Pressure dependence of radiative processes in helium

Volume 28A, n u m b e r 6

PRESSURE

PHYSICS

DEPENDENCE

OF

LETTERS

RADIATIVE

30 December 1968

PROCESSES

IN

HELIUM

P . G U A Z Z O N I , M. P I G N A N E L L I Istituto di F i s i c a d e l l ' U n i v e r s i t h , Milano. I N F N . Sezione di Milano

Received 13 November 1968

The radiative p r o c e s s e s in helium a r e studied. The role of atomic m e t a s t a b l e s t a t e s and of He~ ions is determined.

O u r p u r p o s e w a s to i n v e s t i g a t e t h e l i g h t e m i s s i o n b y h e l i u m u n d e r c o n d i t i o n s of low ion d e n s i ty a n d h i g h p r e s s u r e . These conditions can be obt a i n e d e x c i t i n g h e l i u m g a s b y m e a n s of i o n i z i n g p a r t i c l e s . T h e m e c h a n i s m of t h e s c i n t i l l a t i o n p r o d u c e d b y c~ p a r t i c l e s h a s b e e n i n t e r p r e t e d i n t e r m s of e l e c t r o n - i o n r e c o m b i n a t i o n [1], f o r m a t i o n of m e t a s t a b l e m o l e c u l e s [2] o r a s i m p u r i t i e s e f f e c t s [3]. T h e a p p a r a t u s u s e d h a s b e e n d e s c r i b e d in a p r e v i o u s p a p e r [4]. T h e He w a s i n t r o d u c e d in t h e gas chamber through a charcoal trap at liquid nitrogen temperature; other gases could be introd u c e d , a s c o n t r o l l e d a m o u n t s of i m p u r i t i e s , down to a m i n i m u m v a l u e of 2 × 10 - 4 T o r r . A n 2 4 1 A m (~ s o u r c e w a s p l a c e d in t h e c h a m b e r . T h e a p p a ratus, using a quartz window and a photomultip l i e r 6255 S, w a s s e n s i t i v e f r o m 2000 to 5700 A. To detect the vacuum ultraviolet emission the i n t e r n a l s u r f a c e s of t h e c h a m b e r w e r e c o a t e d b y quaterphenyl as wavelength shifter. The pulse dec a y t i m e of t h e u.v. r a d i a t i o n f o l l o w i n g a n c~ p a r t i c l e e m i s s i o n w a s m e a s u r e d v a r y i n g t h e He p r e s sure. The pressure dependence above 1 arm is s i m i l a r , b u t w i t h h i g h e r a b s o l u t e v a l u e s , to t h a t r e p o r t e d b y T a k a h a s h i e t al. [2]. W e c a n n o t e x plain this disagreement, m o r e o v e r w e find a change in the dependence at lower pressures, as s h o w n i n fig. 1. A d e c a y t i m e i n v e r s e l y p r o p o r t i o n a l to t h e s q u a r e of t h e p r e s s u r e m a y b e d u e , a s s u g g e s t e d b y T a k a h a s h i , to t h r e e - b o d y c o l l i s i o n s b e t w e e n a m e t a s t a b l e He a t o m a n d two g r o u n d s t a t e a t o m s , a c c o r d i n g to t h e p r o c e s s : H e m + + He + He -4 He 2 + H e ~ 3He + h r . A d e p e n d e n c e i n v e r s e l y p r o p o r t i o n a l to t h e p r e s s u r e , a s t h a t w e f i n d b e l o w 0.8 a t m , m a y b e i n s t e a d c a u s e d b y two-body collisions. This is an useful indication of t h e n a t u r e of H e m s t a t e . A n i m p o r t a n t f r a c t i o n of t h e e n e r g y e x p e n d e d i n a t o m i c e x c i t a t i o n i s s t o r e d i n 2xS s t a t e . H o w e v e r 23S s t a t e c a n n o t b e i g n o r e d , a p r i o r i , o n a c c o u n t of e x c i t a t i o n b y 432

lO_

31 o

E

"=L 1

Helium

10

pressure

(atm)

Fig. 1. P r e s s u r e dependence of the decay time of the vacuum u l t r a - v i o l e t radiation. secondary electrons impacts or transfer process e s i n v o l v e d in t h e d e p l e t i o n m e c h a n i s m of n l p s t a t e s [3]. It i s k n o w n [5] t h a t t h r e e - b o d y c o l lisions are the dominant destruction process for 23S s t a t e a b o v e 10 T o r r , s o t h a t a t w o - b o d y m e c h a n i s m p r e v a i l i n g u p to 0.8 a t m m u s t b e d u e to 21S s t a t e d e s t r u c t i o n . W e c a n t h e r e f o r e o b t a i n from the present data the two-body destruction c r o s s s e c t i o n f o r t h i s s t a t e , t h a t r e s u l t s : (8.5 + ± 3 ) x 1 0 - 2 0 c m 2. P h e l p s ' d a t a [5] m a y b e f i t t e d w i t h a v a l u e of (3 + 4 . 5 ) x 1 0 - 2 0 ; t h e o v e r a l l e r ror cannot be easily assigned, but we believe that t h e two d a t a a r e in a g r e e m e n t w i t h i n t h e a c c u r a c y of t h e two m e t h o d s . T h e t h r e e - b o d y d e s t r u c t i o n c o e f f i c i e n t f o r 21S s t a t e r e s u l t s : (6 ± 2 ) x 10 - 3 4 c m 2 / a t o m 2 s e c (at 300OK). F r o m p u b l i s h e d d a t a we c a n h a v e o n l y a n u p p e r l i m i t of 6 x 10 - 3 3 [5]. A v a r i e t y of i m p u r i t i e s e f f e c t s a r e r e p o r t e d in t h e l i t e r a t u r e , h o w e v e r a r e m i s s i n g s y s t e m a t i c d a t a . W e h a v e s t u d i e d t h e s c i n t i l l a t i o n of He,

