Photoelectron spectroscopy of the hexafluorobenzene cluster anions: (C₆F₆ )_n^- (n = 1-5) and I^- (C₆F₆)

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.