Theoretical study of valence excitations in fluoromethanes by high energy electron impact

We report a theoretical study of the electron-impact electronic excitation of fluoromethanes. Generalized oscillator strengths (GOSs) are calculated for the molecules at the equation-of-motion coupled-cluster singles and doubles level. To clarify the influence of nuclear dynamics on the electronic excitations, the effects of internal molecular vibrations are included in the calculation. Comparison with experimental data resolves some inconsistencies in the assignment of the transition bands. Furthermore, it is shown that the lengthening of the CH bond enhances the excitation from the highest occupied orbital to the 3s Rydberg orbital for CHF₃, CH₂F₂, and CH₃F, and that the CH stretching vibrations thus have a significant influence, especially at small momentum transfer where electric dipole transitions are dominant. Since the 2 ¹A₁ state of CHF₃ dissociates into CF₃ + H, the result indicates that the CH stretching vibration causes an increase in the production of CF₃ by electron collision and by photoabsorption.