TIME-RESOLVED PATTERNS FOR ELECTRON-STIMULATED DESORPTION OF METASTABLE ATOMS FROM THIN AR(111) FILMS

Time-resolved desorption patterns of metastable argon atoms have been obtained from thin Ar(111) films condensed on Pt(111) at 15 K. The desorption is stimulated by monochromatic electrons with a fixed energy of 14.5 eV. Combining the information on time-of-flight and angular distributions, five distinct metastable-atom components can be identified. The maxima of their kinetic energies are located at 19, 36, 53, 85, and 345 meV (+/- 10%), with the two lowest-energy signals being more intense. The 53-meV component has a narrow angular distribution of 8-degrees at half maximum with respect to the surface normal, while the others are broad. Different aspects of the basic desorption mechanisms are discussed. The sharpness of the angular distribution of the 53-meV component is interpreted as due to nuclear zero-point motion in an undistorted crystalline environment. The data for the 19-, 36-, and 53-meV components are in qualitative agreement with recent molecular-dynamics studies.