A combined experimental-theoretical study of the vibrational predissociation and product rotational distributions for high vibrational levels of (HeBr2)-Br-79

Pump-probe spectra of HeBr2 in vibrational states upsilon' = 10 and 39 thro ugh 48 of the B electronic state are reported and the fragment rotational d istributions from vibrational predissociation of the cluster are extracted from the measured E(0(g)(+))<--B((3)Pi(0u)(+)) spectra of Br-2. The experim ental results are compared to theoretical calculations on the B<--X spectra using atom-atom model potentials and performing a thermal average over tra nsitions that contribute to the net excitation. Very good agreement between experiment and theory is obtained, except in the region of upsilon' = 44, where the Delta upsilon = -1 channel closes, and in the region of upsilon' = 48 where the Delta upsilon = -2 channel closes. For upsilon' = 43, and up silon' = 44, the agreement is less satisfactory because the dynamics are ex tremely sensitive to details of the potential energy surface due to thresho ld effects associated with the Delta upsilon = -1 channel closing. Similar sensitivity to the potential due to the Delta upsilon = -2 channel closing impairs the agreement between experiment and theory for upsilon' = 48. Belo w upsilon' = 43, the rotational distributions for Delta upsilon = -1 and De lta upsilon = -2 are quite similar, Above upsilon' = 43 the peaks of the ro tational distributions for Delta upsilon = -2 move to higher values of j. T hese results are compatible with the theoretical conclusion that dissociati on shifts from a direct mechanism to one involving intramolecular vibration al distribution in the region of the closing of the Delta upsilon = -1 chan nel. Although the simple additive potential model used in this work succeed s in reproducing most of the experimental data for this system, further imp rovements in the potential energy surface will be required to achieve preci se agreement between experiment and theory for large Br-Br separations. (C) 1999 American Institute of Physics. [S0021-9606(99)03801-5].