The observation of one-phonon assisted selective desorption and adsorption of He atoms in defined vibrational levels on a LiF (001) single crystal surface

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Abstract
By choosing the appropriate incident angle it has been possible to inject the nearly monoenergetic (Δv/v≊1%) He atoms of an incident beam (18 meV) into one of the four bound states of the atom–LiF(001) surface potential. The final angular distributions of the atoms which have been inelastically scattered by phonons out of these bound states have been measured for the 〈100〉 and 〈110〉 directions of LiF (001). For the low-lying states these distributions are peaked at large final angles (≊75°) near grazing and are different from the distributions for the atoms directly inelastically scattered. The time of flight distributions indicated a predominance of annihilation events indicating that the atoms have been ejected by gaining the minimum amount of energy for release from the surface. The initial angle distribution of He atoms inelastically entering the 〈110〉 v=0 ground state could also be measured by monitoring the flux at a unique magic angle at which the atoms can leave this state by the reverse of selective adsorption. The time of flight spectra reveal that the atoms in this case were trapped by a creation process. The two processes of adsorption and desorption are related by microscopic reversibility. The results are discussed in terms of a simple quantum resonance theory.

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