STM-induced H atom desorption from Si(100): Isotope effects and site selectivity

We investigate the scanning tunnelling microscopy-induced H and D atom desorption from Si(100)-(2 x 1):H(D). The desorption of both atoms shows the same energy threshold that corresponds well with the computed sigma --> sigma* excitation energy of the Si-H group, The H desorption yield, however, is much higher than the D yield. We ascribe this to the greater influence of quenching processes on the excited state of the Si-D species. We use wavepacket dynamics to follow the motion of H and D atoms, and conclude that desorption occurs, for the most part, from the 'hot' ground state populated by the quenching process. Site-selective excitation-induced chemistry is found in the desorption of H from Si(100)-(3 x 1):H.