TIME CONSTANT FOR PHONON INDUCED DESORPTION OF HELIUM

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TIME CONSTANT FOR PHONON INDUCED

DESORPTION OF HELIUM

P. Taborek, M. Sinvani, M. Weimer, D. Goodstein

To cite this version:

JOURNAL DE PHYSIQUE

CoZZoque CF, suppllment au n o 12, Tome 42, dlcembre 1981 page C6-852

P. Taborek, M. Sinvani, M. Weimer and D. Goodstein

California I n s t i t u t e o f Technology, Pasadena, CA 91125, U.S. A .

Abstract.- We have made d i r e c t measurements o f the t i m e constant f o r thermal d e s o r p t i o n o f t h i n helium f i l m s as a f u n c t i o n o f s u b s t r a t e temperature and

f i l m b i n d i n g energy. We f i n d t h a t t h e attempt frequency f o r desorption

T ~ i s several orders o f magnitude l a r g e r than p r e v i o u s l y r e p o r t e d values. - ~

Phonons impinging on a s o l i d s u r f a c e which i s covered w i t h a helium f i l m can

cause some o f t h e atoms t o be e j e c t e d . The physics o f t h i s process i s important f o r an understanding o f the dynamics o f desorption and energy t r a n s p o r t a t s o l i d /

helium i n t e r f a c e s . We have used pulsed phonon techniques t o study t h e r a t e of

d e s o r p t i o n from a nichrome surface. The experimental apparatus c o n s i s t s o f a

r e c t a n g u l a r t h i n f i l m nichrome heater deposited on a sapphire c r y s t a l which forms

the bottom o f a vacuum can (see F i g . 1, i n s e t ) . A superconducting t r a n s i t i o n

bolometer w i t h a thermal response time o f

-

10 nsec i s mounted

-

1

m

above the

heater. By a d m i t t i n g known q u a n t i t i e s o f helium gas, t h e thickness o f the f i l m

which covers a l l t h e surfaces i n t h e c e l l can be c o n t r o l l e d i n t h e range 1-3

l a y e r s . The ambient temperature i s 3.5 K and the pressure i s always s u f f i c i e n t l y

low t h a t t h e atoms t r a v e l b a l l i s t i c a l l y t o the d e t e c t o r . A c u r r e n t p u l s e w i t h a

F i g . 1 : Bolometer s i g n a l as a f u n c t i o n o f time f o r various heater pulse widths. Heater temperature = 6.2 K. The i n s e t shows t h e experimen- t a l geometry. 0 4 6 8 10

TIME

(psec)

'Supported i n p a r t by ONR Contract

#

N00014-80-C-0447

I I I I I I I I I I

Pulse W ~ d t h ( p s e c )

I

F i g . 2 : Bolometer s i g n a l as a f u n c t i o n o f t i m e f o r v a r i - ous heater p u l s e widths.

Heater temperature = 8.1 K.

The p u l s e shape i s described

by Eq. 1. No f u r t h e r change

i n t h e s i g n a l i s observed f o r pulses l o n g e r than 1 psec.

known power and d u r a t i o n i s a p p l i e d t o t h e heater, which r a i s e s t h e heater tempera- t u r e and causes d e s o r p t i o n o f t h e f i l m ; t y p i c a l bolometer s i g n a l s a r e shown i n

Figs. 1 and 2. The temperature o f t h e heater f i l m i s a f u n c t i o n o f the power and

can be c a l c u l a t e d from a c o u s t i c mismatch theory(' ) and can a l s o be deduced inde-

pendently from t h e time o f f l i g h t o f the desorbed atoms. The c a l c u l a t e d tempera-

t u r e o f t h e nichrome f i l m and the measured temperature o f the desorbed atoms a r e i n

good agreement. Because t h e heater f i l m i s i n c o n t a c t w i t h a s i n g l e c r y s t a l sub-

s t r a t e , t h e heater temperature can be r a i s e d and lowered w i t h a t i m e constant of

-

10 nsec. When t h e heater temperature i s higher than t h e ambient, the number o f

atoms/cm2 i n t h e helium f i l m n ( t ) r e l a x e s t o a new steady s t a t e value nss i n a c h a r a c t e r i s t i c time T.

For heater pulse widths .s T , t h e d e s o r p t i o n s i g n a l i s a s e n s i t i v e f u n c t i o n of t h e pulse width, b u t f o r l o n g pulses, t h e s i g n a l saturates. By s l o w l y i n c r e a s i n g t h e p u l s e w i d t h from a minimum o f 30 nsec, the p u l s e w i d t h which causes s a t u r a t i o n

i s e a s i l y observed. We have used t h i s technique t o measure T as a f u n c t i o n o f

f i l m thickness and heater temperature. T y p i c a l data shown i n Figs. 1 and 2 g i v e

T

-

1 usec f o r Theater = 6.2 X and T

-

.2 usec f o r Theater = 8.1 X.

Theories f o r the d e s o r p t i o n time T can be q u i t e complicated,(') b u t t h e

r e s u l t s can be described using a simple thermal a c t i v a t i o n model(3) T = T 0 eEbIT

where T~ i s a c h a r a c t e r i s t i c attempt frequency and Eb i s t h e b i n d i n g ener3y of

t h e atom t o t h e s u b s t r a t e . I n order t o comnare our r e s u l t s w i t h previous i n d i r e c t

experimental

measurement^(^)

and t h e o r e t i c a l c a l c u l a t i o n s ( ' ) i t i s necessary t o

measure an average Eb f o r t h e desorbed f i l m s . We have estimated Eb i n two inde-

C6-854 JOURNAL DE PHYSIQUE

temperature o f t h e s u b s t r a t e and t h e atoms desorb w i t h a Maxwellian d i s t r i b u t i o n . The expected bolometer s i g n a l has t h e form

w i t h v = E / t where

L

i s t h e d i s t a n c e t o t h e d e t e c t o r . Eb may a l s o be deduced from t h e chemical p o t e n t i a l o f t h e gas, which can be determined by measuring P and T. Using these methods we f i n d t h a t Eb 2 3 5 K f o r t h e data shown i n Figs. 1 and 2.

From

r

= ro eEb/T, we f i n d ro

<

5 x

l o - '

sec, a t l e a s t two orders o f magnitude

smaller than t h e value deduced i n r e f e r e n c e (4) and explained i n reference (2). The technique used i n t h a t experiment i s very i n s e n s i t i v e t o t h e small t i m e

constants which a r e observed f o r heater temperatures above 6 K. Our measurements

are c o n s i s t e n t w i t h t h e i r data, b u t y i e l d much more p r e c i s e values of T i n t h i s

regime.

Keterences: 1. 0. Weis, Z. Ang. Phy.

26,

325, 1969.

2. Z. W. Gortel, H. J. Kreuzer, and D. Spaner, J. Chem. Phys.

72,

234, 1980.

3. J. Frenkel, K i n e t i c Theory o f L i q u i d s , Dover, New York, 1946.