Theoretical study of K-shell excitations in formaldehyde -: art. no. 022504

The C 1s and O 1s excitation of formaldehyde (H2CO) has been studied within an ab initio framework. The second-order algebraic-diagrammatic construction [ADC(2)] polarization propagator method has been used to calculate energies and oscillator strengths of the electronic transitions. For selected C 1s excited states also multireference configuration-interaction (MRCI) calculations were performed. The vibrational excitations accompanying the electronic transitions have been studied using a linear vibronic coupling model. The theoretical C 1s and O 1s spectra are in excellent qualitative agreement with high-resolution K-shell photoabsorption measurements. The present results support the previous assignments of the C 1s spectrum, while they revise the interpretation of the O 1s spectrum above 537 eV. In contrast to the C 1s case, the main photoabsorption intensity in the O 1s spectrum is due to nd rather than to np Rydberg excitations. For the two lowest singlet excited states, that is, the B-1(1)(C 1s-->pi*) single excitation and the B-1(1)(C 1s,n-->pi*(2)) double excitation, we find vibronic interaction with the (1)A(1)(C 1s-->3s) and (1)A(2)(C 1s-->3d) Rydberg states via the nu (4) out-of-plane bending mode. In addition, the B-1(2)(C 1s,n-->pi*(2)) and the (1)A(1) (C 1s-->3s) states interact via the nu (5) mode. The vibronic coupling leads to a complex spectral pattern in the low-energy part of the C 1s excitation spectrum, allowing one to interpret the finer details of the experiment.