Polarisation effects in electronically inelastic collisions: SiF C²Δ + H₂→SiF B ²Σ⁺ + H₂

Linearly polarised laser excitation was used to prepare aligned, electronically excited samples of SiF molecules. Selected, spectroscopically isolated bandheads in the B-X and C-X systems were excited, and appropriate bands resolved in the dispersed fluorescence. The observed degree of polarisation anisotropy, R, of B-X (3,2) fluorescence from the directly excited SiF B ²Σ⁺, v=3 state agreed, within experimental error, with the theoretical prediction. The value of R for C-X (0,0) fluorescence from the SiF C²Δ, v=0 level excited in the presence of H₂ (and Ar carrier) was reduced from the theoretical prediction, consistent with previous observations of partial rotational redistribution under these conditions. As also previously established, collisions with H₂ induce SiF C→B transfer. The SiF B ²Σ⁺, v′=0 product was found to be significantly less polarised than the SiF C²Δ, v=0 population from which it was produced. Two simple classical models were developed for the degree of depolarisation based on alternative assumptions about the spatial distribution of the additional angular momentum, ΔJ, imparted during the collision process, relative to the initial J. Both predict a degree of depolarisation consistent with the experimental observations in this case. We note that there exists in principle an impulsive dynamical limit in which the product state will remain partially aligned regardless of the relative magnitudes of J and ΔJ.