A PINCHING PULSE
DISCHARGE
LIGHT N.
T.
PHOTOLYTIC
SOURCE
Timofeev
and
P.
A.
Shakhverdov
UDC 535.241.6
The pulse photolysis method [1] is an example of using applied s p e c t r o s c o p y in physical c h e m i s t r y . It is used to study the kinetics of fast photochemical r e a c t i o n s . Essentially this method c o n s i s t s of c r e a t ing instantaneous high concentrations (adequate for s p e c t r o s c o p i c identification) of s h o r t - l i v e d photolysis products. This is done by pulse tube flashes. It is used for studying photokinetic p r o c e s s e s in solutions [2] and the gas phase [3], d e t e r m i n i n g the absorption s p e c t r a of free r a d i c a l s [4] and other intermediate reaction products [5], studying the initial phases of photosynthesis in models [6] and under natural, fullscale conditions [7]. Resolution time is fixed by the duration of the photolytic pulse flash. Glow quenching must be s h o r t e r (time) than the time for establishing equilibrium in the reagent s y s t e m . This time v a r i e s over a wide range (fractions of a nanosecond to s e v e r a l seconds) for different types of relaxation. Let us evaluate, as an example, the flash duration required for o b s e r v i n g the changing concentration kinetics of an intermediate p r o duct with an extinction coefficient e which d i s a p p e a r s by a second o r d e r reaction with rate constant K. Half-life of the s t a r t i n g concentration JR] 0 of the product is 1
tl/2 K
[Rio
If the length of the optical path of the operating cell, in which absorption is m e a s u r e d , is 100 c m and 0.1 is assumed to be the limit of reliably m e a s u r e d optical density tl/2 = - - -
lOS see.
K Table 1 lists the durations of activating impulse calculated by this formula for three values of K close to rate constants of diffusion controlled r e a c t i o n s and four values of e c o v e r i n g most of the possible values of free r a d i c a l extinction coefficients [1, 8]. The number of quantums absorbed in the cell during this t i m e must c o r r e s p o n d to a product concentration change A c and the c o r r e s p o n d i n g optical density change AD = 0.1. Then Ac = AD/~ 9 I = 10-i/~ 9102 9m o l e s / l i t e r = 10-6/e m o l e s / c m 3 and the required number of quantums is N = A c . V - NA/~, where V is the cell volume, ~ is the quantum yield of the p r o c e s s , and NA is A v o g a d r o ' s number. I f V = 100 c m S and r = 1, N = 6. 1019/e quantums. F o r example approximately 4 0 / e J of light energy are required for photolysis in the 200-400 nm visible range for which the average energy of a quantum is 6 . 6 . 1 0 -19 J. This value for different values of e appears in the last line of Table 1. Energy evolved in the cell must be 4 J over 10 -~ sec for o b s e r v i n g an intermediate product d i s a p p e a r ing with a 10 l! l i t e r - mole -1 9 -1 rate constant and 10 liter 9 mole -i 9c m -1 extinction coefficient; 4 . 1 0 -~ J over 10 -4 sec is adequate for a product with a 104 liter 9 mole -1 9c m -1 extinction coefficient. Existing pulse photolysis units with n o r m a l pulse tubes have approximately 10 ~zsec r e s o l v i n g time [9]. The use of pinching (selfcompressing) d i s c h a r g e tubes i m p r o v e s resolution time and achieves g r e a t e r power levels [10]. This work was devoted to developing and studying a powerful photolytic source using pinching d i s c h a r g e which could be used to study the kinetics of photochemical reactions with a resolution time b e t t e r than 1 psec. Attention was devoted p r i m a r i l y to matching the p a r a m e t e r s of the unit and the s o u r c e to get a single short duration light pulse. T r a n s l a t e d from Zhurnal Prikladnoi Spektroskopii, Vol. 20, No. 5, pp. 775-779, May, 1974. nal article submitted March 14, 1973.
Origi-
9 1975 Plenum Publishing Corporation, 227 West 17th Street, New York, N. Y. 10011. No part' o f this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, microfilming, recording or otherwise, without written permission o f the publisher. A copy o f this article is available from the publisher for $15.00.
