Inner-shell electron energy loss spectra of NO 2 at high resolution: Comparison with multichannel quantum defect calculations of dipole oscillator strengths and transition energies

Abstract

High-resolution electron energy loss spectra of the nitrogen ls and oxygen 1s inner shells of NO 2 have been obtained under conditions of high-impact energy (3 keV) and zero-degree scattering angle. The spectra have been placed on an absolute dipole oscillator strength scale. Multichannel quantum defect theory calculations of excitation energies and dipole oscillator strengths for N 1s and O 1s excitation are in quite good agreement with the measured spectra. The calculations are used to aid the assignment of features in the inner-shell spectra. A strong resonance present in both the N 1s and O 1s ionization continua is identified as excitation to the unbound 5b 2 (b 2 *) molecular orbital. A consideration of the present ISEELS work for NO 2 and earlier ISEELS measurements for other molecules shows that additional prominent structures observed in previously reported NO 2 inner-shell photoabsorption spectra obtained using synchrotron radiation are due to the presence of impurities.