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A detailed theoretical analysis of the 1s photoionization of neon is presented. It is found that the most significant many-electron correlation in computing photoionization of inner shells is the rearrangement of the outer shells caused by the inner vacancy. Further noticeable effects are: (i) the polarization of the ion core by the outgoing photoelectron and (ii) the coherent effect of double excitation/ionization. The core polarization increases the photoionization cross section by about 10% at the 1s threshold, and the coherent excitation results in further increases by about 5%. Incoherent excitation of the satellite channel leads to an additional 10% increase in the photoabsorption cross section in the double-ionization threshold region., 6 pages, 3 figures, 22 bibliography

The two-photon above-threshold 3p-ionization of argon for the photon energies exceeding the 3p-ionization threshold is studied using the correlation function technique. Generalized two-photon ionization cross sections were calculated taking into account many-electron correlations. The calculations demonstrate that the two-photon ionization of Ar at the 3p4 threshold is almost entirely a collective process. The decisive contribution in the above-threshold two-photon 3p-ionization at the exciting-photon energies corresponding to the 3p→ed giant resonance comes from the Auger-like many-electron correlations of e′ de″d − 3pef type.

Rearrangement of the electron shells accompanying inner-shell photoionization of the CH4 molecule is studied theoretically. For this purpose, the K-shell photoionization cross section σ1s (ω) and the respective photoelectron angular distribution (PAD) parameter βe1s(ω) are computed in different approximations using the single center method. It is obtained that a strong mixing of the partial ionization channels in the vicinity of the 1s ionization threshold, caused by the the non-spherical part of the molecular potential, results in a qualitative difference between the electronic rearrangement effects in molecules, as compared to atoms.

Photon-induced fluorescence spectroscopy was applied to measure absolute fluorescence emission cross sections from the main 5s- and first satellite levels of XeII upon photoionization from XeI in a broad exciting-photon energy range (23–28 eV) with 20 meV bandwidth of the exciting radiation and in a narrow energy range (23.25–23.50 eV) with a bandwidth of 1.8 meV. The average fluorescence resolution amounted to 0.07 nm. Fluorescence emission cross sections were computed with taking into account many-electron correlations and doubly excited states. Computed cross sections, especially in integrated form, agree well with the results of experiment. Many features observed in the high resolution experiment are identified to originate from nf-Rydberg series.

Total and partial 5p- photoionization cross sections of Xe in the exciting-photon energy range corresponding to the 5s1np Rydberg series were computed including the most important many-electron correlations. Results are compared with available experimental data. The most prominent spectral features observed in the experiments are identified.

The orientation parameter O10 for the 4p4(1D)5p2P3/2 state of KrII populated in resonant Auger transitions from 3d95p/6p photo-excited states was measured at BESSY II for photon energies between 90.8 eV and 92.9 eV. The (1D)5p2P3/2 satellite was populated in the Raman regime, i.e. with a bandwidth of the exciting photons (10 meV) smaller than the natural width of the 3d95p/6p states. Partial cross sections determined from the measurement of the orientation O10 and alignment A20 are compared with results of calculations where the non-resonant part of the photoionization cross section is computed within the sudden approximation. The electron partial wave analysis as the main intention of this investigation shows that this approach yields partial cross sections for the s1/2 (d5/2) waves that are larger (smaller) than the measured ones.

The photoionization and photodissociation of an oxygen molecule are investigated theoretically by the method of coupled differential equations. The molecular orbitals of the core are calculated in the MO LCAO approximation. The molecular orbitals of a photoelectron in the discrete and continuous spectra are determined by the single-center method. The wave functions of vibrational motion of the oxygen molecule are obtained within the diabatic approach. The described method is used for calculating the predissociation and autoionization widths of the 2σ u −1 (c 4Σ u − )n/σg, v Rydberg states of the oxygen molecule. The total cross sections of the resonant photoionization and neutral predissociation calculated for the oxygen molecule in the excitation energy range 20.6–24.8 eV are in good agreement with the available experimental data.

