Isotope effects and spectroscopic assignments in the non-dissociative photoionization spectrum of N₂.

Abstract

Photoionization efficiency spectra of (14)N2, (15)N(14)N, and (15)N2 from 15.5 to 18.9 eV were measured using synchrotron radiation at the Advanced Light Source at Lawrence Berkeley National Laboratory with a resolution of 6 meV, and significant changes in peak energies and intensities upon isotopic substitution were observed. Previously, we reported the isotope shifts and their applications to Titan's atmosphere. Here, we report more extensive experimental details and tabulate the isotope shifts of many transitions in the N2 spectrum, including those for (15)N(14)N, which have not been previously reported. The isotope shifts are used to address several long-standing ambiguities in spectral peak assignments just above the ionization threshold of N2. The feature at 15.677 eV (the so-called second "cathedral" peak) is of particular interest in this respect. The measured isotope shifts for this peak relative to (14)N2 are 0.015 ± 0.001 eV for (15)N2 and 0.008 ± 0.001 eV for (15)N(14)N, which match most closely with the isotope shifts predicted for transitions to the (A (2)Πu v' = 2)4sσ(g) (1)Π(u) state using Herzberg equations for the isotopic differences in harmonic oscillator energy levels plus the first anharmonic correction of 0.0143 eV for (15)N2 and 0.0071 eV for (15)N(14)N. More generally, the isotope shifts measured for both (15)N2 and (15)N(14)N relative to (14)N2 provide new benchmarks for theoretical calculations of interferences between direct and indirect autoionization states which can interact to produce intricate resonant structures in molecular photoionization spectra in regions near ionization thresholds.