Comparative study of K-shell exciton series in condensed neon and nitrogenby electron time-of-flight spectroscopy

Photon absorption and electron emission phenomena in the near-K-edge region are studied for condensed neon and nitrogen, employing electron time-of-flight spectroscopy in combination with high photon-energy resolution. Surface and bulk ionization thresholds and bulk exciton excitation energies, including the (Formula presented)5p transition, are reported for neon multilayers. For condensed nitrogen, we observe three different types of resonant features in the near-K-edge region. Type 1, which is excited by photon energies less than the surface N 1s ionization potential, is clearly of excitonic nature. Resonances of type two are seen for excitation energies which are larger than the surface K edge by more than 3 eV. They parallel maxima seen in the kinetic-energy distribution of secondary electrons and can be well explained by transitions into regions of the conduction band with a high density of states. Peaks of the third category appear between the surface K edge and 1.5 eV above. They have no analogs either in the secondary electron distribution, or in the amplitude of the N 1s photoemission signal. We explain them as bulk excitons converging toward the bottom of the conduction band of solid nitrogen. In our study, we demonstrate that electron time-of-flight spectroscopy is a versatile analytical tool for the study of electronic properties of samples such as those which suffer severely under beam induced damage and charging.