STATE-TO-STATE ROTATIONAL ENERGY-TRANSFER MEASUREMENTS IN SILANE BY INFRARED DOUBLE-RESONANCE WITH A TUNABLE DIODE-LASER

Infrared double resonance spectroscopy has been used to study state-resolved rotational and vibrational energy transfer in vibrationally excited SiH4. Completely specified rotational levels (v,J,C n) are populated by CO2 laser radiation. Subsequent energy transfer is followed by diode laser transient absorption. The total relaxation efficiencies of the initially populated levels for self-collisions and collisions with Ar and CH4 follow the ordering σ(F2) > σ(A2) > σ(E) and are slightly larger than the Lennard-Jones cross sections. State-to-state rotational energy transfer in the v4 vibration of SiH4 is extremely state specific. In addition to a differentiation between the A, E, and F symmetry levels, there is a selectivity with respect to the fine-structure levels within each rotational state. A preference for transfer to other levels of the same Coriolis sublevel of v4 was found. This can be phrased as a Δ(J - R) = 0 propensity rule. Principal pathways, only one per J per symmetry, are identified. Within each rotational level, the principal-pathway final states are closely spaced; this effect is related to the clustering of the rovibrational levels of the dyad. Large changes in J are possible in a single collision between silane molecules. A kinetic master equation has been used to model energy flow among rotational levels in silane, from which state-to-state energy transfer parameters could be extracted. Collision-assisted absorption of two CO2 photons into the triad has also been detected. A simple modification of the kinetic analysis allows us to obtain an estimate for the relaxation rate out of the triad levels. These laser pumping and relaxation processes determine the efficiency with which high vibrational levels of silane may be populated by infrared multiple photon excitation. © 1990 American Institute of Physics.