Volume 13, number 3 CHEMICAL PHYSICS LETTERS

T E A C H E M I C A L L A S E R S F R O M H 2 + CI 2 A N D H 2 + B r 2

I. B U R A K , Y. N O T E R , A .M. R O N N * and A. S Z O K E

Departments o f Chemistry and Physics, Tel-A viv University, TeI.A viv, Israel

Received 13 December 1971

1 March 1972

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HC1 and HBr chemical lasers based on the reactions H + C12 and H + Br2 have been successfully operated using a transverse discharge excitation. Both lasers yielded outputs in the k~,V range and pulse widths of 200--1000 nsec. Os- cillations were obtained on a laxge number of rotation--vibration lines in the 3--2, 2-- I and 1 --0 vibrational bands• Self mode locking of the laser has been observed on most transitions• Oscillations on I - 0 transitions are observed at H2 :C12 and H2 :Br2 ratios greater than 10:1. The main characteristics of the lasers are discussed in terms of the reaction and initiation mechanism.

I n f r a r e d laser a c t i o n on HCI and HBr m o l e c u l e s f o r m e d in c h e m i c a l r e a c t i o n s has b e e n r e p o r t e d b y severa l a u t h o r s , u s ing va r ious c h e m i c a l and i n i t i a t i o n

m e c h a n i s m s [ 1 - 5 ] . M u c h o f th is w o r k is s u m m a r i s e d in s o m e e x c e l l e n t rev iew a r t i c les [ 6 , 7 ] . In this l e t t e r , we r e p o r t the o b s e r v a t i o n o f p o w e r f u l laser a c t i o n on

several v i b r a t i o n - r o t a t i o n a l t r a n s i t i o n s in the 1 - 0 , 2 - - i , and 3 - -2 b a n d s o f b o t h HCI and HBr. Pulses o f

a p p r o x i m a t e l y 1 k W p e a k p o w e r and 200 nsec d u r a - t ion were o b t a i n e d in a t r ansverse d i s cha rge , u t i l i s ing the s imp le s t r e a c t i o n s c h e m e , t ha t o f H 2 r eac t i ng w i t h CI 2 and Br 2 r e spec t ive !y .

T e c h n i c a l g rade H 2 and C12 or Br 2 ( v a p o u r ) we re m i x e d and m a d e to f low t h r o u g h a PVC laser t u b e . The r e a c t i o n was i n i t i a t e d b y an e l e c t r i c d i s cha rge ex- c i t ed t h r o u g h a single row o f 210 p in e l e c t r o d e s w i t h i nd iv idua l l oad res i s to rs [ 8 ] . P o w e r was c o u p l e d o u t t h r o u g h a 2.5 m m ho le in a g o l d - c o a t e d m i r r o r , de-

. t ec t ed w i t h a G e : A u d e t e c t o r ( R a y t h e o n Q K N I 5 6 8 ) having a rise t i m e o f ~ 7 0 nsec , and d i s p l a y e d on a T e k t r o n i x 5 5 0 3 o s c i l l o s c o p e . N o d i spe r s ive e l e m e n t was used in the cav i ty .

A t y p i c a l se t o f r esu l t s on the HCI laser is g iven in tab le 1. A l l laser l ines s h o w d e l a y s and i n t ens i t i e s t yp - ical o f cascades o f se l f -O-s 'wi tched lasers ( see , fo r ex-

:'*")klfrecl P. Sloan Fellow, on leave from the Chemistry De- partment, Polytechnic Ins t i tu te o f Brooklyn, Brooklyn 11201, New York. . . . . .

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Table 1 H 2 + CI 2 laser. C12:3 torr, H2:47.5 tort, He: 36.5 torr, 14

kV excitation. Repeti t ion rate 1 pulse per 5 seconds

Relative Pulse delay Pulse width Transition intensity (mV) (t2sec) (l\vhh) (t~sec)

PI(5) 5 2.5 Pi(6) 1000 I_5 0.2 PI(7) 2000 1.5 0.3 PI (8) 3500 2.0 I- 1.5 PI(9) 5000 2.5 1.5 PI(IO) 5000 4.0 1.5 P l ( l I ) 1200 I0 5.0

P2(4) 100 1.0 0.5 Pz(5) 1000 1.0 0.2 P2(6) 1000 1.5 0.4 P2(7) 750 2.0 0.4 P2(8) 3000 l.O 1.5 P2(9) 4000 2.5 2.0 P2(IO) 600 0.5 0.5