584
TABLE I. Duration (see) and E n e r g y (J) of the Activating P u l s e t{ equired for Studying a t3imolecular with R a t e Constants K and F r e e t l a d i c a l Extinction Coefficients E s, Hter. mole-1. crn-I
K, liter, molerl 9
see -1 lO
10 2
10 a
IO r
10n
10-7
10-6
10-5
10-4
10TM
I0-6
10-~
10-~
10-a
10 2
I0-~
I0-4
lO-a
10-2
4,0
0,4
0,04
0,004
E( 200-400 nm)
F i g u r e 1 is a s c h e m a t i c of the unit. Source 1 is a quartz cylinder with 3 m m thick w a i l s . 60 m m diam e t e r x 100 m m long. Flat b r a s s e l e c t r o d e s with c e n t r a l openings w e r e glued to its end with E D - 5 epoxy r e s i n . Q u a r t z sight g l a s s e s w e r e built into the openings for o b s e r v i n g the d i s c h a r g e along the tube axis. A distance of 70 m m was provided between the e l e c t r o d e s and the sight g l a s s e s to prevent clouding. A c o p p e r cylinder s u r r o u n d s the tube as a r e t u r n conductor to r e d u c e d i s c h a r g e circuit inductance. Axial and r a d i a l openings w e r e provided in the cylinder for r e c o r d i n g the development of d i s c h a r g e d y n a m i c s by a SFR quick scan c a m e r a . The b a t t e r y of high voltage c o n d e n s e r s 2 (30 pF c a p a c i t a n c e , 10 kV o p e r a t i n g voltage) was a s s e m b l e d by connecting low induction type B G K - T s c o n d e n s e r s in p a r a l l e l . Its n a t u r a l inductance was 9.7 nil. It was connected to the source by flat c o p p e r b u s e s which w e r e s e p a r a t e d by a 0.4 m m thick polyethylene film. The b u s e s w e r e 0.2 m m thick x 100 m m wide x 250 m m long. Maximum e n e r g y stored in the b a t t e r y was 1500 J and the s h o r t circuit c u r r e n t was 5.5.105 A. D i s c h a r g e was switched by a s i m p l e t r i g g e r type air b r e a k e r 3. Total circuit inductance was 20 nil. It was m e a s u r e d by the c u r r e n t cycle period during the d i s c h a r g e p r o c e s s through the tube. The c i r c u i t could provide about 3" 105 A m a x i m u m o p e r a t i n g c u r r e n t and c h a r g e r a t e was about 5- 10 li A / s e c at 10 kV applied to the b a t t e r y . C u r r e n t s t r u c t u r e was r e c o r d e d using a Rogovskii belt which supplied its signal to a channel in o s c i l l o g r a p h 7. The s t r u c t u r e of the light pulse was r e c o r d e d on the second channel. It was picked up by an F E K - 0 9 photoe l e m e n t 8 through the opening in the e l e c t r o d e and side of the tube. Air was used as the working g a s in the s o u r c e . The o p t i m u m d i s c h a r g e conditions w e r e found by v a r y i n g air p r e s s u r e (0.1-10 m m Hg) and d i s c h a r g e c u r r e n t . F i g u r e 2 shows o s c i l l o g r a p h s of c u r r e n t and l u m i n e s c e n c e for three p r e s s u r e s at constant energy stored in the b a t t e r y (750 J). The s h o r t light pulse is visible on all o s c i l l o g r a p h s but the shape of the c o m p l e t e light pulse is strongly affected by the p r e s s u r e . At 0.1 m m Hg the s h a r p peak a p p e a r s b e f o r e the c u r r e n t r e a c h e s its m a x i m u m value. The c o m p r e s s i o n r a t e of the p l a s m a column is too g r e a t and a prolonged l u m i n e s c e n c e pulse o c c u r s a f t e r the f i r s t c o m p r e s sion: the m a x i m u m coincides with the next c u r r e n t m a x i m u m . The r e v e r s e is o b s e r v e d at 5 m m Hg:
Fig. 1 Fig. 2 Fig. 1. Schematic of the e x p e r i m e n t a l installation: 1) d i s c h a r g e c h a m b e r , 2) cond e n s e r b a t t e r y , 3) b r e a k e r , 4) power supply a s s e m b l y , 5) firing a s s e m b l y , 6) SFR, 7) OK-17M o s c i l l o g r a p h , 8) FI~K-09, and 9) ISP-66 s p e c t r o g r a p h . Fig. 2. Oscillograph of 1) d i s c h a r g e c u r r e n t and 2) e m i s s i o n pulses at 7 kV o p e r a t i n g voltage and v a r i o u s a i r p r e s s u r e s (mm Hg): a) 0.1, b) 1, and c) 5.
