Kinetics of H2 (D2) desorption from a Ge(100)-2×1:H (D) surface studied using scanning tunneling microscopy and temperature programmed desorption

Abstract

The kinetics of H2 (D2) desorption from a Ge(100)-2×1:H (D) surface was studied using scanning tunneling microscopy (STM) and temperature programmed desorption (TPD). Inspection of STM images of surfaces at the saturation coverage of H (D) (θH(D)≃1.0 ML) revealed a 2×1 monohydride (monodeuteride) phase in which most H (D) atoms were paired on Ge-dimers. By counting the sites of H2 (D2) desorption in STM images taken after desorption of H2 (D2) at temperatures in the range Ts=500−550 K, the desorption of H2 (D2) was found to follow first order kinetics with an activation energy of Ed=1.65±0.1 eV (1.65±0.1 eV) and a pre-exponential factor of νd=(2.7±0.5)×1013 s−1 [(1.2±0.5)×1013 s−1]. These values of Ed and νd were used to simulate TPD spectra for the desorption of H2 (D2) from a Ge(100)-2×1:H (D) surface. The simulated spectra were in good agreement with the experimental TPD spectra. In contrast to the surfaces with saturated H coverage, which are characterized by pairs of H atoms on Ge-dimers, at the low H coverage of θH≃0.05 ML unpaired H atoms as well as paired H atoms were observed on the Ge-dimers on the surface, causing the desorption process to follow second order kinetics. At Ts∼300 K, the singly occupied dimers (SODs) appear to be favored over doubly occupied dimers (DODs). However, upon increasing the temperature (Ts) from 300 to 500 K, most SODs were rapidly converted into the thermodynamically favored DODs by the migration of H atoms. On the other hand, it is observed that even above Ts∼500 K, the onset temperature for H2 desorption from DODs, a non-negligible number of SODs remain on the surface due to the large entropic barrier to pairing. These results suggest that H adsorption in the low coverage is strongly influenced by the energetics of the pairing of H atoms.