On the Dissociation of van der Waals Clusters of X2−Cyclohexane (X = O, Cl) Following Charge-Transfer Excitation in the Ultraviolet

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We report the results of experiments investigating the charge-transfer state photofragments of a van der Waals cluster after excitation through a strong absorption band in the ultraviolet. The O2−cyclohexane cluster has a strong absorption to a charge-transfer state near 226 nm resulting in dissociation yielding O(3P). On the basis of a simple model (Mulliken, R. S. J. Am. Chem. Soc. 1952, 74, 811) and low level ab initio calculations, the location of the charge-transfer absorption for a Cl2−cyclohexane cluster is predicted and the dissociation of this cluster leading to Cl*(2P1/2) and Cl(2P3/2) is also investigated. The translational energy distribution, P(ET) for each cluster is analyzed in terms of two possible dissociation mechanisms. The dissociation may be considered to proceed on the initially accessed charge-transfer state through a harpooning-type mechanism. Alternatively, the dissociation may proceed following a nonadiabatic electronic transition to the neutral excited states of the diatomic subunit of the cluster. For O2−cyclohexane, the P(ET) is consistent with the second dissociation mechanism. We determine from the available data that the likely structure for the vdW complex is analogous to the resting structure of I2−benzene with the O2 bond axis lying above the cyclohexane ring. For Cl2−cyclohexane, we analyze the velocity dependence of the Cl recoil anisotropy and find it increases from nearly isotropic (β ∼ 0) to distinctly anisotropic (β ∼ 1.7−2). The fast, anisotropic Cl atoms result from dissociation of the cluster on the neutral excited states of Cl2. The slow, isotropic Cl atoms likely result from secondary dissociation of the product Cl−cyclohexane cluster. We determine a Cl*(2P1/2)/Cl(2P3/2) branching ratio of 0.53 ± 0.05 and estimate that ∼19% of the observed Cl atoms result from primary dissociation on the initially accessed charge-transfer state. The data suggest that the Cl2−cyclohexane cluster has an axial-like structure following absorption of a photon. Finally, we explain the rapid nonadiabatic hop from the charge-transfer state to the neutral excited states of the diatomic in terms of coupling of the states though a one-electron change.