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

A study of rotationally inelastic collisions of AlH(A (1)Pi) with Ar is rep orted. AlH was prepared by 193 nm photolysis of trimethylaluminum diluted i n slowly flowing Ar at similar to 0.5 Torr total pressure. Specific rotatio nal, Lambda-doublet levels of AlH(A (1)Pi,v=0) were prepared by pulsed dye laser excitation on rotational lines in the AlH A (1)Pi-X (1)Sigma(+) (0,0) band, and final levels were detected by spectrally resolving the fluoresce nce emission. Bimolecular state-to-state and total removal rate constants w ere determined. The state-to-state rate constants display dramatic Lambda-d oublet propensities. In particular, for high initial rotational levels the Delta J=-1 f-->e Lambda-doublet symmetry changing transitions had significa ntly larger rate constants than for Delta J=-1 f-->f Lambda-doublet symmetr y conserving transitions. These Lambda-doublet changing transitions are fac ilitated by curve crossing between the effective potential energy curves co rrelating with different rotational asymptotes. The total removal rate cons tants decrease with increasing initial J, reflecting the increasing rotatio nal spacings. (C) 2000 American Institute of Physics. [S0021-9606(00)00334- 2].