Photoelectron anisotropy and channel branching ratios in the detachment of solvated iodide cluster anions

Photoelectron spectra and angular distributions in 267 nm detachment of the I^-ṡAr, I^-ṡH₂O, I ^-ṡCH₃I, and I^-ṡCH₃CN cluster anions are examined in comparison with bare I^- using velocity-map photoelectron imaging. In all cases, features are observed that correlate to two channels producing either I(²P_3/2) or I(²P_1/2). In the photodetachment of I^- and I^-ṡAr, the branching ratios of the ²P_1/2 and ²P_3/2 channels are observed to be ≈0.4, in both cases falling short of the statistical ratio of 0.5. For I ^-ṡH₂O and I^-ṡCH₃I, the ²P_1/2 to ²P_3/2 branching ratios are greater by a factor of 1.6 compared to the bare iodide case. The relative enhancement of the ²P_1/2 channel is attributed to dipole effects on the final-state continuum wave function in the presence of polar solvents. For I^-ṡCH₃CN the ²P _1/2 to ²P_3/2 ratio falls again, most likely due to the proximity of the detachment threshold in the excited spin-orbit channel. The photoelectron angular distributions in the photodetachment of I ^-, I^-ṡAr, I^-ṡH₂O, and I^-ṡCH₃CN are understood within the framework of direct detachment from I^-. Hence, the corresponding anisotropy parameters are modeled using variants of the Cooper-Zare central-potential model for atomic-anion photodetachment. In contrast, I^-ṡCH ₃I yields nearly isotropic photoelectron angular distributions in both detachment channels. The implications of this anomalous behavior are discussed with reference to alternative mechanisms, affording the solvent molecule an active role in the electron ejection process.