Higher Singlet States Research Articles

Spin-Orbit Coupling in the 3A2—1A1 Transition of Formaldehyde

An approximate quantitative calculation is given for the effect of spin-orbit coupling in perturbing the 3A2 state of H2CO. The spin-orbit coupling mixes the 3A2 state with some higher singlet state, and allows the 3A2←1A1 transition to have nonvanishing probability. The calculations show that the 3A2←1A1, np→π2 transition of H2CO should be perturbed principally by the 1A1←1A1, π1→π2 transition. The bands of the 3A2←1A1 transition should therefore have the structure of a transition which is allowed by orbital symmetry with an oscillating electronic transition moment along the C–O axis (parallel bands). The oscillator strength, f, is calculated to be f3A2→1A1≅1.5×10−7. The natural radiative lifetime τ for the 3A2→1A1 phosphorescence transition is calculated to be τ≅1×10−2sec. The calculated value for τ is in agreement with the experimental values for the related molecules (CH3)2CO, (C6H5)2CO, and C6H5COCH3, insofar as a comparison can be made. It is further shown that the spin-orbit perturbation in the π→π* transitions of aromatic hydrocarbons should be about one thousand times less effective than the spin-orbit perturbation in the n→π* transitions of carbonyl compounds. This is in agreement with the few pieces of experimental data which have been reported thus far.

EMISSION BAND SPECTRA OF NITROGEN: A STUDY OF SOME SINGLET SYSTEMS

Ten bands of Gaydon's and Herman's singlet systems and eight new bands have been photographed under high resolution and analyzed in detail. Two of the new transitions were shown to be [Formula: see text], the upper state being in one case identical with Watson's and Koontz's state g, and one new transition to be 1Δg—ω1Δu in type. It is proposed that the new state 1Δg has the same electron configuration as the [Formula: see text] state. Two bands in the red and one band in the ultraviolet could not be assigned with certainty. Local perturbations in the [Formula: see text] state were observed and shown to be caused by the ν = 1 level of the [Formula: see text] state. Observed pecularities in the rotational structure of most of the upper states are proposed to be indicative of a transition from case b′ to d′ coupling. In some cases pronounced decreases in branch intensities were observed, indicating predissociations probably caused by "forbidden" intercombination processes. Identification of the electronic structure of the higher singlet states in terms of Rydberg orbitals is discussed. Rotational and vibrational constants and excitation energies are presented.