Pressure studies of subthreshold photoionization: CH3I perturbed by CF4 and c-C4F8

Subthreshold photoionization spectra of CH3I perturbed by CF4 and c-C4F8 for variable CH3I number density ρCH3I and perturber number density ρP are presented. The ρCH3I and ρP dependence of the subthreshold structure is discussed in terms of the excited-state processes of electron attachment and associative ionization. The effective rate constants for these processes are determined from the variation in the subthreshold photocurrent as a function of ρCH3I and ρP. These constants are then analyzed with respect to the properties of the excited CH3I Rydberg state. The variation in the effective rate constants is discussed in terms of ground-state perturber properties.