Fast ions in liquid helium

Volume 30A, number 4

PHYSICS LETTERS

c r e a s e d E m a x , and above 1.05°K the peak was not d et ect ab l e. The magnitude and t e m p e r a t u r e dependence of Eg was s i m i l a r to that n e c e s s a r y for ions to c r e ate and bind to v o r t e x r i n g s . S i m i l a r l y , E m c o r r e s p o n d e d to the f i e l d E a of B r u s c h i et al.[2], who found additional s e c o n d sound atten.,ation and ' p e r s i s t e n t c u r r e n t s ' at f i e l d s g r e a t e r than E~ unde r s i m i l a r conditions to our e x p e r i m e n t s . We m e a s u r e d the drift v e l o c i t y VD of ions c r o s s i n g the m e a s u r i n g s p a c e GC. F o r ESG < E g only the n o r m a l n e g a t i v e ion was p r e s e n t , but f or ESG > E g (see fig. 1) we d e t e c t e d a h i g h e r m o b i l i t y ' f as t ion' in addition to the n o r m a l ion. The n o r m a l ions had the usual v e l o c i t y - f i e l d depe nden ce, f o r m i n g v o r t e x r i n g s above a m a x i m u m v e l o c i t y of 30 m / s e c , but the fast ions r e a c h e d a l i m i t i n g v e l o c i t y of 54 m / s e c without showing the fall in v e l o c i t y c h a r a c t e r i s t i c of ions binding to v o r t e x rings. P o s i t i v e ions did not give a fast ion current. The fast ion c u r r e n t a p p e a r s when v o r t e x r i n g s a r e f o r m e d in the s o u r c e r e g i o n SG, s u g g e s t i n g that the fast ion is f o r m e d by the decay of a c h a r g e d c o r t e x r i n g at the grid. Cunsolo and Mar a v i g l i a [5] have shown that a n e g a t i v e l y c h a r g e d v o r t e x r i n g d e c a y s at a l i q u i d - v a p o u r s u r f a c e , t r a n s f e r r i n g all its e n e r g y to the e l e c t r o n which is e j e c t e d into the vapour. If we a s s u m e that in our a p p a r a t u s the v o r t e x r i n g d e c a y s at the g r id , e j e c t i n g an e l e c t r o n into the liquid in the c o l l e c t ing r e g i o n , then we expect the fast ion s ta te to be f o r m e d by e l e c t r o n s with an e n e r g y of a few e l e c t r o n volts. The p h o t o - e j e c t i o n e x p e r i m e n t s of S a n d e r s et al [8,9] may p r o v i d e i n d i r e c t e v i d e n c e f o r a fast ion state. Using a Cunsolo m o b i l i t y c e l l

20 October 1969

o p e r a t i n g at just above the cut off f r e q u e n c y , they m e a s u r e d a c u r r e n t when 'fast c a r r i e r s ' w e r e p r o d u c e d by the incident light. A s s u m i n g the fast c a r r i e r s w e r e e l e c t r o n s p h o t o - e j e c t e d f r o m a Is st at e in the bubble to the continuum, they concluded f r o m t h e i r s p e c t r a that the well depth of an e l e c t r o n t r a p p e d in a bubble was 0.6 eV, c o r r e s p o n d i n g to the f i r s t m a x i m a in this s p e c t r a . H o w e v e r , a f r e e e l e c t r o n in liquid h e l i u m should f o r m a bubble in a t i m e of 10 -11 s e c , which is too s h o r t - l i v e d f o r the mobility c e l l to r e g i s t e r a c u r r e n t . If this f i r s t m a x i m u m was the t r a n s i t i o n f r o m a l s st at e to a fast ion state, then S a n d e r s ' s e c o n d m a x i m u m at 1.2 eV could be the t r a n s i t i o n to the continuum, in a g r e e m e n t with o t h er e x p e r i m e n t a l and t h e o r e t i c a l work [6,7]. The long life t i m e of the fast ion ( > 10 -2 sec) would allow the m o b i l i t y c e l l to m e a s u r e a c u r r e n t . The a m p l i tude of the m a x i m a in the s p e c t r a would depend on the l i f e t i m e of the 'fast c a r r i e r s ' as well as the t r a n s i t i o n p r o b a b i l i t y , in a g r e e m e n t with the much higher f i r s t m a x i m u m . A s s u m i n g that the f a s t ion is not an i m p u r i t y ion, e.g., O-, then two p o s s i b l e a l t e r n a t i v e m o d e l s s u g g e s t t h e m s e l v e s . One, that the fast ion could be an e l e c t r o n in a s m a l l e r bubble than the n o r m a l ion bubble. G r o s s [10] has pointed out that long r a n g e p o l a r i z a t i o n e f f e c t s , when taken into account with the e l e c t r o n i c , s u r f a c e t en si o n and cavity f o r m a t i o n e n e r g y could s h r i n k the bubble c o n s i d e r a b l y . A bubble of r a d i u s l e s s than 11 A should f o r m r o t o n s i n s t e a d of v o r t e x r i n g s at high f i e l d s [4]. The o t h er a l t e r n a t i v e is an u n t r ap p ed e l e c t r o n , s u r r o u n d e d by a cloud of v i r t u a l phonons, a kind of polaron, moving in an e n e r g y band b etween the v a l e n c e and conduction bands. The m e t a s t a b i l i t y of this st at e would then be due to, and depend on the ions v e l o c i t y . T h e s e two a l t e r n a t i v e m o d e l s might of c o u r s e , with r i g o r o u s t h e o r e t i c a l t r e a t m e n t , be s i m i l a r . We wish to thank D. T. M e l d r u m f o r help in c a r r y i n g out some of the e x p e r i m e n t a l work.

