Partial-channel Photoionization Cross Sections Research Articles

Photoionization in molecular oxygen

Theoretical studies are reported of partial-channel photoionization cross sections in molecular oxygen employing the static exchange approximation and Stieltjes-Tchebycheff moment theory techniques. It is found to be particularly important in this prototypical open-shell case to specify the correct ionic parentages of the various multiplet photoionization states that arise in order to account for all the experimental features in the relatively complex photoelectron spectra, and to obtain corresponding partial-channel photoionization cross sections in good accord with measured values. Resonancelike structures in certain of the cross sections are attributed to contributions from diabatic valencelike states that appear in the appropriate photoionization continua, rather than in the corresponding discrete spectral intervals. These and other aspects of the photoionization spectrum in molecular oxygen are contrasted and compared with the results of previously reported studies in molecular nitrogen and carbon monoxide.

Theoretical studies of photoionization in diatomic and polyatomic molecules

A review is given of recent theoretical studies of partial-channel photoionization cross sections in diatomic and polyatomic molecules. Results obtained from separated-channel static-exchange calculations and Stieltjes-Tchebycheff moment-theory techniques are compared with recent photoabsorption, electron-impact excitation, fluorescence production, photoelectron spectroscopy, and dipole (e, 2e) measurements. Various structures in the calculated and measured cross sections as functions of incident photon energy are attributed to final-state wavefunctions of either atomiclike or molecularlike composition. Specifically, σ-orbital cross sections in light diatomic and polyatomic molecules are found to be generally dominated by strong σ → σ^* photoionization resonances of molecularlike origin, whereas π-orbital cross sections in such molecules exhibit strong distinctly 2p → kd atomiclike features. These aspects of molecular photoionization are illustrated with the results of detailed theoretical and experimental studies of partial-channel cross sections in CO, CO_2, and H_2CO.

Theoretical study of ultraviolet photoelectron spectra of carbonyl systems: CO,Cr(CO)6, and CO/Cu(100)

We present calculations of partial-channel photoionization cross sections and photoelectron spectra in the ultraviolet energy region of the carbonyl systems CO, Cr(CO)${}_{6}$, and CO/Cu${}_{14},$ the latter system modeling CO/Cu(100). The dependence on photon energy, including He I and He II spectra, and on the particular ionization channel has been investigated for the three systems by means of the independent-channel static exchange method. It is found that the method is able to reproduce all essential trends and features of the spectra as well as the energy dependence of the cross sections. The COCu${}_{14}$ model can be used to reproduce the organization of states in the spectra of CO/Cu(100) and for the photon energy dependence of the individual channel cross sections, while the relative cross sections between the different parts of the spectra can only be qualitatively assessed due to the limited representation of the metal density of states in the cluster model.

Partial channel photoionization cross sections of polyenes

We present calculations of partial channel photoionization cross sections and photoelectron spectra of polyenes in the ultraviolet wavelength region. The dependences on molecular size, up to ${\mathrm{C}}_{16}$${\mathrm{H}}_{18}$, on photon energy, including He I and He II spectra, and on the particular ionization channel have been investigated using the independent-channel static exchange method. The channel dependence of the He I cross sections, which is quite strong for ethylene, diminishes, as the molecular length is increased, and becomes weak in the He II region. The energy dependence, which is strong in the He I region for all molecules and/or channels, is also insignificant for He II excitation, with the cross sections of all channels being close to each other. The cross sections are remarkably constant for the different \ensuremath{\pi} levels at any given photon energy. Spectral analyses for the first three smallest molecules---ethylene, butadiene, and hexatriene---show that the independent-channel approach is able to reproduce experimental data quite well.

