INTERFERENCE · Interference effects in inner-shell excitation of diatomic molecules studied by polarization analysis of fluorescence from valence-ionized states

Interference effects in inner-shell excitation of diatomic molecules studied by polarization analysis of fluorescence from valence-ionized states

Quantum mechanical interference is an effect fundamental to a wide range of processes and applications in different branches of physics. For many processes in nature it causes dramatic counterintuitive observations, requiring detailed understanding of this effect. Interference effects in molecular spectroscopy are realisations of the macroscopic double slit experiment on the microscopic scale. Therefore, the current project aimed at detailed theoretical and experimental investigations of different types of interference effects in the decay of inner-shell excited molecules. Previous studies have demonstrated, that coherent population and decay of overlapping vibrational resonances, known as lifetime vibrational interference (LVI), is the dominant effect in solid-angle-averaged decay spectra. Another type of interference effect is the electronic state interference (ESI). It consists of coherent population of final decay states via different intermediate electronic resonances and via direct photo-ionisation. ESI in molecules has only been studied in very few publications. In the project we were able to illustrate that angularly resolved decay spectra of core-excited molecules provide conceptually new fundamental information on interference effects of both types. For this purpose, core-excitations of the CO and NO molecules have been investigated experimentally and theoretically. Experiments have been performed by means of photon-induced fluorescence spectroscopy at BESSY II in Berlin. The theory and computational approach for calculating angular distributions of the Auger electrons and fluorescence photons emitted by randomly oriented diatomic molecules excited by polarised radiation have been developed within the project. As result it was shown that angularly resolved decay spectra of core-excited CO are much more sensitive to LVI and ESI than the angularly unresolved ones. Although the weak direct photo-ionisation is practically invisible in the total photo-ionisation cross sections, it induces broad range dispersions of the angular distribution parameters for Auger electrons and for fluorescence photons. Even for well separated vibronic resonances exhibiting very weak LVI in the total cross sections, the LVI gives rise to clearly observable variations of angular distribution parameters across the resonances. We have proven for the first time, that the ESI between the amplitudes for excitation and decay of neighbouring electronic resonances of different symmetries in the core-excited NO molecule plays a crucial role in their angularly resolved decay spectra, although it is strictly symmetry-forbidden in the solid-angle-averaged spectra. The LVI of core-excited NO is very important for the interpretation of both magic-angle decay spectra and angular distribution parameters. Quantitative interpretation of the angularly resolved experiments on NO requires incorporation of the weak direct photo-ionisation in the computations. All scientific results have been published in refereed journals, presented at scientific conferences, and reported to a wide society of specialists. New international collaborations have been established during the duration of project. The researcher has co-supervised diploma and PhD students, and the performed work has considerably contributed to a completion their theses. Angularly resolved spectroscopy of molecules is a very powerful tool in experimental physics. The present project has advanced this branch of physics considerably, and will stimulate further activities in this field. The present results are important for a wide range of the spectroscopic society and are of great fundamental importance. They considerably extend our knowledge on fundamental interference effects, which are relevant for different branches of science, and open a new area of research: partial wave analysis of molecular photo-ionisation by fluorescence polarisation experiments.

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