Photocurrent excitation spectra of methyl iodide/methane fluid mixtures

Abstract

Impurity photocurrents of fluid CH4:CH3I were studied in the region of the (2E1/2)nd Rydberg series of CH3I and also beyond the 2E1/2 ionization potential up to the LiF cutoff. In the gas, a step-like rise of the photocurrent was observed at a photon energy somewhat lower (by about 20 meV) than the 2E1/2 ionization potential, indicating that high members of the (2E1/2)nd Rydberg series of methyl iodide can be ionized by further processes besides the well-known autoionization into the 2E3/2 continuum. At the high-energy side of the 2E1/2 limit further peaks appear, corresponding to simultaneous excitation of (2E1/2)nd Rydberg states and one or two quanta of the ν2(a1) vibration of CH3I. In the photon energy region of 10.8–11.8 eV a series of at least eight peaks were observed, due to excitation of vibronic levels of the Jahn–Teller split 4s Rydberg state of methane. The value of the adiabatic ionization potential 2E3/2 of CH3I in liquid CH4 near the critical point was found to be 8.30 eV, while in neat dilute CH3I gas it is 9.48 eV. The overall variation of 2E3/2 in the liquid phase (from 190 to 132 K, 6.4×1021 to 1.5×1022 cm−3) is 70 meV. In the densest (and coolest) liquid samples studied a series of peaks, with intervals of ≊60 meV appeared in the photon energy range 8.37<hν<8.5 eV on top of the continuous background.