2o

I

0

300

600

EGC(V/crn) Fig. 1. Ion drift velocity in m / s e e versus electric field EGC in V/cm curves for T = 0.92°K. Curve (a), fast ion current; (b) normal ions, forming Reif vortex rings. 252

Reference s 1. S. Cunsolo, Nuovo Cimento 21 {1961) 76. 2. L. Bruschi, B. Maraviglia and P. Mazzoldi, Phys. Rev. 143 (1966) 84. 3. G.W. Rayfield, Phys. Rev. 168 (1968) 222. 4. G.W. Rayfield, Phys. Rev. Letters 16 {1966) 934. 5. S. Cunsolo and B. Maraviglia, Proc. Eleventh Intern. Conf. on Low Temperature Physics (LT 11), St. Andrews (1968), Vol. I, p.265. 6. W. T. Sommer, Phys. Rev. Letters 12 (1964) 271. 7. M.A. Woolf and G. W. Rayfield, Phys. Rev. Letters 15 ~1965) 235.

Volume 30A, number 4

PHYSICS

8. J . A . Northby and T. M. Sanders J r . , Phys. Rev. Letters 18 (1967) 1184. 9. C. Zipfel and T. M. Sanders J r . , Proc. Eleventh In-

RAMAN

SCATTERING

BY

LETTERS

20 October 1969

tern. Conf. on Low Temperature Physics (LT 11), St. Andrews, (1968), Vol. I, p. 296. 10. E . P . Gross, in Quantum fluids, Proc. of Sussex University Symposium (1965), p. 275.

RADIATION

INDUCED

DEFECTS*

I. R. N A I R ** and C. E. H A T H A W A Y Department of Physics. Kansas State University, Manhattan, Kansas 66502, USA Received 18 September 1969

Raman scattering from color center defects induced in KN 3 by ultraviolet irradiation has been observed.

