A soft X-ray emission study of the electronic structure of molecules

Sammanfattning: The electronic structure of molecules has been studied with soft x-ray emission (SXE)spectroscopy. The parity selection rule for electronic transitions using resonant excitation has been verified for homonuclear diatomic molecules by studying N2 and O2. The breaking of the selection rules due to vibronic coupling in polyatomic molecules has been shown to be both energy and bondlength dependent. By detuning the excitation energy below the first absorption resonance in CO2, the symmetry was regained. Comparing the SXE spectra from C2H2, C2H4 and C2H6 it was seen that the symmetry breaking is largest for C2H6 and this was explained to be due to the fact that C2H6 has the longest bond between the carbon atoms.Analyzing SXE spectra from CO it was seen that screening caused by a resonantly excited electron results in energy and intensity shifts compared to the nonresonant spectrum. When polarized light is used for excitation the angular distribution of the emission is nonisotropic. Lifetime vibrational interference affects the band profile, more the largerthe lifetime width and the closer the vibrational levels are. The spectra are reproducedby calculations involving these effects, strengthening the interpretation.The parity selection rule and the angular anisotropy of SXE have been used as a method of assigning unoccupied valence and Rydberg orbitals. Applied to CO2, previously debated Rydberg-associated structures just below the ionization threshold were assigned to be of mainly Σg character.Chemometric techniques, standard in the analysis of UV and IR absorption spectra, have been used for SXE spectra for the first time. Applied to a series of aliphatic molecules, they were shown to be successful and can be used for predicting SXE spectra and for noise removal.