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

摘要

Photoelectron spectra and angular distributions in 267 nm detachment of the I~-centre dot Ar,I~-centre dotH_2O,I~-centre dotCH_3I,and I~-centre dotCH_3CN 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~-centre dotAr,the branching ratios of the ~2P_(1/2) and ~2P_(3/2) channels are observed to be approx=0.4,in both cases falling short of the statistical ratio of 0.5.For I~-centre dotH_2O and I~-centre dotCH_3I,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~-centre dotCH_3CN 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~-centre dotAr,I~-centre dotH_2O,and I~-centre dotCH_3CN 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~-centre dotCH_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.