Photoelectron Spectroscopy of the Hexafluorobenzene Cluster Anions: (C₆F₆)_n⁻ (n = 1 – 5) and I⁻(C₆F₆)

Abstract

Frequency-resolved (2D) photoelectron (PE) spectra of the anionic clusters (C6F6)n–, for n = 1–5, and time-resolved PE spectra of I–C6F6 are presented using a newly built instrument and supported by electronic structure calculations. From the 2D PE spectra, the vertical detachment energy (VDE) of C6F6– was measured to be 1.60 ± 0.01 eV, and the adiabatic detachment energy (ADE) was ≤0.70 eV. The PE spectra also contain fingerprints of resonance dynamics over certain photon energy ranges, in agreement with the calculations. An action spectrum over the lowest resonance is also presented. The 2D spectra of (C6F6)n– show that the cluster can be described as C6F6–(C6F6)n−1. The VDE increases linearly (200 ± 20 meV n–1) due to the stabilizing influence on the anion of the solvating C6F6 molecules. For I–C6F6, action spectra of the absorption just below both detachment channels are presented. Time-resolved PE spectra of I–C6F6 excited at 3.10 eV and probed at 1.55 eV reveal a short-lived nonvalence state of C6F6– that coherently evolves into the valence ground state of the anion and induces vibrational motion along a specific buckling coordinate. Electronic structure calculations along the displacement of this mode show that at the extreme buckling angle the probe can access an excited state of the anion that is bound at that geometry but adiabatically unbound. Hence, slow electrons are emitted and show dynamics that predominantly probe the outer-turning point of the motion. A PE spectrum taken at t = 0 contains a vibrational structure assigned to a specific Raman- or IR-active mode of C6F6.