Theoretical studies of photoexcitation and ionization in H2O

Abstract

Theoretical studies are reported of the complete dipole excitation and ionization spectrum in H₂O employing Franck–Condon and static‐exchange approximations. Large Cartesian Gaussian basis sets are used to represent the required discrete and continuum electronic eigenfunctions at the ground‐state equilibrium geometry, and previously devised moment‐theory techniques are employed in constructing the continuum oscillator‐strength densities from the calculated spectra. Detailed comparisons are made of the calculated excitation and ionization profiles with recent experimental photoabsorption studies and corresponding spectral assignments, electron impact–excitation cross sections, and dipole (e, 2e)/(e, e+ion) and synchrotron‐radiation studies of partial‐channel photoionization cross sections. The various calculated excitation series in the outer‐valence (1b⁻¹₁, 3a⁻¹₁, 1b⁻¹₂) region are found to include contributions from valence‐like 2b₂ (σ*) and 4a₁ (γ*) virtual orbitals, as well as appropriate nsa₁, npa₁, nda₁, npb₁, npb₂, ndb₁, ndb₂, and nda₂ Rydberg states. Transition energies and intensities in the ∼7 to 19 eV interval obtained from the present studies are seen to be in excellent agreement with the measured photoabsorption cross section, and to provide a basis for detailed spectral assignments. The calculated (1b⁻¹₁) X ²B₁, (3a₁⁻¹)²A₁, and (1b₂⁻¹)²B₂ partial‐channel cross sections are found to be largely atomic‐like and dominated by 2p→kd components, although the 2b₂(σ*) orbital gives rise to resonance‐like contributions just above threshold in the 3a₁→kb₂ and 1b₂ →kb₂ channels. It is suggested that the latter transition couples with the underlying 1b₁→kb₁ channel, accounting for a prominent feature in the recent high‐resolution synchrotron‐radiation measurements. When this feature is taken into account, the calculations of the three outer‐valence channels are in excellent accord with recent synchrotron‐radiation and dipole (e, 2e) photoionization cross‐sectional measurements. The calculated inner‐valence (2a₁⁻¹) cross section is also in excellent agreement with corresponding measured values, although proper account must be taken of the appropriate final‐state configuration‐mixing effects that give rise to a modest failure of the Koopmans approximation, and to the observed broad PES band, in this case. Finally, the origins of the various spectral features present in the measured 1a₁ oxygen K‐edge electron energy‐loss profile in H₂O are seen to be clarified fully by the present calculations.