VIBRATIONAL PREDISSOCIATION OF 9,10-DICHLOROANTHRACENE - MIXED AND HOMO RARE-GAS ATOM CLUSTERS

Clusters of the form DCA (R1R2), where DCA = dichloroanthranthracene a nd R1, R2 = Ar, Kr, and Xe were synthesized in a supersonic molecular beam. The mixed clusters were efficiently formed by the coexpansion of DCA in a mixture of two different rare gases with the heavier rare ga s being in the minority. The clusters' vibrational predissociation dyn amics was probed using a nanosecond excimer pumped dye laser spectrosc opy and energy resolved emission diagnostics. DCA was chosen for this study because of its high emission quantum yield and relatively few sp ectral interferences at high vibrational energy. The emission quantum yield of DCA-rare gas atom complexes was found to be unity at the elec tronic origin. The emission quantum yield is greatly reduced upon the increase of the vibrational energy being 0.052 at the 1390 cm-1 vibrat ional level. It was more than three times higher in the DCA-rare gas a tom clusters at the 1390 cm-1 vibration, in comparison with that of th e bare molecule, due to vibrational predissociation. The vibrational p redissociation products have been identified using energy resolved emi ssion. At 1390 cm-1, excess vibrational energy two argon atoms or one xenon atom could dissociate. For DCA(Kr)n, it is not clear whether one or two krypton atoms had dissociated. In DCA(XeAr) or DCA(XeKr) excit ed to the 1390 cm-1 vibration, either one of the two rare gas atoms co uld dissociate, but not both of them. The results indicate that predom inantly the weakest bound rare gas atom dissociates, although its vibr ational modes seems less effectively coupled to the excited skeleton m odes. The dissociation rates were determined by the relative emission intensity before and after the dissociation, which could be spectrally identified. The time scales for vibrational predissociation of all th e various DCA clusters were found to be about 1 ns, independent of the rare gas identity. The results are interpreted by assuming the excita tion of a vibrationally mixed 1390 cm-1 state which undergoes a second ary intramolecular vibrational energy redistribution (IVR) within the DCA chromophore to a combination mode which contains a low lying promo ting vibrational character. This secondary IVR is the ''bottleneck'' p recursor process whose time scale is intramolecular, being independent of the rare gas atom attached to the DCA. After this secondary IVR, t he vibrational energy flows on a much shorter time scale to and betwee n the rare gas atom-DCA vibrational modes, and the weaker bound atom s tatistically dissociates.