Dissociation of a surface bond by direct optical excitation: H-Si(100)

Excitation of the H-Si bond on the Si(100)-(2 X 1):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 similar to 18 degrees 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 similar to 0.4 fs.