5π_{u 3/2}→5d_σR Bands and Newly Observed 5π_{u 3/2}→5dR Bands

The progressions of the bending (ν₂) vibration are not observed in the 5π_{u 3/2}→5d_σR band, from which it follows that the molecular structure is linear and that the force field determining the fundamental frequency of ν₂ vibration is very comparable to that of the ground state.

In the fluorescence excitation spectra in the wavelength regions of λ_ex = 124.75 − 127.25 nm, one can find weak progressions of the bending ν₂ vibration. The 5d Rydberg orbitals are classified as σ_g, π_g and δ_g and the quantum defect for each orbital can be different. At present these weak bands are assigned to the 5d_π or 5d_δ Rydberg bands with different symmetries from that for the strong Rydberg bands observed in the absorption spectrum.

The R value decreases for these bands, but the minima positions of R do not agree with the positions of the band maxima. From the results of fluorescence anisotropies this transition is concluded to be perpendicular to the molecular axis. Therefore this Rydberg orbital has either σ_g or δ_g symmetry. Since the weak Rydberg bands have been observed at the excitation energies lower than the bands assigned as 5π_{u 3/2}→5dR, we would like to assign these bands to 5π_{u 3/2}→5d_σR transitions.