Resonant and Non-Resonant Electron Spectroscopy of Free Molecules and Free Clusters

Abstract: Resonant electron spectroscopy has been performed on the diatomic molecules CO, N2 and HCl. Core-excitations were made to bound and dissociative intermediate electronic states. Fundamental interference phenomena are observed and discussed in the framework of ”X-ray Raman Scattering Theory”. For C1s→π' core-excited CO higher vibrational levels, which are difficult to discerne in a total yield photoabsorption spectrum, are revealed. For N1s→π' core-excited N2 the interaction of the B2Σ+u final state with the neighbouring C2Σ+u state leads to breakdown of the commonly used ”participator” and ”spectator” classification. For negative photon frequency detuning with respect to the same resonance, an interference quenching of a certain vibrational line in the X2Σ+g final state of N+2 has been observed and analysed, showing a novel way to determine the equilibrium bond distance of the core-excited state. The duration time concept for the scattering process is refined in terms of partial and mean duration time, explaining detuning asymmetries for the X2Σ+g, A2Πu and B2Σ+u final states of N+2 . The role of monochromator stray-light on the formation of electron spectra has been investigated in the vincinity of the N1s→π' resonance of N2, a method to drastically reduce undesired ”Stokes spectral features” is demonstrated. The decay of a triply-excited intermediate state in N2, located above the N1s ionisation threshold, has been studied, revealing a ”double spectator” type mechanism. In HCl the decay to the 4σ-1 inner valence region upon excitation to the ultrafast dissociative Cl2p-16σ' intermediate state exhibits a novel type of interference involving ”atomic” and ”molecular” decay channels, giving rise to a ”continuum-continuum interference hole” in the electron spectrum. A selective population of spin-orbit split final state vibrational components has been observed in the decay to the X2Π final state in HCl+ upon photon energy tuning to either of the spin-orbit split components of the Cl2p-16σ' core-excited state. Direct photoelectron spectroscopy on free, neutral Ar, Kr and Xe clusters has been performed and changes in the electronic structure upon cluster formation has been investigated. Band structure formation for some of the inner valence levels is encountered, making a description of these orbitals in the sense of localised or delocalised difficult. The first resonant Auger electron spectra of free rare gas clusters are presented and discussed. A ”spectroscopic-loop” method to decompose complex cluster photoabsorption spectra is experimentally demonstrated.