Subthreshold Photoionization Spectra of CH3I Perturbed by SF6

We present pressure-dependent and temperature-dependent subthreshold photoionization spectra of pure CH3I (up to 200 mbar) and CH3I doped into SF6 (up to 1 bar). At the high pressures studied, no temperature effect is observed for the subthreshold structure, thus ruling out vibrational autoionization of CH3I as an ionization mechanism. Moreover, analysis of photocurrent intensities as a function of CH3I number density (pure CH3I) and SF6 number density (CH3I doped into SF6) shows a quadratic dependence in the former case and a linear dependence in the latter case. This is discussed in terms of dopant (D)/perturber (P) interactions involving the excited state processes D + P 6 D + P and D + P 6 [DP] + e, where D is a discrete Rydberg state of the dopant (CH3I). From the density dependence of the subthreshold structure of CH3I/SF6, the electron scattering length in SF6 is determined and compared to a value recently obtained from autoionizing states in the same system.