Ab initio calculation of the photoelectron spectra of the hydroxycarbene diradicals.

Photoelectron spectra of the cis and trans isomers of HCOH were computed using vibrational wave functions calculated by diagonalizing the Watson Hamiltonian, including up to four mode couplings. The full-dimensional CCSD(T)/cc-pVTZ potential energy surfaces were employed in the calculation. Photoionization induces significant changes in equilibrium structures, which results in long progressions in the nu(5), nu(4), and nu(3) modes. The two isomers show progressions in different modes, which leads to qualitatively distinguishable spectra. The spectra were also calculated in the double harmonic parallel-mode (i.e, neglecting Duschinsky rotation) approximation. Calculating displacements along the normal coordinates of the cation state was found to give a better approximation to the vibrational configuration interaction spectrum; this is due to the effects of Duschinsky rotations on the vibrational wave functions.