Time-of-flight spectroscopy of high-overtone mode-selective vibrational excitation of CF4 and SF6 in collisions with H+(D+) at energies between 10 and 28 eV

Abstract

Time-of-flight spectra for H+(D+)–CF4 and SF6 collisions have been measured with an improved resolution and at higher collision energies (10≤Elab ≤28 eV) compared to earlier work. In the low energy region (≤13 eV) new distinct peaks are resolved for both molecules corresponding to small contributions from the second infrared active ν4 mode in addition to the dominant ν3 mode observed previously. Reexamination of experimental vibrational transition probabilities reveals an almost perfect agreement with a Poisson distribution for both modes up to the n=6 overtone transition of ν3. A simple straight line theory is used to calculate the energy transfer in small angle scattering from the long-range potential in good agreement with a full classical trajectory calculation. With this theory dipole moment derivatives can be determined directly from the observed energy transfers and are found to agree well with previous infrared measurements. At larger collision energies (≥16 eV) an additional low intensity vibrational distribution is identified in the high energy loss tail of the spectra which can be attributed to small impact parameter collisions which probe the repulsive region of the potential. The observed energy transfers are also in good agreement with trajectory calculations indicating that the forced oscillator model is also applicable in the repulsive potential region for the present systems. A closer examination of the high energy loss tail reveals resolved structure which has been assigned to discrete states of the ν3 mode in CF4 up to the n=14 overtone. These new results demonstrate that H+(D+) energy loss scattering can provide spectroscopic information not readily available from other experiments.