Vibrational predissociation of 9,10-dichloroanthracene—Mixed and homo rare gas atom clusters

Abstract

Clusters of the form DCA(R1R2), where DCA=dichloroanthracene and 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 gas being in the minority. The clusters’ vibrational predissociation dynamics was probed using a nanosecond excimer pumped dye laser spectroscopy and energy resolved emission diagnostics. DCA was chosen for this study because of its high emission quantum yield and relatively few spectral interferences at high vibrational energy. The emission quantum yield of DCA–rare gas atom complexes was found to be unity at the electronic origin. The emission quantum yield is greatly reduced upon the increase of the vibrational energy being 0.052 at the 1390 cm−1 vibrational level. It was more than three times higher in the DCA–rare gas atom clusters at the 1390 cm−1 vibration, in comparison with that of the bare molecule, due to vibrational predissociation. The vibrational predissociation products have been identified using energy resolved emission. 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) excited to the 1390 cm−1 vibration, either one of the two rare gas atoms could dissociate, but not both of them. The results indicate that predominantly the weakest bound rare gas atom dissociates, although its vibrational modes seems less effectively coupled to the excited skeleton modes. 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 the various DCA clusters were found to be about 1 ns, independent of the rare gas identity. The results are interpreted by assuming the excitation of a vibrationally mixed 1390 cm−1 state which undergoes a secondary intramolecular vibrational energy redistribution (IVR) within the DCA chromophore to a combination mode which contains a low lying promoting vibrational character. This secondary IVR is the ‘‘bottleneck’’ precursor process whose time scale is intramolecular, being independent of the rare gas atom attached to the DCA. After this secondary IVR, the vibrational energy flows on a much shorter time scale to and between the rare gas atom–DCA vibrational modes, and the weaker bound atom statistically dissociates.