y s i c a l Review.  1, 1913 p.469.  Knauss H.P.  P h y s i c a l Review.  32, 1928 p.417.  Herzberg  G.  12, 1900 p.18.  Bay & Steiner Zeits.f.Elec.Chem. Kiehlu & Aehaiya Proc.Roy,Soc. Sfsoneri- H.  Proc.Nat.Acad.Sci,  35, 1929 p.733. A123, 1927 p.168. 13, 1927 p.100.  Ruark, Eoote, Rudnick & Chenault, Journ.Opt.Soe.Amer.  14, 1927 p.17.  S t r u t t R.J. & Fowler A. Proc.Roy.Society. Mulliken.R„S. P h y s i c a l Review.  A86, 1912 p.105. 26, 1925 p . l .  Foote, Ruark & Chenault, Kaplan J .  P h y s i c a l Review,  25, 1925 p.241.  P h y s i c a l Review,  33, 1929 p.154.  Okubo J . & Ham ada H.  Phil.Mag,  5, 1928 p.372.  S t r u t t R.J.  Proc.Roy.Society.  A85'," ,1911,p.219.  S t r u t t R.J.  Proc.Roy,Society.  A86, 1912 p.262.  S t r u t t R.J.  Proc.Roy.Society.  A86, 1911 p. 56.  Constantinides P.A. Phys.Rev.  30, 1927 p. 95.  Rudy R.  27, 1926 p.110.  Physical Review.  Bonhoeffer K.F. & Kaminsky G. Zeits,f.Phys.Chem,  127, 1927 p.385.  BIBLIOGRAPHY. Contd. (22) . Kheser H.O.  Ann. der Phys.  (23) . Lord Rayleigh.  Proc.Roy.Society.. A151, 1935  (24) . Sponer H.  Zeits,f.Phys.  34, 1925 p.622.  (25).  Wrede E.  Zeits.f.Phys.  54, 1929  (26).  Compton & Boyce.Physical Review.  (27) . Kaplan J . (28).  Physical Review.  Okubo J.& Hamada H. P h i l .  87, 1928 p.717. © 5 G*7 ©  33, 1929 p.145. 42, 1932  © 97 ©  Mag.  15, 1933 p.103.  (29) . Cario G.& Kaplan J . Phys. Rev.  33, 1929 p.189.  (30).  S t r u t t R.J.  Proc.Roy.Society.  A91  f  1^22 p.303.  (31) . Okubo J.& Hamada H. Phys. Rev.  42, 1932 p * y s 5 c  (32).  37, 1931 p.1407,  Kaplan J .  P h y s i c a l Review.  (33) . Okubo J.& Hamada H.lohoku Univ. Sci.Reports. (34).  23, 1934  © 28 *5 ©  Jevons W. "Report on Band Spectra of Diatomic Molecules". Cambridge Univ.Press.  1932 p.200.  Tohoku Univ.Sci.Reports  1932  (35)  Ham ad a H.  J3 e 549  ©  (36)  Lewis B.  P h y s i c a l Review.  31 ^ 1923 p.314.  (37)  S t r u t t R.J.  Proc.Roy.Society.  A86, 1911 p. 2 6 6.  

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