Core level photoionization—decay coincidence spectroscopy of free molecules

Abstract

Coincidence measurements of the outgoing particles in resonant soft X-ray scattering on molecules are presented. In a simplifying picture the first method measures the photoelectron and the soft X-ray photon, whereas the second method measures the photoelectron and the Auger electron in coincidence. In the first experiment the zero-kinetic energy photoelectrons are detected in a time-of-flight electron spectrometer, and photons are collected in a large solid angle by a detector situated close to the interaction region. The spectrum of N 2 shows an adiabatic 1 s line, free from electron-electron post collision interaction effects, together with structures below the ionization limit, associated with Rydberg electron shake-off during the radiative decay. Above the ionization limit the latter process is no longer possible, resulting in a drop at threshold for the total intensity. For the O 2 molecule we find an anomalous quartet/doublet intensity ratio, which we tentatively ascribe to a resonance just below the 4 Σ − threshold. The O 2 data also reveal an inconsistency in the literature values regarding the calibration of the X-ray photoelectron and the soft X-ray absorption energy scales. In the second experiment two cylindrical mirror analyzers are used to measure electron-electron coincidences. The interpretation of several features in the conventional Auger spectrum follows directly from the results, e.g. the decay of the pure 1 s hole almost is entirely contained in the main structures, the additional structure and the spread-out low-energy background being associated with the decay of multiply excited states. Fine structure in the decay of the singlet coupled 1 s −1 π u −1 π g shake-up state is well reproduced by calculations from the literature. The methods open up new perspectives on core hole excitation-emission dynamics.