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NOVEL APPROACHES TO SHORT-WAVELENGTH CHEMICAL LASERS
S. Rosenwaks
To cite this version:
S. Rosenwaks. NOVEL APPROACHES TO SHORT-WAVELENGTH CHEMICAL LASERS. Journal
de Physique Colloques, 1987, 48 (C7), pp.C7-339-C7-342. �10.1051/jphyscol:1987781�. �jpa-00227085�
JOURNAL DE PHYSIQUE
Colloque C7, suppl6ment au n012, Tome 48, dgcembre 1987
NOVEL APPROACHES TO SHORT-WAVELENGTH CHEMICAL LASERS
S. ROSENWAKS
Department of P h y s i c s , Ben Gurion U n i v e r s i t y o f t h e Negev,
~ e e r - S h e v a . IL-84105, I s r a e l
Abstract
-
Short-wavelength chemical l a s e r s (SWCL1s) have been pursued i n many l a b o r a t o r i e s , following t h e g r e a t success achieved with i n f r a r e d chemicall a s e r s u t i l i z i n g simple atom molecule exchange r e a c t i o n . However, a l l e f f o r t s t o o b t a i n SWCL's ( < lum) have f a i l e d t o date. The reasons f o r f a i l u r e and p o s s i b l e ways t o overcome t h e d i f f i c u l t i e s a r e discussed. In p a r t i c u l a r , a novel approach t o o b t a i n i n g SWCL1s based on a premixed f u e l - l a s a n t capable o f producing high energy p u l s e s , i s described. E l e c t r o n i c a l l y - e x c i t e d n i t r o g e n molecules a r e pro- duced by detonation of metal azides. Lasing i s expected v i a energy t r a n s f e r from t h e n i t r o g e n molecules t o metal atoms o r within t h e n i t r o g e n molecules. Prelimin- ary r e s u l t s obtained by d e t o n a t i o n o f metal azides a r e presented.
The search f o r short-wavelength chemical l a s e r s (SWCL'S) began i n t h e e a r l y s i x - t i e s . 1 I t has been pursued i n many l a b o r a t o r i e s , encouraged byehe g r e a t success achieved with i n f r a r e d chemical l a s e r s u t i l i z i n g simple atom-molecule exchangere- a c t i o n s ( f o r a summary on i n f r a r e d chemicals l a s e r s p r i o r t o 1976 s e e r e f e r e n c e 2; f o r l a t e r developments s e e t h e proceedings o f t h e Gas Flow and Chemical La- s e r Symposia, r e f e r e n c e s 3 a - f ) .
U n t i l t h e mid-seventies, most attempts t o o b t a i n SWCL1s u t i l i z e d r e a c t i o n s o f metal atoms with
oxidant^,^
although r e a c t i o n p a t h s not i n v o l v i n g metal atoms were suggested a s we1lO5 A well-known example o f t h i s " d i r e c t " approach t o t h e SWCL i s t h e r e a c t i o nBa + N20 -> BaO* + N 2 , (1)
which was s t u d i e d i n a number o f l a b ~ r a t o r i e s . ~ Extensive e f f o r t s t o o b t a i n a t h r e s h o l d population i n v e r s i o n v i a r e a c t i o n (1) and many o t h e r , analogous,
" d i r e c t 1 ' r e a c t i o n s 5 have f a i l e d t o d a t e . This approach has been l a r g e l y ab- andoned.
A more r e c e n t approach t o t h e SWCL has been based on the " i n d i r e c t n , t r a n s f e r chemical l a s e r approach. This approach was utiliz'ed t o o b t a i n i n f r a r e d chemical l a s e r s , e . g . v i a energy t r a n s f e r from chemically-produced, v i b r a t i o n a l l y - e x c i t - ed HF t o C02, which, i n t u r n , l a s e d a t 10.6 microns .2 A High-efficiency e l e c t r o n i c - t r a n s x t i o n i n f r a r e d t r a n s f e r chemical l a s e r has i n f a c t been developed.
