A General Approach for Calculating Strongly Anharmonic Vibronic Spectra with a High Density of States: The X̃ 2 B 1 ← X̃ 1 A 1 Photoelectron Spectrum of Difluoromethane.

Due to a low-lying fragmentation channel, the X̃2 B1 ← X̃1 A1 photoelectron spectrum of difluoromethane is dominated by strong anharmonicity effects. We have used a time-independent eigenstate-free Raman wave function approach (RWF) to calculate the entire spectrum. Vibronic transitions with the most significant Franck-Condon factors were determined by employing our recently developed residual-based algorithm for the calculation of eigenpairs (RACE). An analysis of the factors controlling the accuracy of the predicted band shape is provided. The calculated spectrum is in very close agreement with experimental results.