Average Energies of Combining States and Purely Electronic Transition Frequencies in Vibronic Spectra

The electronic 0–0-transition frequency (ν₀₀) could be determined from not only the frequency dependence of spectral transition cross sections σ(ν) but also their relative changes with temperature by using the Franck–Condon principle, thermally equilibrated sublevel populations of the initial electronic state, and temperature independent elementary transition probabilities. In this instance, ν00 corresponded to a minimum in the dependence of the total average energies of the combining states on frequency or an extremum in the function ±hν/2 – ∂ ln [σ(ν)]/∂(1/kT), where the "+" sign refers to absorption; the "–" sign, to emission. The method was tested using gas-phase spectra of several compounds and showed better precision for determining ν₀₀ than the frequency dependence of the spectra, especially for overlapping spectra.