This l a s e r i s t h e chemical i o d i n e l a s e r , i n which e l e c t r o n i c a l l y e x c i t e d O2 i s chemically produced and t r a n s f e r s i t s -1 eV energy t o i o d i n e atoms,which l a s e a t 1.315 microns. 7
The l a s t few years have seen e x t e n s i v e e f f o r t s t o achieve t r a n s f e r chemical l a s e r s a t s h o r t wavelengths ( e l m i ~ r o n ) . 3 ~ - ~ One example i s t h e r e a c t i o n
NF(b) + IF(X) -> NF(X) + IF(B)
,
(2)where t h e e x c i t e d NF ( i n i t s e l f a p o s s i b l e l a s a n t ) i s produced i n r e a c t i v e mix- t u r e s comprised o f molecules containing N, F and H s p e c i e s . I F ( B ) i s a po- t e n t i a l l a s a n t i n t h e v i s i b l e wavelength r e g i o n s 8
Most e f f o r t s t o o b t a i n SWCL's, i n c l u d i n g r e a c t i o n scheme ( 2 ) , have concentrated on systems s u i t a b l e f o r CW operation. These e f f o r t s have so f a r been unsuccess- f u l d e s p i t e t h e f a c t t h a t i n many cases t h e energy t r a n s f e r Trocess i s very e f f i - c i e n t . The main d i f f i c u l t y i n o b t a i n i n g a CW SWCL l i e s i n t h e f a c t t h a t t h e over a l l r e a c t i o n r a t e ( t o produce e i t h e r t h e l a s a n t s p e c i e s l l d i r e c t l y l l o r t h e r e s e r - v o i r s p e c i e s f o r a t r a n s f e r l a s e r ) i s l i m i t e d by t h e d i f f u s i v e mixing rate.Quench-
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1987781
C7-340 JOURNAL DE PHYSIQUE
i n e of both l a s i n g and r e s e r v o i r s p e c i e s i n p o t e n t i a l SWCL mixtures i S o f t e n s i g - n i f i c a n t l y f a s t e r t h a n mixing r a t e s t h a t can be achieved u s i n g c u r r e n t n o z z l e t e c h nology a t p r e s s u r e s needed f o r high i n v e r s i o n d e n s i t i e ~ . ~ The quenching can be homogeneous ( r a d i a t i v e o r c o l l i s i o n a l
-
i n c l u d i n g chemical r e a c t i o n s ) o r h e t e r - ogeneous (on t h e w a l l s ) . Even if l a s i n g i s observed, t h e primary bottleneck t o e f f i c i e n t CW SWCL l a s i n g w i l l i n most c a s e s be c o l l i s i o n a l quenchingby t h e many s p e c i e s p r e s e n t i n suggested r e a c t i o n mixtures. I t should be noted t h a t o p e r a t i o n o f CW chemical mixing l a s e r s i s l i m i t e d t o low p r e s s u r e s (usually up t o a few t e n s of t o r r ) by t h e f a c t t h a t t h e lower l i m i t o f t h e p e r m i s s i b l e mixing t i m e i s d e t e r - mined by t h e d i f f u s i o n r a t e . High power o p e r a t i o n of t h e s e l a s e r s t h e r f o r e r e - q u i r e s t h e use o f l a r g e vacuum pumps.S p e c i e s with l o n g e f f e c t i v e l i f e t i m e s a r e t h e obvious c a n d i d a t e s f o r CW mixing l a s e r s . The o n l y r e p o r t e d , e s t a b l i s h e d example o f a chemically-produced, e l e c t r o - n i c a l l y - e x c i t e d r e s e r v o i r molecule which i s quenching r e s i s t a n t i s O ~ ( ~ A ) . I t s e a s e of chemical p r o d u c t i o n , long r a d i a t i v e l i f e t i m e , and low p r o b a b l l l t y f o r quenching by c o l l i s i o n s w i t h many s p e c i e s and with v a r i o u s w a l l m a t e r i a l s g a make i t an i d e a l energy r e s e r v o i r f o r CW t r a n s f e r l a s e r s . However, i t s energy c o n t e n t ( 1 eV) l i m i t s i t s use t o i n f r a r e d l a s e r s u n l e s s e x c i t a t i o n o f - h i g h - l y i n g metastable e l e c t r o n i c s t a t e s v i a m u l t i p l e c o l l i s i o n s with O ~ ( ' A ) molecules can b e c o n t r i v e d . This p o s s i b i l i t y has been explored r e c e n t l y i n our
labor at or^.^
0 2 ( l h ) has been r e a c t e d with both pb9a and B i g c atoms and r e a c t i o n s with o t h e r atoms has been considered as w e l l . 9 b For Pb, and 3 ~ 2 s t a t e s have been observed a s well as h i g h l y e l e c t r o n i c a l l y e x c i t e d PbO s p e c i e s . For B i , t h e 3 ~ 3 , 2 s t a t e a s well as e x c i t e d B i 2 and B i O have been observed.A very a t t r a c t i v e energy r e s e r v o i r t h a t has been explored f o r t h e p a s t s e v e r a l y e a r s i s t h e N 2 molecule i n i t s lowest e x c i t e d s t a t e , ~ ~ 2 % . This s t a t e has an energy c o n t e n t of 6.2 eV and a r a d i a t i v e l i f e t i m e o f 2 seconds. Furthermore, i n e x t e n s i v e i n v e s t i g a t i o n s c a r r i e d o u t i n our l a b o r a t o r y i t has been f o u n d t h a t energy t r a n s f e r from N2(A) t o a number o f atomic s p e c i e s i s both e f f i c i e n t and s t a t e s e - l e c t i v e , i . e . , t h e r e i s a p r e f e r e n t i a l population o f e l e c t r o n i c l e v e l s t h a t f u l - f i l l c e r t a i n p r o p e n s i t y r u l e s . 1 ° The p o s s i b i l i t y o f p r e f e r e n t i a l energy t
from N2(A) t o s p e c i f i c molecular s t a t e s has a l s o been explored r e c e n t l y ,
F83Pr
However, t h e r e i s a very s e v e r e problem i n u s i n g N2(A) a s an energy r e s e r v o i r . N2 (A) s u f f e r s from s e l f - d e s t r u c t i o n v i a t h e energy-pooling r e a c t i o n
N2(A) + N2(A) 4 N2* + N2, (3)
which i s very e f f i c i e n t , t h e r a t e c o n s t a n t b e i n g a t l e a s t g a s - k i n e t i c . 1 4 A s a r e - s u l t , p r o d u c t i o n o f high c o n c e n t r a t i o n s o f N2(A) f o r high-power CW mixing l a s e r s i s i m p r a c t i c a l , even i f a r e a c t i o n scheme which produces "clean" N2(A) is found.