Calculations of giant resonances and cross section profiles of valence ionizations of cubane by the LCAO Density Functional Stieltjes Imaging approach

Valence partial channel photoionization cross sections for each final symmetry of cubane have been evaluated at the local density level. The muffin-tin potential approximation is avoided by the use of large basis set LCAO calculations and the Stieltjes Imaging approach. The “giant resonances” which are expected from symmetry considerations have only actually been determined for outer valence ionizations, while they are not present in the inner valence. Minimal basis set calculations have been successfully used throughout the discussion in order to assess the position and the intensity of the transitions to virtual valence orbitals. Eventually, we analyze the total cross sections for each final state and their ratios, which might be compared with further experimental data that are not yet available. The previous partial final symmetry analysis allows the assignment of the features which could be experimentally observable,

Absolute Cross Sections for Molecular Photoabsorption, Partial Photoionization, and Ionic Photofragmentation Processes

A compilation is provided of absolute total photoabsorption and partial-channel photoionization cross sections for the valence shells of selected molecules, including diatomics (H2, N2, O2, CO, NO) and triatomics (CO2, N2O), simple hydrides (H2O, NH3, CH4), hydrogen halides (HF, HCl, HBr, HI), sulfur compounds (H2S, CS2, OCS, SO2, SF6), and chlorine compounds (Cl2, CCl4). The partial-channel cross sections presented refer to production of the individual electronic states of molecular ions and also to production of parent and specific fragment ions, as functions of incident photon energy, typically from ∼20 to 100 eV. Total photoabsorption cross sections above the first ionization threshold are reported from conventional optical measurements obtained using line and continuum sources and from ‘‘equivalent-photon’’ dipole (e,e) electron scattering experiments. Partial photoionization cross sections for production of electronic states of molecular ions are obtained from photoelectron spectroscopy and from dipole (e,2e) coincidence measurements. Photoionization mass spectrometry and dipole (e, e+ion) coincidence studies provide measurements of partial cross sections for ionic photofragmentation (i.e., dissociative photoionization). Photoelectron anisotropy factors, which together with electronic partial cross sections provide cross sections differential in photon energy and in ejection angle, are also reported. There is generally good agreement between cross sections measured by the physically distinct optical and dipole electron-impact methods. The cross sections and anisotropy factors also compare favorably with selected ab initio and model potential (X-alpha) calculations which provide a basis for interpretation of the measurements.

Theoretical studies of photoexcitation and ionization in hydrogen sulfide

Computational studies are reported of photoexcitation and partial-channel photoionization processes in the valence shell of H 2S employing single- and coupled-channel, vertical-electronic, static-exchange approximations. Detailed assignments are provided on the basis of the calculations of all prominent features observed below the first ionization threshold in recent photoabsorption studies. Comparisons are also made with experimental partial-channel photoionization cross sections obtained from electron-impact measurements in the dipole limit, and with branching ratios obtained from synchrotron-radiation photoelectron spectroscopy studies. The calculated discrete valence-shell excitation series are in generally good agreement with recent high-resolution VUV spectral measurements, although some of the present assignments are not in accord with those suggested on basis of the experimental data alone, nor with those based on earlier computational studies. The origins of these discrepancies are indicated and clarified. Configuration mixing is seen to influence the absorption intensities in the H 2S valence-shell spectrum, particularly in 1A 1 → 1A 1 and 1A 1 → 1B 2 symmetries due to the presence of intravelence excitations in these cases. The calculated valence-shell partial-channel photoionization cross sections are in good accord with the recent electron-impact study, and with branching ratios obtained from synchrotron-radiation measurements. Agreement with previous computational studies is not entirely satisfactory, however. The discrepancies are attributed to use of Coulomb and local-potential approximations in the earlier calculations. As in previous reports in this series on molecular photoabsorption and photoionization in polyatomic molecules, the general natures of the excitation and ionization spectra in H 2S are clarified on basis of the presence of valence-like (3b 2(σ *), 6a 1(γ *)) and diffuse orbital contributions to both discrete and continuum states.

