Symmetry-resolved soft x-ray absorption spectroscopy: its application to simple molecules

This paper reviews our recent progress on the angle-resolved photoion-yield spectroscopic (ARPIS) technique applied to the K-shell excitations of the simple linear molecules, N2, CO, C2H2, O2, CO2, N2O, OCS and CS2. This spectroscopic technique enables us to distinguish symmetries of electronic states; e.g., the ΔΛ = 0 parallel and ΔΛ = ±1 perpendicular transitions for the diatomic molecules can be distinguishable. We call this technique symmetry-resolved inner-shell excitation spectroscopy. The complete symmetry resolution is rationalized by the axial-recoil fragmentation. Moreover, the angular distribution of fragment ions is directly related to the molecular orientation upon the photoabsorption and the vibrations; therefore, the ARPIS measurement can be used to reveal complicated vibronic couplings in polyatomic molecules in terms of anisotropy in fragmentation. We have studied Rydberg-valence mixings, vibronically induced 1s → Rydberg excited states, and strong bending-mode couplings in Renner–Teller split 1s →π* states for the linear molecules through the ARPIS spectra.