P. teidererIns t i t u t f . Physik, Johannes-Gutenberg-Wniversl t%t $1)-6500 K a i n z , W,Germany Our understanding of the phonon processes a t t h e interfaces between two aedla i s s t i l l quite rudimentary, Particularly notorious i s the Helium-solid P n t e r f a c e , where the a c c o u s t i c i m p e d a n c e s d i f f e r by a l a r g e a m o u n t . It i s w e l l known that the Kapitza resistance, i.e. the thermal boundary resistance between Helium and a s o l i d , i s usually up t o 100 times smaller than predict e d by the a c o u s t i c theory a s f o r m u l a t e d by KhaPatnikov [ I ] , However, it was shown by J. WEBER e t al.[ 2 ] t h a t there was no discrepancy at surfaces of f x e s h l y cleaved crystals, i.e. at surf aces of exceptional good quality. Thus ft i s clear t h a t surface irregularities must be responsible for the anomalous Kapitza resistance. Little progress has b e e n made, however, in t h e understanding of how these irregularities m e d i a t e the anomalously s t r o n g phonon t r a n s n i s s i o n .In t h i s Paper we describe a technique which a l l o w s the in situ modificat i o n of a surface. S f m v l t a n e o u s l y the phonon loss into H e l i u m c a n b e measured. From t h e effect 05 the surface rnodifScations on the phonon losses, we can gain i n s i g h t into the nature and the sole of the surface irregularities.We u s e the method of laser annealing which was introduced recently 131. Thfs technique u s e s high-power l i g h t p u l s e s from a ruby laser to irradiate a Si s u r f a c e * The energy of t h e pulse t s sufficfent to melt momentarily a thin (less t h a n 5000 8 thickness) surface layer. It i s known f r o m several studies (31 t h a t the m o l t e n layer r e c r y s t a l l i z e s in the same structure as the bulk and t h a t t h e surface is free of impurities.Our e x p e r i m e n t a l set up i s shown in Fig, 1.a. W e u s e d a vacuum c h a m b e r which was immersed i n liquid Helium at 1 K. The sample, a S i c r y s t a l , (100) o r i e n t e d , of 4 m m thickness, was sealed to one side of t h e chamber. The test s u r f a c e o f t h e s a m p l e was a t the v a c u u m side. It was polished by the supplier and w i t h o u r v i s i b l e scratches, The phonon generator and the detector were placed about 0.5 m m apart on t h e outer side of t h e c y s t a l . As generator w e used a Sn film w h i c h was heated by irradiation w i t h l i g h t p u l s e s from an SeNe laser. We assume that the emitted phonon spectrum has a broad frequency band with a maximum at 2 4 , the energy gap of t h e Sn. W e used phonon pulses of 100 n s duration. These p u l s e s were, after r e f l e c t i o n from the test surface, d e t e c t e d w i t h an A 1 tunnel junction. The i n t e n s i t y of t h e reflected phonons depends , of course, o n the l o s s e s at the t e s t surface. The r e l a t i v e s i g n a l change during f f l l i n g t h e chamber w i t h Helium could be measured w i t h a'trotlt 1 % p r e c i s i o n...