Photoionization of carbonyl sulfide: threshold resonance features in outer-valence-shell partial cross sections

Computational studies are reported in the static-exchange approximation of the outer-valence-shell (3π −1, 9σ −1, 2π −1, 8σ -1) partial-channel photoionization cross sections of carbonyl sulfide. The calculated values are found to be in general agreement with corresponding dipole (e,2e) and synchrotron-radiation PES studies, although quantitative discrepancies between theory and experiment remain in specific channels and spectral intervals. Projection procedures and Stieltjes-orbital diagnostics aid in the identification of resonance-like 4π(π *) and 10σ(σ *) features present in near threshold regions, and of a higher-lying 11σ(σ *) feature in the (8σ −1) channel, clarifying the origins of the spectral shapes of the measured cross sections.

Theoretical studies of valence-shell photoexcitation and ionization cross sections in nitric oxide

Theoretical studies in static-exchange approximation are reported of photoexcitation and partial-channel photoionization cross sections of the outer-valence-shell 2π, 1π, 5σ, and 4σ electrons in nitric oxide. The calculations account satisfactorily for observed Rydberg series and for the spectral shapes of corresponding experimental photoionization cross sections. Multiplet-specific static-exchange potentials are employed for construction of 2π −1, 5σ −1, and 4σ −1 excitation series and partial cross sections. These are required particularly for (5σ −1)b 3 Π and (5σ −1)A 1Π partials in order to obtain values in agreement with recent synchrotron-radiation studies, which exhibit resonance maxima having different kinetic energies in the two channels. Additionally, the multiplet-specific calculations predict triplet-singlet cross sectional maxima in the 5σ −1 channels that differ noticeably from a statistical 3:1 ratio, also in good accord with measured values. Slater-average potentials are employed for the many channels that arise from 1π ionization, providing results in generally good agreement with dipole (e, 2e) measurements. Good agreement also obtains between measured values and the calculated 2π and 4σ cross sections, the present results thereby providing an overall clarification of the partial-channel photoionization cross sections of NO. Stieltjes orbitals in body-frame point-group symmetry provide three-dimensional graphical diagnostics of various resonance features present in the photoionization cross sections. The 6σ(σ*) antibonding orbital is found merged in kσ final-state continua, accounting for the presence of strong photoionization resonance features, and for the absence of corresponding discrete excitations. Computational aspects of the construction of Stieltjes cross sections and orbitals are discussed, and the origins of apparent discrepancies among calculated NO partials in the literature are clarified.

Photoionization of carbon disulfide: theoretical studies of outer-valence-shell partial cross sections

Studies in vertical-electronic, static-exchange approximation are reported of the outer-valence-shell (2π g −1, 2π u −1, 5σ u −1, 6σ g −1) partial-channel photoionization cross sections of CS 2. The origins of strong features present in the calculated profiles, and in corresponding experimental cross sections, are attributed to the 6σ u(σ ∗) and 7σ g (σ ∗) orbitals of Mulliken on basis of Stieltjes orbital diagnostics.

Theoretical studies of photoionization in nitric oxide: Multiplet-specific spectral shapes of (5σ −1)b 3II and (5σ −1) A 1II partial cross sections

Multiplet-specific calculations in static-exchange approximation are reported of (5σ −1)b 3II and (5σ −1)A 1II partial-channel photoionization cross sections in nitric oxide. The calculated cross sections exhibit resonance peaks at significantly different kinetic energies in the two multiples channels, clarifying the origins of similar behaviors observed in recent synchrotron- radiation studies.

Theoretical studies of photoexcitation and ionization in H2O

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.

