The C 1s and O 1s excitation of formaldehyde (H2CO) has been studied within
an ab initio framework. The second-order algebraic-diagrammatic constructi
on [ADC(2)] polarization propagator method has been used to calculate energ
ies and oscillator strengths of the electronic transitions. For selected C
1s excited states also multireference configuration-interaction (MRCI) calc
ulations were performed. The vibrational excitations accompanying the elect
ronic transitions have been studied using a linear vibronic coupling model.
The theoretical C 1s and O 1s spectra are in excellent qualitative agreeme
nt with high-resolution K-shell photoabsorption measurements. The present r
esults support the previous assignments of the C 1s spectrum, while they re
vise the interpretation of the O 1s spectrum above 537 eV. In contrast to t
he C 1s case, the main photoabsorption intensity in the O 1s spectrum is du
e to nd rather than to np Rydberg excitations. For the two lowest singlet e
xcited 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 th
e (1)A(1) (C 1s-->3s) states interact via the nu (5) mode. The vibronic cou
pling leads to a complex spectral pattern in the low-energy part of the C 1
s excitation spectrum, allowing one to interpret the finer details of the e
xperiment.