LASER-INDUCED DESORPTION OF H-2 FROM SI(111) 7 X 7

Laser excitation can lead to desorption when silicon surfaces are exposed to laser pulses at a frequency above the band gap. To determine whether a thermal or a photochemical desorption mechanism is operative, this study compared experimental measurements and numerical calculations for the laser-induced desorption (LID) of H-2 from Si(111)7 x 7. The surface-temperature rise on Si(111)7 x 7 produced by laser pulses from a TEM-00 Q-switched ruby laser at lambda = 694 nm was calculated using temperature-dependent physical and optical parameters and a finite penetration depth. The laser-induced surface heating results were then used to calculate laser-induced thermal desorption (LITD) yields for H-2 from Si(111)7 x 7. The calculated H-2 LITD yields as a function of laser energy, surface temperature, and hydrogen coverage were compared with the corresponding experimental H-2 LID measurements. The excellent agreement between the calculated and measured LID results indicated that a thermal desorption mechanism can describe the LID of H-2 from Si(111)7 x 7. Photochemical LID was also considered as an alternative desorption mechanism. Although significant electron-hole pair densities were estimated at the silicon surface following laser excitation, an unambiguous distinction between thermal and photochemical desorption mechanisms was difficult because the optical properties of silicon vary dramatically with temperature. In addition, the surface heating calculations indicated that silicon surface melting can occur given typical experimental conditions used for LID from silicon surfaces.