Theoretical studies of inner-valence-shell photoionization cross sections in N 2 and CO

Theoretical studies in the intensity-borrowing sudden approximation are reported of inner-valence-shell photoionization cross sections in N 2 and CO. The required ionic-state energies and spectroscopic amplitudes are obtained from appropriate Green's-function and configuration-interaction calculations, and previously devised Stieltjes-Tchebycheff moment-theory techniques are employed in determinations of corresponding continuum dipole transition moments in the static-exchange approximation. Comparisons are made of the Green's-function calculations in the two-particle-hole Tamm-Dancoff approximation with wavefunction results obtained from single-excitation and polarization configuration-interaction calculations. Detailed descriptions are given of the calculated spectroscopic intensity distributions and of the hole-particle configurational compositions of the corresponding inner-valence-shell ionic states. and comparisons are made with previously reported wavefunction studies in N 2 + and CO +. Spectroscopic assignments are suggested on basis of the present calculations for the strong features observed recently in higher-resolution inner-valence-shell photoelectron spectra. The corresponding calculated partial-channel photoionization cross sections for the designated C 2Σ g +, F 2Σ g +, G 2Σ g +, and (2σ g −1) 2Σ g + bands in N 2 and C 2Σ +, D 2Π, F 2Σ +, G 2Σ +, and (3σ −1) 2Σ + bands in CO are found to be in good quantitative accord with dipole (e, 2e), (e, e + ion), and synchrotron-radiation studies.

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 7a1(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.

Extended-basis-set rpae calculations of molecular photoionization cross sections: theoretical studies of V π-V σ interaction in molecular nitrogen

Theoretical studies in the time-dependent Hartree-Fock approximation (RPAE) employing L 2 moment methods are reported of the effects of continuum V π-V σ configuration mixing upon the partial-channel photoionization cross sections in molecular nitrogen.

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 (${\mathrm{N}}_{2}$, CO, ${\mathrm{O}}_{2}$, ${\mathrm{F}}_{2}$) and polyatomic (${\mathrm{H}}_{2}$O, ${\mathrm{H}}_{2}$CO, ${\mathrm{O}}_{3}$) 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 $2{\ensuremath{\pi}}_{u}({\ensuremath{\pi}}^{*})$, $5{\ensuremath{\sigma}}_{g}({\ensuremath{\sigma}}^{*})$, and $4{\ensuremath{\sigma}}_{u}({\ensuremath{\sigma}}^{*})$ virtual valence orbitals, and from more diffuse discrete and continuum Rydberg orbitals. The $2{\ensuremath{\pi}}_{u}({\ensuremath{\pi}}^{*})$ orbital is found to contribute to discrete excitation series, whereas the $5{\ensuremath{\sigma}}_{g}({\ensuremath{\sigma}}^{*})$ and $4{\ensuremath{\sigma}}_{u}({\ensuremath{\sigma}}^{*})$ 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 $(1{\ensuremath{\pi}}_{g}^{\ensuremath{-}1})X^{2}\ensuremath{\Pi}_{g}$, $(1{\ensuremath{\pi}}_{u}^{\ensuremath{-}1})A^{2}\ensuremath{\Pi}_{u}$, $(3{\ensuremath{\sigma}}_{u}^{\ensuremath{-}1})B^{2}\ensuremath{\Sigma}_{u}^{+}$, and $(4{\ensuremath{\sigma}}_{g}^{\ensuremath{-}1})C^{2}\ensuremath{\Sigma}_{g}^{+}$ 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 $1{\ensuremath{\pi}}_{g}^{\ensuremath{-}1}$ and $1{\ensuremath{\pi}}_{u}^{\ensuremath{-}1}$ 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 $2{\ensuremath{\sigma}}_{u}^{\ensuremath{-}1}$ and $3{\ensuremath{\sigma}}_{g}^{\ensuremath{-}1}$ 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 ($1{\ensuremath{\sigma}}_{g}^{\ensuremath{-}1}$, $1{\ensuremath{\sigma}}_{u}^{\ensuremath{-}1}$) cross section exhibits both the expected $5{\ensuremath{\sigma}}_{g}({\ensuremath{\sigma}}^{*})$ and $4{\ensuremath{\sigma}}_{u}({\ensuremath{\sigma}}^{*})$ 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 ${\mathrm{O}}_{2}$, and the origins of the similarities and differences in the cross sections in these cases are clarified.

