A theoretical study of the 1B1(O 1s -> pi*) and 1A1(O 1s -> 3s) excited states of formaldehyde

The O 1s excited states of formaldehyde have been investigated theoretically using the restricted open-shell Hartree-Fock method for electronic structure calculations and a linear vibronic coupling model for studying excited state nuclear dynamics. The results are compared with vibrationally resolved experimental data, published previously. In its 1B1(O 1s -> pi*) state the molecule is found to be unstable with respect to the out-of-plane bending mode v4(b1). The latter is a manifestation of vibronic coupling between the 1B1(O 1s -> pi*) state and the next excited state, 1A1(O 1s -> 3s). The dynamical calculations taking into account this interaction are in good agreement with experiment, whereas a simple Poisson intensity distribution reproduces only the main features of the spectral envelope of the 1B1(O 1s -> pi*) resonance. For the 1A1(O 1s -> 3s) state, the Poisson distribution gives poor agreement with experiment, whereas the vibronic coupling model yields a more satisfactory description.