The photoionization cross sections for the 4p shell of ions of the Kr isoelectronic sequence Rb+, Sr2+, and Y3+ are calculated. The configuration interaction theory and the perturbation theory are used to describe the many-electron effects. The relativistic effects are taken into account in the Pauli-Fock approximation. The calculated resonance structure of photoionization cross sections for the 4p shell in the region below the 4s threshold associated with the autoionization of the 4s-np singly excited states and the 4p4p-nln′l′ doubly excited states reproduces the results of recent measurements of total photoabsorption cross sections for the Rb+, Sr2+, and Y3+ ions. It is found that, as the nuclear charge in the isoelectronic sequence increases, the ratio between the direct and correlation parts of amplitudes of the 4s-(n/ɛ)p transition changes and, as the consequence, the minimum of the photoionization cross section of the 4s shell shifts from the continuous spectrum to the region of states of discrete spectrum. This accounts for the strong changes in the shape of the 4s-np resonances in the photoionization cross sections for the 4p shell of Rb+, Sr2+, and Y3+, as well as the distinction between the shapes of the 4s-6p 1/2 mirror resonance in the partial 4p 1/2 and 4p 3/2 photoionization cross sections for the Y3+ ion which do not suppress each other in the total photoionization cross section, as is the case for similar resonances in Rb+ and Sr2+.

The photoionization cross sections of the 4p shell and the 4s main level and 4p 4(3 P) 5s 4 P 5/2, 3/2 satellite subvalence levels of KrII have been calculated in the 4s-near-threshold range of excitation energies from 28.48 to 28.70 eV. The calculation takes into account the core relaxation by the methods of the theory of non-orthogonal orbitals, the interaction between resonant states through autoionization channels by solving the complex secular equation, and the interaction between the channels of the continuous spectrum in all orders of the perturbation theory by the K-matrix method. Good quantitative agreement between the energy-integrated theoretical and experimental photoionization cross sections for the satellite levels has been obtained for the first time. It is shown that only simultaneous consideration of the above-mentioned effects leads to such agreement. The resonant structure of the photoionization cross sections in this excitation energy range is related to the autoionization decay of the 4p 45s(4 P 1/2)np and 4p 45s(2 P 3/2)np Rydberg series. The specificity of this process is that both series manifest themselves not independently but owing to their strong electrostatic interaction with the prominent 4p 4(1 D)5s 2 D 5/2 6p 3/2 resonance, which lies in this excitation energy range.

The dynamics of the processes of alignment and orientation of atoms subjected to the action of polarized radiation has been studied theoretically in the energy range of excitation of autoionization resonances. The alignment parameters A20 for the 4p45p states of Kr II populated through the excitation and autoionization decay of Kr I 3d9np resonances depend on energy due to the interference between different resonance channels and the channel of direct photoionization. It is predicted that the orientation parameter O10 of some Kr II 4p45p states and the parameter of the angular distribution of photoelectrons βe1 strongly depend on energy. The absolute photoionization cross sections are calculated for the population of the Kr II 4p45p states in the case where the energy of excitation photons corresponds to the first four 3d9np resonances. A good agreement between the calculated and measured photoionization cross sections proves that it is important to take into account the interference between different resonance channels in order to appreciate the dynamics of the Auger decay of Kr I 3d9np resonances.

The alignment parameter A20 and the orientation parameter O10 for the 4p4(1D)5p2F7/2 and the 4p4(1D)5p2D5/2 states of Kr+ populated in resonant Auger transitions from 3d95p/6p photo-excited states of Kr were measured at BESSY II for photon energies between 91 eV and 92.8 eV. The method of fluorescence polarimetry was applied. A bandwidth of the exciting photons of 10 meV allowed to study A20 and O10 within the natural line width. Interference effects for resonances overlapping due to lifetime broadening were predicted and observed even when separated by 15 times the natural line width. The relative contribution of the three allowed electron partial waves was derived from A20 and O10 as a function of the exciting-photon energy and compared with the calculated contributions. The resulting good overall agreement between experiment and prediction is considered as a noticeable improvement of the understanding of excitation and decay of the prominent Kr 3d95p resonances.

