Photoabsorption in formaldehyde: Intensities and assignments in the discrete and continuous spectral intervals

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 (!YO) calculations. Vertical one-electron dipole spectra for the 2b 2(n), lb 1(7T), 5a1(a), I b2, and 4a 1 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)-msa1, 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 frorn valencelike a 1a*(CO), a 1a*(CH), and b 2a*(CH)/7Ty *(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 1A 1 -+ B1 , B2 , and A 1 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 B 1 and B2 components, and larger discrepancies apparently due to channel coupling are present in the A1-+ A1 cross section. Partialchannel 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 ka 1<r*(CO) resonance in the (5a 11 ) A1 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.