Time-resolved photoelectron spectroscopy and imaging studies of anion dynamics.

摘要

Femtosecond photoelectron spectroscopy (FPES) is used to study the dynamics of gas-phase anions embedded in a cluster of solvent molecules. The technique is also extended to time-resolved photoelectron imaging (TRPEI), where dissociation of I₂− is the first system studied.; FPES was used to investigate charge-transfer-to-solvent dynamics in I −(S)_n clusters (S = H₂O, D₂O, NH₃, CH₃OH, and Xe) and vibrational energy redistribution in I₂−(S) _n (S = CO₂, Ar). In the charge-transfer-to-solvent systems, the solvent molecules support excited states close to the electron detachment threshold. The dynamics subsequent to excitation differ substantially with solvent type and number, with the hydrogen-bonded solvents showing evidence for partial solvation of the excess electron and rearrangement on multiple potential surfaces. In the vibrational relaxation studies, I₂ − within a cluster is coherently vibrationally excited using femtosecond stimulated emission pumping, and its time-dependent oscillation frequency and spectrum are used to extract rates and amounts of energy transfer to the solvent network. For CO₂ clusters, the timescale for energy loss depends only weakly on the number of solvent molecules and initial excitation energy; in contrast, the argon clusters show much greater variation, with faster relaxation occurring at higher excitation energies and in smaller clusters.; The modifications to the apparatus necessary to transform FPES into TRPEI are also described. The TRPEI method affords superior energy resolution and the ability to monitor photoelectron angular distributions as well as energies. The technique is first utilized to study the photodissociation of I₂ −. The time-dependent angular distribution of photoelectrons changes non-monotonically during the dissociation, indicating a sensitive dependence on the dynamics and on the electronic structure of the anion. The changes at long times (600–800 fs) are proposed to reflect localization of the extra electron onto one of the I atoms.