Theoretical investigations of total and partial-channel photoabsorption cross sections in molecular formaldehyde are reported employing the Stieltjes-Tchebycheff (S-T) technique and separated-channel static-exchange (IVO) calculations. Vertical one-electron dipole spectra for the 2b 2(n), 1b1(π), 5a1(σ), 1b2, and 4a1 canonical molecular orbitals are obtained using Hartree-Fock frozen-core functions and large basis sets of compact and diffuse normalizable Gaussians to describe the photoexcited and ejected electrons. The calculated discrete excitation spectra provide reliable zeroth-order approximations to both valence and Rydberg transitions, and, in particular, the 2b2(n) →nsa1, npa1, npb2, and nda2 IVO spectra are in excellent accord with recent experimental assignments and available intensity measurements. Convergent (S-T) photoionization cross sections in the static-exchange (IVO) approximation are obtained for the 15 individual partial channels associated with ionization of the five occupied molecular orbitals considered. Resonance features in many of the individual-channel photoionization cross sections are attributed to contributions from valencelike a1σ*(CO), a 1σ*(CH), and b2σ*(CH) /πy*(CO) molecular orbitals that appear in the photoionization continua, rather than in the corresponding one-electron discrete spectral intervals. The vertical electronic cross sections for 1A1→1B1, 1B 2, and 1A1 excitations are in generally good accord with previously reported CI (S-T) predictions of continuum orbital assignments and intensities, although some discrepancies due to basis-set differences are present in the 1B1 and 1B 2 components, and larger discrepancies apparently due to channel coupling are present in the 1A2→1A 2 cross section. Partial-channel vertical electronic cross sections for the production of the five lowest parent-ion electronic states are found to be in general agreement with the results of very recent synchrotron-radiation photoelectron branching-ratio measurements in the 20 to 30 eV excitation energy interval. Most important in this connection is the tentative verification of the predicted orderings in intensities of the partial-channel cross sections, providing support for the presence of a strong ka1σ*(CO) resonance in the (5a1-1)2A1 channel. Finally, the total vertical electronic cross sections for absorption and ionization are in general accord with photoabsorption measurements, photoionization-mass-spectrometric studies, and the previously reported CI (S-T) calculations. Although further refined calculations including vibrational degrees of freedom and autoionization line shapes are required for a more precise quantitative comparison between theory and experiment, the present study should provide a reliable zeroth-order account of discrete and continuum electronic dipole excitations in molecular formaldehyde. © 1978 American Institute of Physics.
