Collision-free infrared multiphoton dissociation of silane

We have measured an upper limit of 17 J/cm(2) for the threshold fluence for collision-fret CO2 laser IRMPD of silane by detecting the SiH2 product in its electronic and vibrational ground state. The threshold drops to similar to4 J/cm(2) when silane molecules are irradiated by all lines from a pulse d NH3 laser. This value further drops to less than 2 J/cm(2) when the molec ules are pre-excited to the first Si-H stretch overtone. Using the NH3 lase r on the 868 cm(-1) line, the dissociation yield of pre-excited SiH4 reache s saturation at a laser fluence of 6 J/cm(2). The substantial difference in the fluence threshold and optimal dissociation frequency of the ground and excited molecules allows us to perform highly selective dissociation of pr e-excited silane in the presence of a large excess of ground-state molecule s.