Photoabsorption In Carbon Monoxide: Stieltjes-tchebycheff Calculations In The Separated-channel Static-exchange Approximation

Theoretical investigations of total and partial-channel photoabsorption cross sections in carbon monoxide are reported employing the Stieltjes-Tchebycheff (S-T) technique and separated-channel static-exchange calculations. Pseudospectra of discrete transition frequencies and oscillator strengths appropriate for individual excitations of each of the six occupied molecular orbitals are constructed using Hartree-Fock core functions and normalizable Gaussian orbitals to describe the photoexcited and ejected electrons. Use of relatively large basis sets of compact and diffuse functions insures the presence of appropriate discrete Rydberg states in the calculations and provides sufficiently dense pseudospectra for the determination of convergent photoionization cross sections from the S-T technique. The calculated discrete vertical electronic excitation spectra are in very good agreement with measured band positions and intensities, and the partial-channel photoionization cross sections are in correspondingly good accord with recent electron-electron (e,2e) coincidence, synchrotron-radiation, and line-source branching-ratio measurements. Predicted resonance features in the X, B, O2s -1, and carbon K-shell channels are in particularly good agreement with the positions and intensities in the measured cross sections. A modest discrepancy between experiment and theory in the A-channel cross section is tentatively attributed to channel-coupling mechanisms associated with opening of the 1π shell. The total vertical electronic S-T photoionization cross section for parent-ion production is in excellent agreement with recent electron-ion coincidence measurements. Comparisons are made between ionization processes in carbon monoxide and in the previously studied nitrogen molecule, and similarities and differences in the respective cross sections are clarified in terms of conventional molecular-orbital theory. © 1978 American Institute of Physics.6972992300

Photoexcitation And Ionization In Ozone: Stieltjes-tchebycheff Studies In The Separated-channel Static-exchange Approximation

Theoretical studies are reported of total and partial-channel photoexcitation/ionization cross sections in ozone employing Stieltjes-Tchebycheff (S-T) techniques and the separated-channel static-exchange approximation. As in previously reported investigations of excitation and ionization spectra in diatomic and polyatomic molecules employing this approach, vertical electronic dipole transition spectra for the twelve occupied canonical Hartree-Fock symmetry orbitals in ozone are constructed using large Gaussian basis sets, appropriate computational methods, and noncentral static-exchange potentials of correct molecular symmetry. Experimental rather than Koopmans ionization potentials are employed when available in construction of transition energies to avoid the incorrect ionic-state orderings predicted by Hartree-Fock theory, and to insure that the calculated series have the appropriate limits. The spectral characteristics of the resulting improved-virtual-orbital discrete excitation series and corresponding static-exchange photoionization continua are interpreted in terms of contributions from valencelike 7a 1(σ*), 2b1(πx*), and 5b2(σ*) virtual orbitals, and appropriate diffuse Rydberg functions. The 2b1(πx*) valence orbital apparently contributes primarily to discrete or autoionizing spectra, whereas the 7a1(σ*) and 5b2(σ*) orbitals generally appear in the various photoionization continua. Moreover, there is also evidence of strong 2p→kd atomiclike contributions to ka2 final-state channels in the photoionization continua. The calculated outer-valence-shell 6a1, 4b2, and 1a2 excitation series are compared with electron impact-excitation spectra in the 9 to 13 eV interval, and the corresponding partial-channel photoionization cross sections are contrasted and compared with the results of previously reported studies of photoionization in molecular oxygen. The intermediate- and inner-valence-shell excitation series and corresponding photoionization cross sections are in general accord with quantum-defect estimates and with the measured electron-impact spectra, which are generally unstructured above ∼22 eV. Of particular interest in the intermediate-valence-shell spectra is the appearance of a strong σ→σ* feature just above threshold in the 3b2→kb2 photoionization cross section, in qualitative agreement with previously reported studies of the closely related 3σg→kσu cross section in molecular oxygen. Finally, qualitative comparisons are made of the calculated K-edge excitation and ionization spectra in ozone with recently reported photoabsorption studies in molecular oxygen. © 1981 American Institute of Physics.7484581459

Photoexcitation And Ionization In Carbon Dioxide: Theoretical Studies In The Separated-channel Static-exchange Approximation

Theoretical studies are reported of total and partial-channel photoexcitation and ionization cross sections in carbon dioxide. As in previously reported studies of discrete and continuum dipole spectra in diatomic (N2, CO, O2, F2) and polyatomic (H2O, H2CO, O3) molecules in this series, separated-channel static-exchange calculations of vertical-electronic transition energies and oscillator strengths and Stieltjes-Tchebycheff moment methods are employed in the development. Detailed comparisons are made of the static-exchange excitation and ionization spectra with photoabsorption, electron-impact excitation, and quantum-defect estimates of discrete transition energies and intensities, and with partial-channel photoionization cross sections obtained from fluorescence measurements and from tunable-source and (e,2e) photoelectron spectroscopy. The spectral characteristics of the various discrete series and continua are interpreted in terms of contributions from compact 2u(*), 5g(*), and 4u(*) virtual valence orbitals, and from more diffuse discrete and continuum Rydberg orbitals. The 2u(*) orbital is found to contribute to discrete excitation series, whereas the 5g (*) and 4u (*) orbitals generally appear in the photoionization continua as resonancelike diabatic valence features. Good agreement obtains between the calculated discrete excitation series and the results of a recent analysis of the available spectroscopic data. The calculated outer-valence-shell (1g-1)Xg2, (1u-1)A u2, (3u-1)B u+2, and (4g-1)C g+2 partial-channel photoionization cross sections are in good accord with measured values, and clarify completely the origins of the various structures in the observed spectra. There is evidence, however, of coupling among scattering states associated with 1g-1 and 1u-1 ionic channels, giving rise to moderate disagreement with tunable-source photoelectron and fluorescence measurements over a portion of the spectrum. In the inner-valence-shell region, the calculated 2u-1 and 3g-1 cross sections are in qualitative accord with the observed many-electron spectral intensities, and provide a basis for quantitative interpretation when combined with appropriate intensity-borrowing calculations. The calculated carbon and oxygen K-edge cross sections are in good agreement with available cross sections obtained from electron-impact and photoabsorption measurements. It is of particular interest to find the oxygen K-edge (1g-1, 1u-1) cross section exhibits both the expected 5g(*) and 4u(*) resonancelike features. Finally, comparisons are made throughout of the discrete and continuum spectra in carbon dioxide with the results of previously reported studies in CO and O2, and the origins of the similarities and differences in the cross sections in these cases are clarified. © 1981 The American Physical Society.23121823

