Direct photodesorption of atomic hydrogen from Si(100) at 157 nm: Experiment and simulation

Atomic hydrogen photodesorbs from the Si(100)-(2 x 1):H monohydride surface at 157 nm, with a cross section of similar to 3 x 10(-21) cm(2). We determine using polarized light that the transition dipole moment for the optical excitation is oriented at similar to 18 degrees from the surface normal, as expected for the sigma --> sigma* transition within the H-Si bond. This result unambiguously establishes the dissociation of the H-Si surface bond by direct optical excitation. The dynamics of this process is characterized by a translational energy of [E-trans] = 0.24 +/- 0.02 eV for atomic H and an isotope effect in photodesorption yield of y(H)/y(D) = 10 +/- 3. We carry out time-dependent quantum mechanical simulation on ab initio potential energy surfaces. The simulation suggests a subfemtosecond lifetime in the excited state and a quantum yield for atomic hydrogen desorption of the order 10(-5). This study provides an extreme example for bond-selective photochemistry in large chemical systems. In addition to its significance in understanding ultrafast surface dynamic processes, the direct photodesorption process demonstrated here may also find applications as resistless photolithography in semiconductor technology.