POLARIZATION DEPENDENCE OF PHONON INDUCED DESORPTION OF He ATOMS AT LOW TEMPERATURES

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POLARIZATION DEPENDENCE OF PHONON INDUCED DESORPTION OF He ATOMS AT LOW

TEMPERATURES

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

To cite this version:

P. Taborek, M. Sinvani, M. Weimer, D. Goodstein. POLARIZATION DEPENDENCE OF PHONON

INDUCED DESORPTION OF He ATOMS AT LOW TEMPERATURES. Journal de Physique Col-

loques, 1981, 42 (C6), pp.C6-855-C6-857. �10.1051/jphyscol:19816253�. �jpa-00221341�

JOURNAL DE PHYSIQUE

Colloque C6, suppldment au n o 12, Tome

ddcembre 1981 page

POLARIZATION DEPENDENCE OF PHONON INDUCED DESORPTION OF He ATOMS AT

LOW TEMPERATURES*

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

C a l i f o r n i a I n s t i t u t e o f TechnoZogy, Pasadena,

91125, U.S.A.

Abstract.- We d i r e c t l y observe t h e d e s o r p t i o n o f He atoms from a t h i n f i l m on the surface o f a s o l i d by b o t h l o n g i t u d i n a l and transverse b a l l i s t i c phonons. B a l l i s t i c propagation o f the He atoms, sound i n t h e vapor and second sound i n t h e b u l k l i q u i d a r e a l l s e o a r a t e l y observed. Comparison o f t h e data reveals t h a t t h e e f f i c i e n c y o f conversion o f L and T phonons i s t h e same f o r these t h r e e d i f f e r e n t modes o f coupling.

The heat p u l s e technique has p r e v i o u s l y been used by several authors t o study t h e transmission o f thermal energy, v i a l o n g i t u d i n a l (L) and transverse (T)

b a l l i s t i c phonons generated i n a s o l i d , across an i n t e r f a c e w i t h b u l k s u p e r f l u i d he1 ium. Schwanenburg and blolter" ) f i r s t made a d i r e c t measurement o f t h e r e l a t i v e L/T phonon transmission c o e f f i c i e n t i n t o He I 1 by observing two peaks i n t h e c o l l e c t i v e second sound s i g n a l corresponding t o t h e two i n c i d e n t phonon p o l a r i z a - t i o n s . I n a l a t e r experiment Salemink, van Kempen and wyder(*) s t u d i e d t h e conver- s i o n o f both L and T b a l l i s t i c phonons i n c i d e n t from t h e s o l i d t o f i r s t sound ( b a l l i s t i c phonon propagation) i n He 11.

Using e s s e n t i a l l y s i m i l a r techniques we have s t u d i e d t h e d e s o r p t i o n o f t h i n f i l m s (1-3 l a y e r s ) o f He adsorbed on t h e surface o f sapphire. The experimental arrangement c o n s i s t s o f a r e s i s t i v e t h i n f i l m Nichrome (- .1 nun2) heater evaporated onto a s i n g l e c r y s t a l sapphire c y l i n d e r 57 nun i n diameter and 9.5 mm t h i c k whose c r y s t a l l o g r a p h i c y - a x i s coincides w i t h t h e d i r e c t i o n o f phonon propagation. The f a r s i d e o f t h e c r y s t a l i s i n a vacuum can where He gas may be admitted. The adsorhed f i l m thickness i s c o n t r o l l e d v i a t h e He gas pressure. The e n t i r e c e l l i s immersed i n a helium bath a t about 2 K. P u l s i n g t h e heater, b a l l i s t i c l o n g i t u d i n a l and transverse phonons propagate through t h e c r y s t a l a r r i v i n g a t t h e f a r s u r f a c e a t d i f f e r e n t times (see i n s e t Fig. 1 ) . He atoms a r e desorbed, and t h e i r a r r i v a l t i m e spectrum detected by a superconducting bolometer above t h e c r y s t a l surface i n t h e gas. When t h e mean f r e e path i n t h e gas i s l o n g compared t o t h e d i s t a n c e t o t h e bolometer t h e desorbed atoms t r a v e l b a l l i s t i c a l l y t o t h e d e t e c t o r . I n t h i s low pressure regime we observed two peaks i n t h e d e s o r p t i o n s i g n a l ( F i g . 1 ) . The d i f f e r e n c e i n a r r i v a l t i m e f o r these two peaks i s p r e c i s e l y t h e same as t h e d i f f e r e n c e i n times o f f l i g h t across t h e sapphire o f t h e two phonon p o l a r i z a t i o n s , as measured by a bolometer evaporated d i r e c t l y on t h e surface o f t h e c r y s t a l . The

* Supported i n p a r t by ONR Contract #N00014-80-C-0447

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19816253

C6-856

Fig. 1 : Bolometer signal vs. time.

a: Desorbed atoms travel b a l l i s t i c a l l y t o bolometer in gas. T = 2 K, pulse 200 nsec, heater power 2 watts, distance t o bolometer 5.8 x cm. Arrival time of peaks corresponds t o two phonon polarizations s h i f t e d by TOF through c r y s t a l .

Bottom: Phonons propagating through c r y s t a l : bolometer on opposite face.

T = 2 K , pulse 30 nsec.

0

6

TIME (psec)

Fig. 2 : Bolometer signal vs. time.

w: Second sound T

2 K , pulse 250 nsec, heater power 2 watts, distance t o bolometer 2.8 x cm.

Bottom: Sound in gas, T = 2.86 K, pulse 100 nsec, heater power 2 watts, gas pressure 5 t o r r . Arrival time of peaks f o r sound and second sound separated by TOF difference of two phonon polariza- tions through c r y s t a l . Absolute times coincide with TOF through crystal and speeds of sound and second sound a t temperatures indicated.

detector must be s u f f i c i e n t l y close t o t h e crystal t o resolve the signal due t o L and T phonons in s p i t e of pulse broadening e f f e c t s .

As t h e gas pressure in the c e l l i s increased the desorption signal undergoes

a t r a n s i t i o n t o c o l l e c t i v e motion in the gas, t h a t i s , sound. Two sound peaks,

generated by the L and T phonons a r e likewise observed (Fig. 2). Finally, when

the c e l l i s f i l l e d with bulk liquid He I1 two second sound pulses a r e observed. (1

The most s t r i k i n g feature of t9ese observations i s t h a t the r e l a t i v e transmission

c o e f f i c i e n t of L and T phonons, as deduced from the amplitude of the two peaks, i s

very nearly the same f o r three very d i f f e r e n t modes of heat transport -- b a l l i s t i c

propagation in the gas, c o l l e c t i v e propagation in the gas, and c o l l e c t i v e propa-

g a t i o n i n t h e l i q u i d . Further, t h i s r a t i o i s r o u g h l y equal

t o t h e r e l a t i v e amplitude o f L and T phonon pulses i n t h e c r y s t a l . Thus t h e c o u p l i n g o f these two p o l a r i z a t i o n s across t h e i n t e r f a c e i s approximately eflual.

The s i m i l a r i t y o f t h e L/T r a t i o f o r desorption, sound, and second sound generation suggests t h a t the same mechanism o f c o u p l i n g across t h e i n t e r f a c e operates i n a l l t h r e e cases.

References: 1. T. J. 6. Schwanenburg and J. Molter, Phys. Rev. L e t t . 3, 693 (1973).

2. H. W. M. Salemink, H. van Kempen, and P. Wyder, Phys. Rev. L e t t . 41, 1733 (1978).

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