Subthreshold photoionization spectra of CH3I perturbed by SF6

We present pressure- 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 (C H3I doped into SF6) shows a quadratic dependence in the former case and a l inear dependence in the latter case. This is discussed in terms of dopant ( D)/perturber (P) interactions involving the excited state processes D* + P --> D+ + P- and D* + P --> [DP](+) + e(-), where D* is a discrete Rydberg s tate 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 s ame system. (C) 1999 Published by Elsevier Science B.V. All rights reserved .