ZEKE spectroscopy with coherent vacuum ultraviolet radiation: The X 2∑g+ and A 2∏u states of N2+ in the 15.5 eV to 17.7 eV photon energy range

Abstract

The zero kinetic energy (ZEKE) photoelectron spectrum of nitrogen has been recorded using coherent vacuum ultraviolet light for single-photon excitation to the various ionization thresholds. Ionization thresholds from the lowest (15.58 eV) to 17.7 eV were studied. In this energy range, both Franck–Condon allowed (v+=0 and 1) and Franck–Condon forbidden (v+=4, 5, 6, and 8) levels of the N2+ X 2∑g+ ground electronic state were studied. The rotational intensities of these bands were studied, and a comparison made with previous ab initio work on the photoionization of N2. The role of complex resonances in the alteration of rotational linestrengths is discussed with reference to a high resolution photoionization spectrum recorded in the region of the v+=1 threshold. It was also possible to compare the relative intensities of all of these vibrational bands, to investigate the mechanism for observation of Franck–Condon forbidden vibrational bands in ZEKE spectroscopy. These results are discussed in terms of resonant autoionization, which is commonly observed in threshold photoelectron spectroscopy. The v+=0, 1, 2, and 4 levels of the A 2∏u excited state of N2+ were also studied, and the rotational structure was compared with ab initio and experimental results for the same state in the isoelectronic system, CO+. The band intensities were also measured, and compared with the Franck–Condon factors for excitation to the A state vibrational levels for the v=0 state of N2. A comparison with the v+=0 band of the X state is used to obtain an estimate of the relative signal yield for these excited state thresholds. For most of the bands observed in the present study, there are previous lower resolution threshold photoelectron spectra recorded using synchrotron radiation, and a comparison is made with these previous results.