CORE-LEVEL PHOTOIONIZATION-DECAY COINCIDENCE SPECTROSCOPY OF FREE MOLECULES

Coincidence measurements of the outgoing particles in resonant soft X- ray scattering on molecules are presented. In a simplifying picture th e first method measures the photoelectron and the soft X-ray photon, w hereas the second method measures the photoelectron and the Auger elec tron in coincidence. In the first experiment the zero-kinetic energy p hotoelectrons are detected in a time-of-flight electron spectrometer, and photons are collected in a large solid angle by a detector situate d close to the interaction region. The spectrum of N-2 shows an adiaba tic 1 s line, free from electron-electron post collision interaction e ffects, together with structures below the ionization limit, associate d with Rydberg electron shake-off during the radiative decay. Above th e 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 tentati vely ascribe to a resonance just below the (4) Sigma(-) threshold. The O-2 data also reveal an inconsistency in the literature values regard ing the calibration of the X-ray photoelectron and the soft X-ray abso rption energy scales. In the second experiment two cylindrical mirror analyzers are used to measure electron-electron coincidences. The inte rpretation of several features in the conventional Auger spectrum foll ows directly from the results, e.g, the decay of the pure is hole almo st is entirely contained in the main structures, the additional struct ure 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 1s(-1)pi(u)(-1)pi(g) shake-up state is well reproduced by calculations from the literature. The methods open up new perspect ives on core hole excitation-emission dynamics.