Photoexcitation and ionization in molecular fluorine: Stieltjes–Tchebycheff calculations in the static-exchange approximation

Theoretical investigation of outer (1πg, 1πu, 3σg) and inner (2σu, 2σg) valence-shell electronic photoexcitation and ionization cross sections in molecular fluorine are reported employing separated-channel static-exchange calculations and Stieltjes–Tchebycheff (S–T) moment-theory techniques. The discrete vertical electronic 1πg excitation series are found to be in good agreement with recent spectral assignments and previously reported theoretical studies, and those for 1πu, 3σg, 2σu and 2σg excitations are in general accord with position and intensity estimates based on quantum-defect analysis. Certain of the partial-channel photoionization cross sections in F2 are seen to exhibit resonancelike features similar to those reported recently in related S–T studies of photoionization in N2, CO, and O2. The resonances can be attributed to valencelike and pre-Rydberg diabatic states that cross the outer limbs of appropriate Rydberg series and corresponding ionic-state potential curves as functions of internuclear coordinate, giving rise to large continuum transition intensities at the ground-state equilibrium internuclear separation. In contrast to the situation in N2, CO, and O2, however, there is no evidence of a resonance like σ→σ* feature in the 3σg→kσu photoionization channel in F2. Rather, this resonance in F2 appears as a strong N→Vg transition below the 3σg ionization threshold, and the corresponding partial-channel photoionization cross section is seen to be structureless. Although experimental studies of partial-channel photoionization cross sections are apparently unavailable for comparison, the calculations reported here should provide reliable approximations to the dipole excitation/ionization spectra in F2, and are helpful in understanding and clarifying the dependences of photoionization spectra in light diatomic molecules on shell occupancy and equilibrium internuclear separation when compared with the results of previous studies of photoionization in N2, CO, and O2.

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, σ-orbital cross sections in light diatomic and polyatomic molecules are found to be generally dominated by strong σ - σ^* 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, C0_2, and H_2CO, the results of which are in generally good accord with corresponding measured values.

Photoexcitation and ionization in molecular oxygen: Theoretical studies of electronic transitions in the discrete and continuous spectral intervals

Theoretical studies of valence-electron (1πg, 1πu, 3σg) photoexcitation and ionization cross sections in molecular oxygen are reported employing separated-channel static-exchange calculations and the Stieltjes–Tchebycheff moment-theory technique. As in previously reported investigations of photoexcitation and ionization in small molecules following this approach, canonical Hartree–Fock orbitals, large Gaussian basis sets, and many-electron eigenstates of correct symmetry are used in defining appropriate noncentral static-exchange potentials and in computations of the appropriate discrete and continuum transition strengths. It is particularly important in molecular oxygen to incorporate the appropriate ionic parentages of the various photoionization multiplet states in order to obtain the correct partial-channel cross sections. The calculated discrete series associated with 1πg excitation are found to be in good agreement with available experimental assignments and previously reported theoretical studies, and the predicted states associated with 1πu and 3σg excitations are in general accord with assignments for the higher series based on spectral and quantum-defect analysis. Although the observed photoelectron spectra and photoionization cross sections are relatively complex, the calculated total vertical electronic photoabsorption cross section and the partial-channel photoionization cross sections for production of X 2πg, a 4πu, A 2πu, 2 2πu, 3 3Πu, b 4Σg−, and B 2Σg− ionic states are found to be in good accord with recent synchrotron radiation, line-source, electron-impact, and (e,2e) dipole oscillator-strength measurements when proper account is taken of the parentages of the various multiplet states. The partial-channel cross sections exhibit resonancelike structures that can be attributed to contributions from diabatic valencelike virtual states that appear in the appropriate photoionization continua, rather that in the corresponding discrete spectral intervals. These features in the dipole spectrum of molecular oxygen are discussed and are contrasted and compared with the results of previously reported related studies in molecular nitrogen and carbon monoxide.

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.