A combined experimental-theoretical study of the vibrational predissociation and product rotational distributions for high vibrational levels of He79Br2

Pump-probe spectra of HeBr2 in vibrational states upsilon' = 10 and 39 through 48 of the B electronic state are reported and the fragment rotational distributions from vibrational predissociation of the cluster are extracted from the measured E(0(g)(+))<--B((3)Pi(0u)(+)) spectra of Br-2. The experimental results are compared to theoretical calculations on the B<--X spectra using atom-atom model potentials and performing a thermal average over transitions 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 upsilon' = 44, the agreement is less satisfactory because the dynamics are extremely sensitive to details of the potential energy surface due to threshold 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. Below upsilon' = 43, the rotational distributions for Delta upsilon = -1 and Delta upsilon = -2 are quite similar, Above upsilon' = 43 the peaks of the rotational distributions for Delta upsilon = -2 move to higher values of j. These results are compatible with the theoretical conclusion that dissociation shifts from a direct mechanism to one involving intramolecular vibrational distribution in the region of the closing of the Delta upsilon = -1 channel. Although the simple additive potential model used in this work succeeds in reproducing most of the experimental data for this system, further improvements in the potential energy surface will be required to achieve precise agreement between experiment and theory for large Br-Br separations. (C) 1999 American Institute of Physics. [S0021-9606(99)03801-5].