Dissociation of a Surface Bond by Direct Optical Excitation: H-Si(100)

Excitation of the H-Si bond on the $\mathrm{Si}\left(100\right)-(2\times 1):\mathrm{H}$ surface at 157 nm leads to the desorption of atomic hydrogen. Quantitative measurement using polarized light shows that the transition dipole moment is oriented at $\sim 18°$ from the surface normal, in agreement with the H-Si bond direction. This result unambigously establishes the direct photodesorption mechanism. A comparison of the isotope effect in the photodesorption cross section with a time-dependent quantum mechanical simulation reveals an excited-state lifetime of $\sim 0.4\mathrm{fs}$.