On the nature of high CF4 vibrational levels excited by proton energy loss spectroscopy

Abstract

The forced harmonic oscillator model, which has been applied to proton energy loss spectroscopy (PELS), is extended here to probe the nature of CF4 vibrational levels excited by the proton beam. At each overtone of the triply degenerate ν3 vibration, there is a collection of vibrational sublevels which differ in the way amplitude is distributed among the four C–F bonds. The model predicts that PELS populates a broad distribution of these sublevels with a propensity for the highest energy, most delocalized vibrations in each overtone band. Comparison is made with infrared multiphoton absorption which excites delocalized vibrations in SF6 and with single photon overtone absorption which selects the most localized states. Peak positions in a simulated spectrum match the experiment up to 8ν3 but for higher overtones the anharmonic shift is up to 0.05 eV less than observed. A demanding test of the model must wait for higher resolution experiments and better spectroscopic constants.