Autoionization spectra of $\mathrm{CH_3I}$ in dense gaseous methane, ethane, and propane observed by photoconduction

The influence of light alkane perturbers (up to densities 5×10$^{20}$ cm$^{−3}$ for methane and ethane and 2×10$^{20}$ cm$^{−3}$ for propane, respectively) on the autoionization spectrum of CH$_3$I nd’($^2E_{1/2}$) Rydberg series was investigated by photoconductivity excitation spectroscopy. In the experiments, we recorded photocurrents due to electronic transitions from the absorber molecule to the conduction state of each perturber. The absorption maxima of the perturbed autoionization Rydberg states appeared as maxima in the photocurrent spectrum. The asymptotic pressure shift of the Rydberg lines equals the shift of the second vertical ionization potential I($^2E_{1/2}$) of the perturbed absorber. The energy shift also provides the electron scattering length A in methane, ethane, and propane. The data obtained are compared with results in the literature on photoinjection, electron beam, and swarm experiments in pure alkanes