The interaction between resonances through autoionization continua and the interaction between autoionization continua were investigated theoretically and experimentally for the photoionization process of the 4p- and 4s-shells and for the population of 4p4 (3P)5s 4PJ, 4p4 (3P)5s 2PJ and 4p4(3P)4d 4DJ satellites of KrII. Cross sections for the satellite production and the angular distribution parameter of the fluorescence radiation were measured by photon-induced fluorescence spectroscopy after excitationx with linearly polarized monochromatized synchrotron radiation at exciting-photon energies between 28.45 eV and 29.95 eV with an exciting-photon energy resolution of 10 meV (FWHM). Measured cross sections are in good agreement with the computed ones. A refined assignment of resonances in the proximity of the 4s14p65p resonance was performed. It was concluded that there is a strong influence of the core rearrangement, of the interaction between resonances through the autoionization continua and of the interaction between autoionization continua, on the investigated processes on the basis of the observed good overall agreement between the computed and measured quantities.

Photoionization cross-sections for the 4p 4 ( 3 P ) 5s 4 P 5 / 2 , 3 / 2 , 1 / 2 satellites and 4s, 4p main levels of Kr II in the exciting-photon energy range between 28.48 and 28.70 eV with extremely narrow bandwidth (1.7 meV at 28.55 eV) of the monochromatized synchrotron radiation were measured utilizing the photon-induced fluorescence spectroscopy. The observed resonances were assigned to the 4p 4 5s( 4 P 1 / 2 )n p and 4p 4 5s( 2 P 3 / 2 )n p Rydberg series on the basis of calculations performed with taking into account core relaxation and interaction between many resonances and many continua. The calculation shows that the resonance structure in the photoionization channels exists due to 4p 4 ( 1 D ) 5s 2 D 5 / 2 6 p 3 / 2 promoter state which also strongly perturbs the above Rydberg series.

Parameters of the resonant 4p 4p –3d e l Auger effect (RA) following the 3d– n p ( n = 5 , 6 ) excitation in Kr were calculated with taking into account the interference between several resonant and direct non-resonant transition amplitudes. For the first time all individual lines of the extended RA spectrum which comprises both the 4 p 4 ( 1 D ) 5p and the 4 p 4 ( 1 D ) 6p groups of final ionic states were considered. It was revealed that each group contains individual lines where the interference contributions have different signs thus providing a weak interference effect on the average over the whole group. Interference effects are found to be more pronounced in the angular distribution of the RA products.

The photoabsorption spectrum of Kr-like Y3+ ions in the very soft x-ray/extreme ultraviolet spectral range where single (4s–np) and double (4p4p–nln'l') electron excitations dominate has been measured using a dual laser plasma photoabsorption technique. The corresponding photoionization cross section has been computed within the framework of the configuration interaction Pauli–Fock approach. The calculations confirm that the 4s–5p resonance drops below the 4p ionization threshold so that the first member of the 4s–np photoionization resonance series becomes 4s–6p. Also, whereas the first such series member in Rb+ and Sr2+ appears as a window resonance, in Y3+ it switches to a normal, slightly asymmetric absorption peak. In addition, although the 4s–5p1/2 resonances in the 4p51/2 and 4p53/2 continua are mirror images in Rb+ and Sr2+ and hence cancel each other, they exhibit quite different shapes in Y3+ and hence we observe the residual signature of the 4s–6p1/2 resonance as a weak partner to the main 4s–6p3/2 resonance.

The competition between autoionization and neutral dissociation (ND) of the vibronic Rydberg states 2σ−1u(c4Σ−u)(ns/nd)σg3Σ−u(v = 0, 1) has been investigated using an experiment of photon-induced fluorescence spectroscopy (PIFS) and the simultaneous measurement of the photo ion yield. The experiments were performed with monochromatized synchrotron radiation from BESSY II with a very narrow bandwidth of ΔE = 1.5 meV at 23 eV where it was possible to observe single rotational branches. The experiments are compared to ab initio calculations for the partial autoionization and dissociation widths and to the corresponding simulations of the spectroscopic shape of the rovibronic branches. For more precise calculations of the dissociation widths, diabatic potential curves of the 2σ−1u(c4Σ−u) states of the O+2 are extracted from the adiabatic ones and improved calculations for molecular orbitals of the Rydberg and Auger electrons are used.

