Knowledge Bank

The van der Waals complexes $He-I2$ and $He-Br2$ are both known to undergo rapid vibrational predissociation where the halogens are excited to high vibrational levels of the B states. The dissociation of the van der Waals bond leads to vibrational relaxation within the B state, which may then be detected by spontaneously emitted $fluorescence.1,2$ In contrast, both $I2$ (B) and $Br2$ (B) suffer efficient electronic quenching in collisions with He at room $temperature.3$ Quenching is known to occur via collisionally induced predissociation. These two observations can be reconciled if the quenching cross sections are collision energy dependent. In this talk, we shall present the results of quenching measurements made in a free jet expansion at temperatures around 15 K. For $I2$ (B) + He the quenching cross section was found to decrease from its room temperature value of $0.92\backslash AA2$ to $0.1\backslash AA2$ at 15 K. Measurements on the $\alpha$ (B) + He quenching are in progress, and the results of both studies will be presented.