In this Thesis studies of highly excited states in gas-phase atoms and molecules (He, Ne, N2, O2, N2O and CO2) using a variety of synchrotron-radiation based techniques are presented. The three techniques used most frequently are X-ray-emission-threshold-electron coincidence (XETECO), X-ray emission spectroscopy (XES) and X-ray photoelectron spectroscopy (XPS) and they are all given a brief introduction.
The fluorescence yield (FY) from doubly excited states in helium near the N=2 threshold(s) has been investigated in weak static external magnetic and electric fields, but also in a field free environment. The FY spectra in weak static magnetic fields show the importance of including the diamagnetic interaction in the theoretical models. The presence of weak static electric fields shows that even weak fields (as low as 44 V/cm) has a great impact on the observed FY spectra. Resonant XES spectra from some of the first doubly excited states in helium has been recorded in a field free environment, and compared to theory.
The XETECO technique is presented and the first XETECO results from Ne, N2, O2, CO2 and N2O are shown, together with interpretations of possible threshold dynamics. I show that XETECO can be interpreted as threshold photoelectron spectra free from post collision interaction, and can hence be compared to above threshold XPS measurements. The observed below-threshold structures in the XETECO spectra are discussed and given a tentative explanation. The results from the analysis of the N2O XETECO spectrum lead to further investigations using XPS. Results showing the vibrational parameters and vibrationally resolved cross-sections and asymmetry parameters for N2O are presented together with theoretical predictions.