585
tcomv psec
J
0
J a
J
0
Jr, cm
o,oe
b
[
[
40~
I
l
I
M~/~ (g/cm)i/4
Fig, 3 Fig. 4 Fig. 3. I.~minescence brightne ss distribution along the d i s c h a r g e c h a m b e r d i a m e t e r for different m o m e n t s in t i m e at a i r p r e s s u r e (ram Hg): a) 1 and b) 5. Fig. 4. C o n s t r i c t i o n t i m e as a function of a i r m a s s per unit length of d i s c h a r g e c h a m b e r at 6 kV o p e r a t i n g voltage. pinching o c c u r s much l a t e r than the f i r s t c u r r e n t pulse. The o p t i m u m f o r m of the light pulse is o b s e r v e d at 1 ram Hg p r e s s u r e : all the l u m i n e s c e n c e is concentrated in a single n a r r o w peak with 0.7 ~sec half width. Light intensity, in this c a s e , is c o n s i d e r a b l y g r e a t e r than at low and high p r e s s u r e and the pulse position coincides (time) with the f i r s t c u r r e n t m a x i m u m . The r a t i o of the n a r r o w peak amplitude to the background of m o r e extended radiation changes f r o m 2-2.5 for low p r e s s u r e to 25-30 for the o p t i m u m c a s e , T h i s d i s c h a r g e development is c o n f i r m e d by quick scan photography using an SFR c a m e r a o p e r a t i n g at a scanning r a t e of 106 f r a m e s / s e c . F i g u r e 3 shows the d i s c h a r g e l u m i n e s c e n c e distribution along the tube. This w a s obtained by m i c r o p h o t o m e t r i c a t i o n of SFR n e g a t i v e s for v a r i o u s time i n t e r v a l s a f t e r d i s charge initiation. At 5 ram Hg the d i s c h a r g e is p r e s s e d to the tube walls by the skin effect for the f i r s t t h r e e m i c r o s e c o n d s , It is c o n s t r i c t e d a f t e r four m i c r o s e c o n d s and a 3 m m d i a m e t e r l u m i n e s c e n t pinch a p p e a r s along the tube axis. The pinch l u m i n e s c e n c e is only slightly b r i g h t e r than the background b r i g h t n e s s . The pinch a p p e a r s during the second m i c r o s e c o n d a f t e r the d i s c h a r g e at the o p t i m u m p r e s s u r e (1 m m Hg) and d i s a p p e a r s a f t e r about 1 p s e c . Its b r i g h t n e s s and r e l a t i v e intensity a r e c o n s i d e r a b l y g r e a t e r . Pinch d i a m e t e r is 7 m m and pinch effect r a t e is 2.2.106 c m / s e c . D i s c h a r g e radiation s p e c t r a (in the 200-600 nm range) a r e also v e r y different at v a r i o u s o p e r a t i n g p r e s s u r e s . The s p e c t r u m c o n s i s t s of a l a r g e n u m b e r of lines at low (0.1 m m Hg) and high (5 m m Hg) p r e s sure while at 1 m m Hg a s t r o n g solid background is o b s e r v e d , However a l a r g e n u m b e r of e m i s s i o n lines is p r e s e n t in the s p e c t r u m at this p r e s s u r e . T h e i r intensity d e c a y s m o r e slowly than the solid background intensity. T h e s e t i m e s for the f i r s t c o n s t r i c t i o n of the p l a s m a at v a r i o u s a i r p r e s s u r e s in the tube a r e in good a g r e e m e n t with t h e o r e t i c a l d e t e r m i n a t i o n s c a r r i e d out on the a s s u m p t i o n that a s i m p l e model of p l a s m a column m o v e m e n t is valid [11]. According to this model the f a s t - m o v i n g c u r r e n t flows in a v e r y thin l a y e r of p l a s m a close to the inner surface of the tube due to skin effect in the initial s t a g e s . The r e s u l t a n t e l e c :trodynamic f o r c e s , d i r e c t e d to the a x i s , a r e not c o m p e n s a t e d by the internal p r e s s u r e of the i n e r t gas. The d e s c e n d i n g p l a s m a l a y e r e n t r a i n s the gas b e f o r e it and c a r r i e s it to the tube axis. Rapid heatup of the p l a s m a is o b s e r v e d at the m o m e n t of m a x i m u m constriction: this is accompanied by intense radiation. T h i s p r o c e s s can be r e p e a t e d s e v e r a l t i m e s giving a s e r i e s of light pulses. The t i m e for the f i r s t c o n s t r i c tion is r e l a t e d to the d i s c h a r g e p a r a m e t e r s by the following function in the l i n e a r law a p p r o x i m a t i o n of c u r r e n t buildup I(t) = (dI/dt)0t for the f i r s t stage of the p r o c e s s tconst = 1.5 [
rc
(dI/dt)o
]~/2M1/4
w h e r e r is the d i s c h a r g e c h a m b e r radius and M is the gas m a s s per unit length of tube. This is equal to 0.6 # s e c at an initial p r e s s u r e of 0.1 m m Hg and 7 kV voltage. T h i s is an close a g r e e m e n t with the e x p e r i m e n t a l value (0.7 #sec). The c o r r e s p o n d i n g values a r e 1.1 and 1.2 # s e c at 1 m m Hg p r e s s u r e . F i g u r e 4 shows the e x p e r i m e n t a l value of tconst as a function gas m a s s M. Constriction t i m e is p r o p o r t i o n a l to the fourth root of gas m a s s per unit tube length in a c c o r d a n c e with the above f o r m u l a . This investigation thus showed that a pinching d i s c h a r g e photolytic pulse s o u r c e has excellent t i m e c h a r a c t e r i s t i c s u n d e r conditions when the buildup r a t e of the d i s c h a r g e c u r r e n t and c o m p r e s s i o n r a t e of 586
the plasma column are equal. It has been demonstrated [12] that light efficiency of a pinching discharge is about 1% in the 200-600 am spectral range when carried out under comparable conditions. Therefore several joules of light energy can be produced in the 200-400 nmrangewhenl500 J of energy are accumulated. Table 1 shows that the source developed can be used to study a wide range of fast photochemical reactions. In conclusion, the authors express their gratitude to N. N. Ogurtsova and I. V. Podmoshenskii for a number of valuable critical comments. LITERATURE 1.
2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12.
CITED
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587
