Collision-Free Infrared Multiphoton Dissociation of Silane

We have measured an upper limit of 17 J/cm2 for the threshold fluence for collision-free CO2 laser IRMPD of silane by detecting the SiH2 product in its electronic and vibrational ground state. The threshold drops to ∼4 J/cm2 when silane molecules are irradiated by all lines from a pulsed NH3 laser. This value further drops to less than 2 J/cm2 when the molecules are pre-excited to the first Si−H stretch overtone. Using the NH3 laser on the 868 cm-1 line, the dissociation yield of pre-excited SiH4 reaches saturation at a laser fluence of 6 J/cm2. 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 pre-excited silane in the presence of a large excess of ground-state molecules.