3πg→7σu (Allowed Valence) and 4πu→7σu (Forbidden Valence) Transitions
The fluorescence anisotropy spectrum reveals a decrease in R value in the wavelength region 125 − 140 nm, while the quantum yield for excimer formation keeps rising except for the Rydberg bands as the photon energy is raised. One may have to call for some other excited states than the ones called for to assign the absorption spectrum, such as the allowed intravalence (10σg→7σu) and Rydberg transitions. Therefore to explain the lowering of the R value observed, allowed valence 3πg→7σu and the forbidden valence 4πu→7σu transitions are called for though they do not show conspicuous bands in the absorption spectrum. The 3πg→7σu transition is dipole- allowed and has a parallel transition moment along the molecular axis. The 4πu→7σu transition can be allowed if combined with anti-symmetric stretching or bending vibrations. The transition moments induced by the anti-symmetric stretching and bending vibrations are perpendicular and parallel to the molecular axis, respectively. The decrease in the R value at around 130 nm where no conspicuous absorption band is observed, may be explained by invoking these intravalence transitions.
The strong broad 10σg→7σu absorption band has a maximum at 158 nm (7.84 eV). If it is assumed that a shift in the excitation energy for any other bands is estimated using the difference in the ionization energies determined by the photoelectron spectrum of XeF2, then the 3πg→7σu excitation would appear at 8.54 eV (145.2 nm). In the similar manner the excitation energies for 4πu 3/2→7σu and 4πu 1/2→7σu would appear at 9.79 eV (126.7 nm) and 10.19 eV (121.7 nm), respectively. Considering the facts that the potential energy surface for the antibonding state 7σu is of repulsive nature, the 10σg→7σu absorption band is very broad, and the excitation energy may shift with the change in the structure, it may be likely that the bands giving rise to these transitions shift to and appear in the region from 125 to 140 nm.