Experimental study of rotationally inelastic collisions of AlH(A 1Π) with Ar: State-to-state rate constants and Λ-doublet propensities

Abstract

A study of rotationally inelastic collisions of AlH(A 1Π) with Ar is reported. AlH was prepared by 193 nm photolysis of trimethylaluminum diluted in slowly flowing Ar at ∼0.5 Torr total pressure. Specific rotational, Λ-doublet levels of AlH(A 1Π,v=0) were prepared by pulsed dye laser excitation on rotational lines in the AlH A 1Π–X 1Σ+ (0,0) band, and final levels were detected by spectrally resolving the fluorescence emission. Bimolecular state-to-state and total removal rate constants were determined. The state-to-state rate constants display dramatic Λ-doublet propensities. In particular, for high initial rotational levels the ΔJ=−1 f→e Λ-doublet symmetry changing transitions had significantly larger rate constants than for ΔJ=−1 f→f Λ-doublet symmetry conserving transitions. These Λ-doublet changing transitions are facilitated by curve crossing between the effective potential energy curves correlating with different rotational asymptotes. The total removal rate constants decrease with increasing initial J, reflecting the increasing rotational spacings.