P3(4) 10 2 P3(5) 1000 1.0 0.8 P3(6) 1500 1.5 1.0 P3(7) 1000 1.5 1.0 P3(8) 2000 1.5 1.5 P3(9) 120 5.0 2.0

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a m p l e , the t r i p l e t • P 3 ( 8 ) - + P 2 ( 9 ) - + P l ( i 0 ) a n d fig. l ) ) i The h ighes t r e p e t i t i o n r a t e for w h i c h HC1 laser action:.il has been o b s e r v e d was 1 pu l se pro:-2 s e c o n d s . T h e I~01~.

b a n d osc i l l a t e s a t H:~ :CI 2 r a t io s g r e a t e r t han 10: I and;~!~

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Volume 13, number 3 CHEMICAL PHYSICS LETTERS 1 March 1972

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Table 2 1-I 2 + B r 2 laser. B r2 :9 ton', I-I2:108 ton', 22 kVexcl ta t lon .

Repeti t ion rate 2 pulses per second

Relative Pulse delay Pulse width Transit ion intensity, (mV) (~sec) (fwhh) (tasec)

Fig. 1. HBr laser pulse. 100 nsec per division, time axis is from left to right. 2 ~ 1 laser transition precedes the 1 ~ 0

transition by approximately 400 nsee.

Pt(4). P~(5) Pl(6)

Pl(7)

P2(4) P2(5) P2(6) P2(7)

P3(4) P3(5) P3(6) P~(7) P3(8)

80 1.2 0.5 2300 !.5 0.5 300O 1.5 0.5 1250 2.5 1.0

2000 0.8 - 3000 - -

200 -

8 - - - - -

600 - - 2500 - -

500 - - 8 - - - -

at pulse intervals longer than the pump-out time of the tube (~5 sec). Decrease of the repetition rate be- yond t ha t gave h igher i n t e n s i t y fo r t r a n s i t i o n . A d d i - t ion o f n o b l e gases inc reases the p o w e r b y 20% and

l eng thens the i n d i v i d u a l pulses . T h e s h a r p e s t pu l ses are o f 200 nsec f w h h and the p o w e r is h ighes t a t

about 15 kV across the 1.5 cm interelectrode gap. Our results with HBr are summarised in table 2. It

shou ld be n o t e d t ha t even at r e p e t i t i o n ra tes as h igh as 2 p e r s e c o n d , the 1 - 0 t r a n s i t i o n s s t i l l o sc i l l a t e as s t r o n g l y as the 2- -1 t r a n s i t i o n s . H e l i u m m a y be a d d e d up to 200 to r r w i t h l i t t l e e f f e c t o n the laser a c t i o n . Pulses o f the HBr laser are s t r o n g e r a n d n a r r o w e r t h a n

those o f HCI. T h e y r e a c h ~ 1 kW p e a k p o w e r and 150 nsec fwh.h in a c a v i t y a p e r t u r e d to a l l ow o n l y low t ransverse m o d e s to osc i l l a t e . T h e laser ga in u n d e r o u r c o n d i t i o n s is ~ 1 0 % p e r m e t e r . I t was e s t i m a t e d us ing a Brews te r angle p l a t e w h i c h was r o t a t e d ins ide the cav i ty and was c o m p a r e d to a C O 2 : N 2 : H e laser o p e r - a t ed u n d e r i d e n t i c a l c o n d i t i o n s . A t y p i c a l pu l se o f the P l ( 6 ) l ine is s h o w n in fig. 2.

Mode l o c k i n g , vis ible on fig. 2, is o b s e r v e d o n b o t h the 2--1 and 1 - 0 t r a n s i t i o n s in H g r . I t is caused b y the p re sence o f a " s a t u r a b l e a b s o r b e r " in the s ame v o l u m e as the las ing m e d i u m . P r e s u m a b l y , the spa t ia l - ly n o n - u n i f o r m d e c o m p o s i t i o n o f t he r e a c t a n t s causes va r i a t i ons in the r e a c t i o n ra tes a long and ac ross the

row o f p ins : T h u s a t the t i m e o f a laser pu l se , some : parts of the mode volume present a loss, lowering t h e t h r e s h o l d s u f f i c i e n t ! y to cause m o d e lock ing .

It is well "known that the formation of HCI (HBr) f r o m H 2 + CI 2 (H 2 + Br2) p r o c e e d s p r i n c i p a l l y b y the cha in r e a c t i o n s :

H + CI 2 --> HCI + CI H = - 4 5 k c a l , ( I )

CI + H 2 - - > H C I + H H = 1 k c a l , ( 2 )

H + Br 2 --> HBr + Br H = - 4 1 . 1 k c a l , (3 )

B r + H 2 ~ H B r + H H = 1 6 . 7 k c a l . ( 4 )

Fig: 2. PI(6) hser ffansition in HBr. Time scale 100 nsec/divi- s/on. Mode locking is Clearly visible across the full prof'de.