The l i m i t a t i o n s i n h e r e n t i n t h e schemes d e s c r i b e d above t o o b t a i n high power S W C L I ~ have l e d us t o a novel approach based on a "premixed" s o l i d f u e l - l a s a n t . The core o f our approach i s t o bypass t h e mixing and quenching problems s o d e t r i m e n t a l t o o p e r a t i o n o f high-power SWCL's by u s i n g "premixed" f u e l l a s a n t systems capable of producing high energy l a s e r p u l s e s . To i l l u s t r a t e our approach, t h e d i s s c u s s i o n w i l l d e a l with a s p e c i f i c system; o t h e r examples have been considered b u t w i l l n o t b e d i s c u s s e d h e r e . Our model f u e l - l a s a n t molecule i s l e a d a z i d e , Pb (N3)2. The
l a s a n t i s t h e Pb atom and t h e f u e l i s t h e N2 molecule, e x c i t e d t o t h e A s t a t e . The chemical and l a s i n g k i n e t i c s scheme i n shorthand i s t h e following:
p b ( ~ ~ ) ~ detonation, Pb + N3, N 2 , N
,
(4)N 3 + N3 4 N2* + 2N2, (5)
N + NJ 4 N2* + N 2
,
(6)Pb + N2* -> Pb* + N 2 , (7)
Pb* hv(laser1, pb. (8)
Pb(N3)1z i s a w e l l known e x p l o s i v e t h a t can he han3led. s a f e l y with adequate precau- t i o n s aqd can be detonated upon f l a s h i n i t i a t i o n . 1 6 Reactions (5) and (6), where
N and N a r e ground s t a t e s p e c i e s and N: a t r i p l e t s t a t e , a r e e n e r g e t i c a l l y and s ~ i n algowed. Measurements i n s e v e r a l l a b o r a t o r i e s i n d i c a t e t h a t t r i p l e t n i t r o g e n i s indeed a product o f (5) and (6) . I 7 Since a l l t r i p l e t s end up a s N2(A)
,
decomposition o f Pb (N3) produces N 2 (A)
.
Since t h e f u e l and t h e l a s a n t o r i g i n a t e i n t h e same molecule, t h e r e i s no need f o r mixing. Furthermore, t h e r e a c t i o n sequence (4) -> (7) f a v o r s f a s t energy t r a n s f e r from N2(A) t o Pb, t h u s p r e v e n t i n g t h e buildup o f high c o n c e n t r a t i o n s o f N2(A) and l a r g e l y e l i m i n a t i n g t h e d e t r i m e n t a l e f f e c t o f t h e energy p o o l i n g r e a c t l o n ( 3 ) .
We have i n i t i a t e d l e a d a z i d e . b y s h o r t ( -10ns )
,
l a s e r p u l s e s a t wavelengths from 266 t o 1064nm a t e n e r g i e s from a few t o s e v e r a l hundred m i l i j o u l e s . Pre- l i m i n a r y r e s u l t s i n d i c a t e t h a t e l e c t r o n i c a l l y e x c i t e d N 2 i s formed following t h e i n i t i a t i o n and t h a t n o n s t a t i s t i c a l e x c i t a t i o n t r a n s f e r t o high l y i n g s t a t e s o f Pb t a k e s p l a c e . Moreover, c a l c u l a t i o n s based on a coupled k i n e t i c - h y d r o - dynamic model p o i n t o u t t h a t t h e i n i t i a t i o n should r e s u l t i n p o p u l a t i o n i n v e r s i o n between p o t e n t i a l l a s e r l e v e l s of Pb. A comprehensive d e s c r i p t i o n o f t h e e x p e r i - ments and modeling w i l l be t h e s u b j e c t o f f u t u r e p u b l i c a t i o n s .References
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