Femtosecond photoelectron spectroscopy: A new tool for the study of anion dynamics.

摘要

A new experimental technique for the time-resolved study of anion reactions is presented. Using femtosecond laser pulses, which provide extremely fast (∼100 fs) time resolution, in conjunction with photoelectron spectroscopy, which reveals differences between anion and neutral potential energy surfaces, a complex anion reaction can be followed from its inception through the formation of asymptotic products. Experimental data can be modeled quantitatively using established theoretical approaches, allowing for the refinement of potential energy surfaces as well as dynamical models.; After a brief overview, a detailed account of the construction of the experimental apparatus is presented. Documentation of the data acquisition program is contained in the Appendix. The first experimental demonstration of the technique is then presented for I₂− modeled using a simulation program which is also detailed in the Appendix. The investigation of I₂− photodissociation in several size-selected I₂− (n = 6–20) and I₂− (CO₂) _n (n = 4–16) clusters forms the heart of the dissertation. In a series of chapters, the numerous effects of solvation on this fundamental bond-breaking reaction are explored, the most notable of which is the recombination of I₂− on the ground state in sufficiently large clusters. Recombination and trapping of I₂− on the excited state is also observed in both types of clusters. The studies have revealed electronic state transitions, the first step in recombination, on a ∼500 fs to ∼10 ps timescale. Accompanying the changes in electronic state is solvent reorganization, which occurs on a similar timescale. Over longer periods (∼1 ps to >200 ps), energy is transferred from vibrationally excited I₂− to modes of the solvent, which in turn leads to solvent evaporation. These effects become more important as cluster size increases. In addition, differences in timescale and mechanism are observed between clusters of Ar, which binds to I− and I₂− rather weakly, and CO₂, whose large quadrupole moment allows substantially stronger binding to these anions.