J. Braun et al., MULTIPHONON HE ATOM SCATTERING FROM XE OVERLAYERS ON CU(111) AND CU(001) SURFACES, The Journal of chemical physics, 106(23), 1997, pp. 9922-9929
We have studied the scattering of lie atoms from ordered monolayers of
Xe atoms adsorbed on Cu(111) and Cu(001) surfaces. Both Xe overlayers
, the commensurate (root 3 x root 3)R30 degrees on Cu(lll) at substrat
e temperature above 47 K and the incommensurate one on Cu(001), exhibi
t weak diffraction. On both surfaces the Xe overlayers sustain Einstei
n-like, vertically polarized Xe vibrations which can be multiply excit
ed and annihilated even at low He atom incident energies and substrate
temperatures. In spite of the anharmonic Xe-substrate potentials, the
energies of multiphonon excitations are not found to exhibit any noti
ceable anharmonic shifts in either system. The results of the measurem
ents are compared to theoretical energy and lateral momentum resolved
scattering distributions, which were calculated by using the recently
developed formalism for treating multiphonon scattering in the collisi
on regimes, in which both the motion of the scattered particle and sur
face vibrations must be treated quantum mechanically. We can interpret
the multiphonon scattering spectra and obtain good agreement with exp
eriments by assuming linear He atom-phonon coupling, delocalized phono
ns in Xe adlayers, and by employing the one-phonon interaction paramet
ers determined from the He-Xe interaction potentials. Effects of the q
uantum recoil on the motion of the scattered He atom, which give rise
to a difference between phonon emission and absorption probabilities,
and thereby non-Poissonian scattering distributions, were assessed, (C
) 1997 American Institute of Physics.