Optimized time-dependent perturbation theory for pulse-driven quantum dynamics in atomic or molecular systems

We present a time-dependent perturbative approach adapted to the treatment of intense pulsed interactions. We show there is a freedom in choosing secular terms and use it to optimize the accuracy of the approximation. We apply this formulation to a unitary superconvergent technique and improve the accuracy by several orders of magnitude with respect to the Magnus expansion.Comment: 4 pages, 2 figure

Photoexcitation And Ionization In Acetylene

Theoretical studies are reported of photoexcitation and ionization processes in the acetylene molecule. The calculations performed employ Franck-Condon and static-exchange approximations, and implicitly invoke vibrational and rotational closure. Detailed comparisons with recent spectral assignments, electron-impact excitation studies, and total and partial-channel cross-sectional measurements indicate that the theoretical results provide a useful first approximation to the complete dipole excitation/ionization spectrum of C2H2. The calculated discrete spectra are seen to include contributions from virtual 4σg(n/σ*) and 1πg(π*) orbitals, which appear an valence interlopers in the 1πu→ nsσg, 2σu → nsσg, 1σu → nsσg and 2σu → ndπg, 1σu → ndπg Rydberg series, respectively, in good accordance with spectral measurements. By contrast, the 3σu(σ*) virtual valence orbital gives rise to prominent diabatic resonance features above threshold in the 3σg → kσu, 2σg → kσu, and 1σg → kσu cross-sections, results that are in good quantitative accordance with corresponding experimental studies. Prominent features in the measured (1πu -1)X2Πu partial cross-section, which is in good accordance with the present values, are attributed to autoionization 2σu → 1πg(π*) and 4σg(n/σ*) intravalence transitions. © 1982.251127Price, (1935) Phys. Rev., 47, p. 444Wilkinson, (1958) J. Mol. Spectrosc., 2, p. 387Herzberg, (1963) Discuss. Faraday Soc., 35, p. 7Nakayama, Watanabe, (1964) J. Chem. Phys., 40, p. 558Greene, Barnard, Duncan, Rydberg Terms of Acetylene (1971) The Journal of Chemical Physics, 54, p. 71Jungen, (1973) Chem. Phys., 2, p. 367Colin, Herman, Kopp, Les Spectres des Molecules Simples au Laboratoire et en Astrophysic (1977) Proc. 11th Int. Astrophys. Colloq., Université de Liege, pp. 355-384Robin, (1974) Higher Excited States of Polyatomic Molecules, , Academic Press, New YorkÅsbrink, Fridh, Lindholm, (1978) Chem. Phys., 27, p. 159Turner, Baker, Baker, Brundle, (1970) Molecular Photoelectron Spectroscopy, , Wiley, New YorkCavell, Allison, (1978) J. Chem. Phys., 69, p. 159Dixon, McCarthy, Weigold, Williams, (1977) J. Electron Spectrosc. Relat. Phenom., 12, p. 239Bradshaw, Eberhardt, Levinson, Domcke, Cederbaum, (1980) Chem. Phys. Lett., 70, p. 36Davis, Shirley, The prediction of core-level binding-energy shifts from CNDO molecular orbitals (1974) Journal of Electron Spectroscopy and Related Phenomena, 3, p. 157Lassettre, Skerbele, Dillion, Ross, (1968) J. Chem. Phys., 48, p. 5066Hitchcock, Brion, (1977) J. Electron Spectrosc. Relat. Phenom., 10, p. 317R. Unwin, I. Khan, N. V. Richardson and A. M. 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Theoretical studies of partial-channel photoionization cross sections in diatomic and polyatomic molecules

An account is given of recent theoretical studies of partial-channel photoionization cross sections in diatomic and polyatomic molecules employing separated-channel static-exchange calculations and Stieltjes-Tchebycheff moment-theory techniques. The calculated cross sections, which refer to production of specific parent molecular-ionic states upon removal of electrons from individual orbitals, are compared with the results of photoabsorption, electron-impact-excitation, fluorescence-production, photoelectron spectroscopy, and dipole (e, 2e) measurements. Various structures in the calculated and measured partial-channel cross sections as functions of incident photon energy are identified as having either atomiclike or molecularlike origins. Specifically, o-orbital cross sections in light diatomic and polyatomic molecules are found to be generally dominated by strong α — α* photoionization resonances of molecularlike origin, whereas π-orbital cross sections in such molecules exhibit distinctly 2p → kd atomiclike features. These aspects of molecular photoionization are illustrated by detailed theoretical studies of partial-channel cross sections in CO, CO2, and H2CO, the results of which are in generally good accord with corresponding measured values