Theoretical studies of photoexcitation and ionization in H2O

Abstract

Theoretical studies are reported of the complete dipole excitation and ionization spectrum in H 2 0 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 synchrotronradiation studies of partial-channel photoionization cross sections. The various calculated excitation series in the outer-valence (lb 1 - B 2 partial-channel cross sections are found to be largely atomic-like and dominated by 2p-kd components, although the 2b 2 (u*) orbital gives rise to resonance-like contributions just above threshold in the 3a 1 --+kb 2 and lb 2 -+kb 2 channels. It is suggested that the latter transition couples with the underlying lb 1 -+kb 1 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 innervalence (2al 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 Ia 1 oxygen K-edge electron energy-loss profile in H 2 0 are seen to be clarified fully by the present calculations.

Excited states of the water molecule: Analysis of the valence and Rydberg character

“…The present study attempts to throw more light into this aspect of the excited states of water which we consider has been somewhat overlooked. 11,14,[21][22][23] Despite earlier conclusions which emphasized the Rydberg character of the band, [17][18][19] the experimental evidence does suggest some valence character for the lowest-energy band of the gas-phase electronic spectrum of water. 11,14,23 The band is broad ͑6.8-8.2 eV͒, with poorly defined vibrational progressions, and has its maximum absorption around 7.4 eV.…”

Cross sections and oscillator strengths for electron-impact excitation of the ÃB11 electronic state of water

“…[16][17][18][19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34] In particular, the peak positions, profile shapes, and widths used in our deconvolution were gleaned and fixed as much as possible from the earlier work. [16][17][18][19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34] An example of our spectral deconvolution is also given in Fig. 1͑a͒.…”

“…20,[35][36][37][38][39] The present OOS value of f = 0.0459± 0.0069 is in excellent agreement with the photoabsorption results from Lee and Suto 42 and Yoshino et al, 43,44 and the f sum-rule adjusted value recommended by Berkowitz,58 and in good accord with the highresolution dipole ͑e , e͒ value from Chan et al 32 We therefore believe that the OOS for the à 1 B 1 excitation of H 2 O is well established. This is important as the OOS represents a good test 62 for any structure calculation seeking to accurately represent the wave functions for this excitation.…”

“…[16][17][18][19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34] These studies are not simply noted here for completeness, as we shall shortly see they were vital in enabling us to deconvolve the respective electronic-state contributions to the measured energy-loss spectra. A summary of the major results from Refs.…”

“…6 In both the molecular spectroscopy case and for the scattering calculations, an important first test to verify the quality of target wave functions is to compare the length and velocity forms of calculated oscillator strengths to corresponding accurate experimental values. For the optical oscillator strength ͑OOS͒ for excitation of the à 1 B 1 electronic state from the X 1 A 1 ground state, there are theoretical 20,[35][36][37][38][39] and experimental 32,[40][41][42][43][44] data available in the literature. In general we find differences in the OOS values within the theories and within the experiments and between the existing theories and experiments.…”