Photoabsorption in formaldehyde

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Abstract
Theoretical investigations employing configuration‐interaction calculations and recently devised moment‐theory techniques are reported of the vertical electronic dipole excitation and ionization spectra in molecular formaldehyde. A double‐zeta basis of contracted Gaussian‐lobe functions, supplemented with appropriate polarization, diffuse, and bond functions, is employed in the construction of Fock spectra in C2v symmetry for X 1A1 and (n→π*)3A2 states near the ground‐state equilibrium geometry. The 50 occupied and virtual Fock orbitals obtained in each case are used in configuration‐interaction calculations of 200‐term eigenvectors of appropriate symmetry for each of the principle‐axis polarization directions, and for the lowest‐lying molecular ionic states. The ionization energies, discrete vertical transition frequencies and oscillator strengths, and associated approximate configurational assignments obtained are in general accord with experimental determinations and with the results of previously reported more elaborate state‐specific theoretical calculations. Spectral moments calculated from 200‐term pseudospectra of transition frequencies and oscillator strengths provide Stieltjes and Tchebycheff vertical electronic photoionization profiles in good agreement with appropriately averaged photoionization–mass‐spectrometric measurements of the cross section for parent H2CO+ ion production. Discrepancies between the total and partial photoionization cross‐section measurements in formaldehyde are indicated. Approximate configurational assignments above the first ionization threshold are made on basis of the calculated eigenvectors, and comparisons with experimental assignments are provided. It is suggested that vibronically preionized π–π* excitation gives rise to a feature at ∼13 eV in the measured photoionization spectrum for H2CO+ production not accounted for by the calculated vertical photoionization cross section.