Abstract

The upper electronic states in the near-ultraviolet transition (²Σ_g ←²Σ_u⁺) of dihalide radical ions $\left({\text{Cl}}_2^{\overline{\cdot }},{\text{Br}}_2^{\overline{\cdot }},{\text{I}}_2^{\overline{\cdot }}\right)$ are known to be dissociative. We have observed the resonance Raman spectra of these highly reactive species (and also $\text{Cl}{\text{Br}}^{\overline{\cdot }}$) prepared by pulse radiolysis methods in aqueous solution. The resonance Raman observations combined with low photodissociation yields (10-24%) indicate that the excited state lifetimes are of the order of vibrational period of the radical molecules. From the extrapolated convergence of the vibrational progressions in the Raman spectra, the bond dissociation energies are evaluated as 1.6, 1.3, and 0.9 eV for ${\text{Cl}}_2^{\overline{\cdot }},{\text{Br}}_2^{\overline{\cdot }},\text{and}{\text{I}}_2^{\overline{\cdot }}$, respectively. Overtone linewidths in the resonance Raman spectra are observed to increase linearly with the vibrational quantum number, and loss of discreteness is evident at much lower energies than the dissociation limits. The dissociation energies calculated on the basis of solvation energies of the product ions are considerably lower than the spectroscopic values and compare well with the energies corresponding to the loss of discrete vibrational structure. The above observations can be reconciled on the basis of the solvation dynamics of the dissociative Σ_g⁺ state of the radical ions at ~10^-12-s time scale.

© 1986 Optical Society of America