PHOTOCURRENT EXCITATION-SPECTRA OF METHYL-IODIDE METHANE FLUID MIXTURES

Impurity photocurrents of fluid CH4:CH3I were studied in the region of the ((2)E(1/2))nd Rydberg series of CH3I and also beyond the (2)E(1/2 ) ionization potential up to the LiF cutoff. In the gas, a step-like r ise of the photocurrent was observed at a photon energy somewhat lower (by about 20 meV) than the (2)E(1/2) ionization potential, indicating that high members of the ((2)E(1/2))nd Rydberg series of methyl iodid e can be ionized by further processes besides the well-known autoioniz ation into the (2)E(3/2) continuum. At the high-energy side of the (2) E(1/2) limit further peaks appear, corresponding to simultaneous excit ation of ((2)E(1/2))nd Rydberg states and one or two quanta of the ups ilon(2)(a(1)) vibration of CH3I. In the photon energy region of 10.8-1 1.8 eV a series of at least eight peaks were observed, due to excitati on of vibronic levels of the Jahn-Teller split 4s Rydberg state of met hane. The value of the adiabatic ionization potential (2)E(3/2) of CH3 I 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 (2)E( 3/2) in the liquid phase (from 190 to 132 K, 6.4X10(21) to 1.5X10(22) cm(-3)) is 70 meV. In the densest (and coolest) liquid samples studied a series of peaks, with intervals of approximate to 60 meV appeared i n the photon energy range 8.37 <h upsilon<8.5 eV on top of the continu ous background.