Ultrafast ionization and fragmentation of molecular silane

The ionization and fragmentation of molecular silane is examined here with laser intensities ranging between 7×1012 and 1×1015W/cm2 at 624nm. The ionization potential of silane determined using both multiphoton ionization (MPI) and tunneling ionization (TI) models agrees with the vertical ionization potential of the molecule. In addition, the application of the tunneling ionization model is extended here to the fragments of silane to determine their appearance potentials. MPI values for SiH3⁺, SiH2⁺, SiH⁺, Si⁺, as well as H2⁺ and H⁺ are consistent with vertical potentials, whereas the TI measurements are found to be in accord with adiabatic potentials. The tunneling appearance potentials observed for the fragments H2⁺ and H⁺ are lower than reported for other techniques. In fact, the appearance potential measurements for these species resulting from silane are lower than their ionization potentials. The fragmentation rate of silane is determined to be nearly 20 times larger than the ionization rate. The main precursor for producing amorphous silicon (a-Si:H) thin films, SiH3⁺ is the dominant fragmentation product making up roughly a third of the total ion yield, a substantial increase from other techniques.