K l e i m a n [1] s u g g e s t e d R a m a n s t u d i e s of F - c e n t e r s m i g h t l e a d to a b e t t e r u n d e r s t a n d i n g of t h e i r l o n g r a d i a t i v e l i f e t i m e . R a m a n s t u d i e s of F c e n t e r s in a l k a l i h a l i d e s w e r e r e p o r t e d by W o r l o c k and P o r t o [2]. H e n r y [3] f u r t h e r a n a l y z e d t h e s e d a t a to s h o w that the f r e q u e n c y s p e c t r u m of p h o n o n s m e a s u r e d by t h e R a m a n s c a t t e r i n g is the s a m e a s the f r e q u e n c y s p e c t r u m of the p h o n o n s t h a t b r o a d e n the F - b a n d . The d a t a p r e s e n t e d in t h i s c o m m u n i c a t i o n r e p r e s e n t the o b s e r v a t i o n of a Raman-active localized mode associated with a radiation induced defect. S t u d i e s of i r r a d i a t i o n - i n d u c e d c o l o r c e n t e r s in a l k a l i - m e t a l a z i d e s , XN3, h a v e b e e n c a r r i e d out by n u m e r o u s i n v e s t i g a t o r s [4]. An N 2 o r l i n e a r N 4 s t r u c t u r e h a s b e e n p o s t u l a t e d on the b a s i s of the E S R d a t a [7]. B r y a n t [8,9] has o b s e r v e d i n f r a r e d a b s o r p t i o n b a n d s at 1723 c m -1 and 1638 c m -1 in u l t r a v i o l e t i r r a d i a t e d s o d i u m and p o t a s s i u m azide, respectively. A cyclic, triangular N3 configuration was postulated for the color center f r o m i n f r a r e d s t u d i e s on s u c h i n d u c e d a b s o r p t i o n b a n d s f o r 15N i s o t o p i c a l l y s u b s t i t u t e d s a m p l e s of t h e s e a l k a l i a z i d e s . The a l k a l i a z i d e s a r e an i d e a l s y s t e m in w h i c h to a t t e m p t to o b t a i n R a m a n d a t a on c o l o r c e n t e r s . U l t r a v i o l e t i r r a d i a t i o n of an a l k a l i a z i d e at 77OK produces characteristic color center absorption b a n d s n e a r 380 n m and 600 nm. T h e u s e of a H e - N e l a s e r (632.8 nm) as a R a m a n e x c i t a t i o n source can enhance the Raman cross-section s i n c e the p r o c e s s is in n e a r r e s o n a n c e with the 600 n m e l e c t r o n i c a b s o r p t i o n band. S m a l l , c l e a r , n e a r p e r f e c t c r y s t a l s of p o t a s -

s i u m a z i d e m e a s u r i n g 1 to 3 m m on a s i d e and up to 1 m m t h i c k w e r e g r o w n by s l o w e v a p o r a t i o n of aqueous solutions. Single crystals were mounted in a d e w a r and m a i n t a i n e d at 77°K. The e x c i t i n g radiation from a modulated helium-neon laser (632.8 n m , 30 mW) was f o c u s e d into the c r y s t a l and the R a m a n s p e c t r u m of t h e c r y s t a l a n a l y z e d w i t h a Spex m o d e l 1401 m o n o c h r o m e t e r and phases e n s i t i v e a.c. d e t e c t i o n s y s t e m . T h e c r y s t a l was t h e n c o l o r e d by p l a c i n g an u n f i l t e r e d P e n - R a y m e r c u r y a r c at a d i s t a n c e of a p p r o x i m a t e l y 3 c m f r o m the c r y s t a l and i r r a d i a t i n g t h r o u g h a q u a r t z window for 7 minutes. The principal radiation f r o m t h i s l a m p i s the 253.7 n m line with an a b s o l u t e i n t e n s i t y of a p p r o x i m a t e l y 4 × 10 . 2 w a t t / (cm 2. s t e r a d i a n ) [ 1 0 ] . A s s u m i n g o n e - h u n d r e d p e r c e n t q u a n t u m e t ~ i c i e n c y in the p r o d u c t i o n of c o l o r c e n t e r s and a m a x i m u m p e n e t r a t i o n depth of 500 m i c r o n s , an u p p e r bound m a y be s e t at one i n d u c e d d e f e c t f o r e v e r y 102 a z i d e ions. T h e R a m a n s p e c t r u m b e t w e e n 125 c m -1 and 975 c m -1 of a s i n g l e c r y s t a l of KN 3 at 7 7 ° K f o r the y ( x x ) z - g e o m e t r y 10 is shown in fig. l a . T h i s s p e c t r a l r e g i o n f o r the s a m e g e o m e t r y a f t e r 7 m i n u t e s i r r a d i a t i o n of the c r y s t a l at 77OK is shown in fig. lb. The i n t e n s i t y of the s c a t t e r e d l i g h t d i m i s h e s a p p r e c i a b l y upon c o l o r i n g the c r y s t a l . An i n c r e a s e in s e n s i t i v i t y by a f a c t o r of 25 w a s r e q u i r e d to r e c o r d the s p e c t r u m shown * Supported in part by the Department of Defense Themis P r o g r a m through the Office of Naval Research. ** Present Address: Department of Physics, Northwestern University, Evanston, Illinois. 253