The valence shell electronic states of trimethylene oxide studied by photoabsorption and ab initio multireference configuration interaction calculations

The absolute photoabsorption cross section of trimethylene oxide has been measured between threshold and 30 eV using monochromated synchrotron radiation. Below the ionization threshold the spectrum exhibits numerous sharp peaks associated with Rydberg states belonging to series converging onto the [X~²B₁] limit. At excitation energies above the ionization threshold at 9.680 eV intravalence transitions play a dominant role, resulting in the appearance of prominent broad bands. Ab initio multireference configuration interaction calculations have been performed to obtain excitation energies for valence electron transitions into Rydberg or virtual valence orbitals. These theoretical predictions have enabled assignments to be proposed for most of the structure due to Rydberg series converging onto the [X~²B₁] limit. The calculations show that configuration interaction is important in the description of some of the excited states. The photoabsorption spectrum of trimethylene oxide has been compared with those of ethylene oxide, ethylene sulphide and trimethylene sulphide to identify and characterize the principal Rydberg excitations that these related three- or four-membered ring molecules have in common.