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The h0t reacti0n [react ions( i ) or (3)] is fast and sup. :plies thlelm01ecules:in vibrationally excited states. The endothermic reaction [reactions (2) or (4)] produces molecules in the ground state only. It is slower than t h e f o r m e r b y a f a c t o r of 102 [9,10] in the H 2 + C12 system and by l011 in the H2+ Br 2 system. Electron excitation is expected to be less selective than flash initiation ina chemical laser. Thus, in an electricdis- charger both H and CI atoms are formed. Indeed, from our observation of a high intensity 1 ~ 0 laser

transit ion in HCI, it can be estimated using the known rates (1) through (4) that under our experimental con- d i t ions tile fractional decomposition of H 2 is at least 1] lOthat of C12, and probably higher.

In addition, in the time of 3gtsec du~/lg which lasing occurs, only the fast reactions ( I ) and (3)can proceed t ° an appreciable extentaSignificant chafn propagation by the cold reactions (2) and (4) proceeds only after the laser pulse is over.

I r t comparison, the flash-initiated laser based on the H 2--:CI 2 reaction proceeds almost exclusively from the photolytic decomposition of CI 2. Thus the

Cold reaction (2) starts the chain propagation and vi' brationally cold HCI molecules are formed during the pr0cessof population inversion.

The observed difference in the repetition rate for whichperformance o f the HC1 and HBr lasers was achieved is also explained in terms of the chemical kinetics of the two systems.

The chain length in the H 2 + C12 system is much greater than the H 2 + Br 2 system. Thus, after the electric pulse, the reaction H 2 + CI 2 proceeds tO com- p!eti0n in thewhole VolUme of the laser tube w h i l e

H B r molecules are produced only i n t h e region of the ;eleciric discharge. In the case Of HCI, then~ the entire gas volume has to be pumped out between pulses.

Sifiee HBr presents a well resonan(sYstem with the :0001 Vibration o f CO2, we performed experiments : Similar to th0se:of Cool et al: [11] . In these eXl~eri- : me nts~iWe:ini!rrdticedl CO:z into the iaser tube t o g e t h e r with: Hzand Bi., .iAf 10~ ~eoneentrations of CO 2 , the ) HBr 4-~ emissionwas quenched and: at higher concen- : traiio~iS, s:t~ongl i0.6-/~ laser action was observed.

: CHEMICAL PHYSICS LETTERS " 1 March 1972

The nearly resonant V - V piocess can be shown to : . . . . .

co <oooo> - .~HB,.?v=r,&I)+CO2(000D+I&EI ' - , , ~ ,, ,

by the following experiment. The pressure o f H 2 is raised to approximately 50 tort, where no direct ex- citation of CO 2 a t 4 0 tort is observed.The addition o f f - 5 torr of Br2 results in very intense 10.6/z emis- sion. Thenumber of 10~6/~ photons emitted from an optimal mixture of Br2, H 2 and CO 2 is approximately 20 times greater than the 4 # radiation emitted from the H2, Br 2 system.

In summary, we report that efficient laser action was obtained in a transverse discharge structure on a fair number of HC1 and HBr transitions using tech- nical grade gases. We interpret our observations from the well-known steps in the chain reaction, the non- specificity Of its initiation, and our choice of operat- ing conditions.

References

[1 ] J.V.V. Kasper and G.C. Pimentel, Phys. Rev. Letters 14 (1965) 352.

[2] P.H. Corneil and G.C. Pimentel' J. Chem. Phys. 49 (1968) 1379. /

[3] T.F. Deutsch, IEEE J. Quantum Electron. QE~3 (1967) 419.

[4] A. Hermy, F. Bourein, I. Arditi, R. Charneau and J. Menard, Compt. Rend. Acad. Sei. (Paris) 267B (1968) 616.

[51 C.B. Moore, IEEE J. Quantum Electron. QE-4 (1968) 52.

[6] C.B. Moore, Advan. Chem. Phys., to be published. [7l M.D. Dzlxidzhoev. V.T. Platonenko and R.V. Khokhlov,

. . . : . .

Soviet Phys r Uspekhi 13 (1970)247. [81 J.A. Beaulieu, Appl. Phys. Letters 16 (1970) 504. [9] V.I. Gorshkov, V. Gromov, V.I. Igoshin, E.L. Koshelev,

E.P. Markin and H.N. Oraevskii, Appl. Opt. 19 (1971) !781. . : , . . . -

[101 S~ Bertson, The foundatlon'of chemical kinetic, (McGraw-Hill, New York , 1960). : . . . .

[1!I T.A. C001 and R.R. Stephens, !" Chem. Phys. 52