Extrinsic photoconduction and Rydberg states due to a methyl iodide impurity in xenon

Photoconductivity in Xe due to a CH₃I impurity is presented as a function of incident photon energy at Xe number densities with range (3--15)×{}10¹⁹ cm^-3, with constant CH₃I concentrations. The Xe absorption spectrum between 9.5 and 10.6 eV, including both the strongly perturbed allowed transitions and molecular bands associated with forbidden lines, appears as dips in the photocurrent. At photon energies below 9.5 eV the photocurrent increases exponentially with photon energy, and members of the nd and ns Rydberg series of CH₃I appear as steps. From the shift of the step positions with density the electron scattering length in xenon was determined as a=-0.324 nm. The photocurrent versus photon energy curves shift more strongly with density than the superimposed Rydberg lines as a consequence of adiabatic polarization effects. The results provide clear evidence that photoconduction below the vertical ionization potential takes place by ionization of Rydberg states.