Knowledge Bank

The mechanism of the one-atom cage effect in the photodissociation of $I2$-X complexes has been investigated, where X is Ar or Kr. Valentini and $Cross1$have demonstrated that excitation of $I2-Ar$ above the dissociation limit of the $I2B$ state results in fluorescence from uncomplexed B state $I2$; 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 $Tellinghuisen3$, 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 $I2$-Ar and $I2$-Kr in the bound region of the B state. In particular this allows us to observe that excitation of $I2$-Ar or $I2$-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 $I2$. 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 $I2$-Kr. The observed one-atom cage effect is thus accomplished via the mixing of electronic states of $I2$ 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.