Comparative study of K-shell exciton series in condensed neon and nitrogen by 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 1s(-1)5p transition, are reported for neon multilayers. For condensed nitrogen, we observe three different types of resonant features in the near-g-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 Is 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.