Poole-Frenkel ionization of excitons in solid Kr

During vacuum-UV excitation of the excitons in solid Kr photoconductivity nas observed. For applied voltages below 50 V the photocurrent spectrum reflected the number of excitons produced in the bulk, whereas at higher voltages the contribution of excitons in a thin surface layer, including longitudinal and surface excitons, dominated the spectrum. These findings can be explained under the assumption of a Poole-Frenkel effect in the electrical fields produced by surface charges generated during the irradiation.