EVIDENCE FOR NONADIABATIC PROCESSES IN THE PHOTODISSOCIATION CAGE EFFECT OF VAN DER WAALS COMPLEXES OF $I_{2}$

1. J.J. Valentini and J.B. Cross, J. Chem. Phys. 77, 572(1982). 2. J.A. Beswick, R. Monol, J.-M. Philippoz and H. van denBergh, J. Chem. Phys. 86, 3965 (1987). 3. J. Tellinghuisen, J. Chem. Phys. 58, 2821 (1973).Author Institution: Department of Chemistry, Harvard UniversityThe mechanism of the one-atom cage effect in the photodissociation of $I_{2}$-X complexes has been investigated, where X is Ar or Kr. Valentini and $Cross^{1}$have demonstrated that excitation of $I_{2}-Ar$ above the dissociation limit of the $I_{2}B$ state results in fluorescence from uncomplexed B state $I_{2}$; the authors suggested that a purely mechanical mechanism is responsible. An alternative mechanism has been proposed by Beswick $et. al.^{2}$ whereby the excitation places the complex not on the B state potential but on the repulsive $\Pi_u$ potential; coupling onto the B state and ejection of the Ar would then lead to the observed fluorescensce. The repulsive $\Pi_{u}$ state has been mapped by $Tellinghuisen^{3}$, and it carries a transition intensity comparable to that for the B state in the spectral region near the B state dissociation limit. In order to distinguish between the possible mechanisms we have conducted high resolution laser induced absoption and fluorescence measurements on $I_{2}$-Ar and $I_{2}$-Kr in the bound region of the B state. In particular this allows us to observe that excitation of $I_{2}$-Ar or $I_{2}$-Kr to the $\Pi_{u}$ state, between the relatively sharp bound state B $\leftarrow$ X transitions, induces a significant continuous fluorescence baseline. The fluorescence is red-shifted suggesting that it originates from B state $I_{2}$. These results completely support the very specific mechanism proposed by Beswick et. al. Furthermore it is observed that a considerably higher efficiency is exhibited by Kr than Ar for the ``one-atom cage effect”, which is consistent with the virtually complete quenching of fluorescence observed upon excitation to the bound B state levels of $I_{2}$-Kr. The observed one-atom cage effect is thus accomplished via the mixing of electronic states of $I_{2}$ induced by the van der Waals complex partner. Given the T-shaped structure of these complexes this appears to be a mechanism for effectively coupling perpendicular nuclear motions. The efficiency of the processes will be discussed