Volume 28A, number 6

PHYSICS LETTERS

at p r e s s u r e s r a n g i n g f r o m 1 to 41 a t m , in m i x t u r e with: N2, O2, CO, CO 2, NO, N20. The gene r a l t r e n d s a r e : a) the s p e c t r a l d i s t r i b u t i o n c o r r e s p o n d s to v i b r a t i o n a l bands of the m o l e c u l a r ion of the i m p u r i t y , b) the a m p l i t u d e of the light pulse is n e a r l y independent f r o m the i m p u r i t y p r e s s u r e until quenching v a l u e s a r e r e a c h e d , c) the a m p l i t u d e is independent f r o m the He p r e s s u r e e x c e p t f o r N 2 and CO m i x t u r e s , f o r which is i n v e r s e l y p r o p o r t i o n a l to this p r e s s u r e , d) the d e c a y t i m e depends only f r o m the p a r t i a l p r e s s u r e of the i m p u r i t y , and is i n v e r s e l y p r o p o r t i o nal to this p r e s s u r e ( t i m e s of the o r d e r of 15 + 70 p s e c at 10-3 T o r r ) . At low c o n c e n t r a t i o n s the d e c a y t i m e due to i m p u r i t y m a y be much longe r of that of u.v. e m i s s i o n . T h i s fact m e a n s that the e n e r g y s o u r c e f o r i m p u r i t y e x c i t a t i o n i s diff e r e n t f r o m that of u.v. r a d i a t i o n . A r e so e x c l u d ed P e n n i g e f f e c t s on i m p u r i t y m o l e c u l e s p r o d u c e d by He a t o m s in m e t a s t a b l e s t a t e s . Taking a l s o i n to account that the e n e r g y expended in i o n i z a t i o n g o e s m a i n l y in He~. s t a t e s , the m o s t p r o b a b l e m e c h a n i s m i s the Charge exchange: He~. + M -~ 2He + + M +v ~ 2He + M + + hv; w h e r e M +v is the m o l e c u l a r ion of the i m p u r i t y in a v i b r a t i o n a l e x c i t e d s t a t e . T h i s m e c h a n i s m w a s s u g g e s t e d by Bennet

30 December 1968

[3] f o r H e - N 2 m i x t u r e at 350 T o r r . Th e m a x i m u m e n e r g y a v a i l a b l e f o r t r a n s f e r in He~, in its h i g h e r e x c i t e d s t a t e s is about 20.5 eV. A d e g r e e of v i b r a tional e x c i t a t i o n of He~ ions, depending on the p r e s s u r e , might show up through a v a r i a t i o n of the yield of the i m p u r i t y ions, when the e n e r g y r e q u i r e d to p r o d u c e the a p p r o p r i a t e st at e i s n e a r to 20 eV. T h i s is the c a s e of the B2Z ÷ s t a t e s in N~. and CO + (18.8 and 19.7 eV r e s p e c t i v e l y ) . The c r o s s s e c t i o n s f o r the c h a r g e exchange deduced f r o m the t i m e d e p e n d e n c e (point d), a r e of the o r d e r of 10-14 c m 2. The e x p e r i m e n t a l data on h e l i u m m i x t u r e s will be given in a m o r e d e t a i l e d paper.

References 1. R.J. Esterling and N. H. Lipman, Rev. Sci. Instr. 36 (1965) 493. 2. T. Takahashi. S. Kubota and T. Doke. Phys. Letters 23 (1966) 321. 3. W.R. Bennet. Ann. of Phys. 18 (1962} 367. 4. P. Guazzoni and M. Pignanelli. Nuovo Cimento 40 0965) 454. 5. A.V. Phelps. Phys. Rev. 99 (1955) 1307 and Phys. Rev. 117 (1960) 619.

PHONON DISPERSION ON AN ARGON SINGLE

MEASUREMENTS CRYSTAL AT 4.2°K

H. E G G E R , M. GSANGER and E. LUSCHER Physik-Department der Teehnischen Hochsehule M~nchen. Germany and B. D O R N E R Kernforschungsanlage dMich. Institut fl~r FestkOrper- und Neutronenphysik. Germany Received 18 November 1968

Phonon dispersion measurements by inelastic neutron scattering on an argon single crystal at helium temperature have been performed.

A single c r y s t a l of A r of about 45 cm 3 v o l u m e with f cc s t r u c t u r e h a s been grown f r o m A r with the n a t u r a l i s o t o p i c m i x t u r e (~ coh = 0.40 b a r n , a inc = 0.25 barn) and an i m p u r i t y c o n c e n t r a t i o n < 4 ppm. The g r o w i n g technique, including handling and t e s t i n g of the c r y s t a l , h a s a l r e a d y b e e n d e s c r i b e d [1]. The s p e c i m e n was mounted in a

c r y o s t a t such that it was c o m p l e t e l y s u r r o u n d e d by a l u m i n i u m w a l l s and could be cooled to liquid helium temperature. The n eu t r o n m e a s u r e m e n t s w e r e p e r f o r m e d with the t r i p l e a x i s s p e c t r o m e t e r at the F R J - 2 r e a c t i o n in J~lich. The c r y o s t a t was mounted on a s p e c i a l g o n i o m e t e r for o r i e n t a t i o n of the s a m p l e 433