The energy dependence of the alignment parameter A20 for the Kr II 4p45p(E 1J1) states following excitation in the Raman regime with the exciting-photon energy passing through the Kr I 3d9np resonances was investigated theoretically and experimentally. Interference between the resonance and direct photoionization channels explains the dependence of A20 on the excitation energy. Experimentally, A20 was determined after the analysis of 5p–5s fluorescence decay of the 4p45p(E 1J1) states excited with monochromatized synchrotron radiation. The band pass of the synchrotron radiation was set to 10 meV which is smaller than the natural width of the 3d9np resonances (~83 meV). Additionally, a strong energy dependence was predicted for the orientation parameter O10 as well as for the angular distribution parameter of photoelectrons, βe, for several 4p45p(E 1J1) ionic states.

Absolute photoionization cross sections for the population of the Kr II 4p4np states when the exciting-photon energy corresponds to the first four 3d9np resonances were calculated beyond the two-step model for the first time. Good agreement between computed and measured photoionization cross sections proves the importance of taking into account the interference between different resonance channels in understanding the dynamics of the Auger decay of the Kr I 3d9np resonances.

The orientation parameter, O10, and the alignment parameter, A20, of the Kr II 4p4(1D)5p 2F7/2 state populated in the resonant Auger decay on the Kr I 3d5/295p 3/2 (J = 1) resonance were measured, using photon-induced fluorescence spectroscopy (PIFS) and a Stokes parameter analysis. These parameters allow a partial wave analysis yielding a dominating contribution of the g9/2 electron partial wave. An excellent agreement with calculated values is obtained. A special feature of the present experiment is the measurement and analysis of circularly polarized light in a PIFS study.

Absolute cross sections for the total fluorescence emission between 97 and 131 nm by atomic oxygen formed after the photoexcitation of 2σ-1u(c4Σ-u)nσg3Σ-u Rydberg states in O2 and their successive predissociation were measured for the first time. The dissociation lifetimes and fluorescence yields were estimated using a stepwise model and the potential curve of the O2+2σ-1u(c4Σ-u) state known from Beebe et al (Beebe N H F, Thulstrup E W and Andersen A 1976 J. Chem. Phys. 64 2080). The calculated lifetimes of the vibrational levels v = 0 and 1 of the 2σ-1u(c4Σ-u) state are 3.4×10-12 and 6.3×10-14 s, respectively. They agree with the measured data of Evans et al (Evans M, Stimson S, Ng C Y and Hsu C-W 1998 J. Chem. Phys. 109 1285) and are consistent with the results of the present experiment. The calculated lifetimes support the model of `fast' dissociation of the O2+2σ-1u(c4Σ-u), v states in contrast to the earlier model of `slow' dissociation of Tanaka and Yoshimine (Tanaka K and Yoshimine M 1979 J. Chem. Phys. 70 1626).

Energy dependences of the total and partial ns-photoionization cross-sections in the vicinity of the ns-subvalence shell threshold were measured for the rare-gas atoms using photon-induced fluorescence spectroscopy. A resolution of 4.5, 3, and 1 meV was attained for Ar, Kr and Xe, respectively. The experimental cross-sections exhibit many well-resolved resonances of different shapes which are connected with the photoionization through the doubly excited atomic states. Calculated photoionization cross-sections of Ar and Kr show that some resonances are determined by doubly excited states including strongly delocalized n′f-excited electrons.

The alignment of the 5p4 6p levels after the decay of the Xe I 4d9 5/2 6p(J = 1) resonance was investigated experimentally and theoretically. The angular distribution of the visible fluorescence radiation was used to measure the alignment. To calculate the measured quantities the multi-step model was applied, taking account of the relativistic effects within the Pauli-Fock approximation. The influence of the cascade effects on the population of the 5p4 6p levels was estimated.

The absolute cross sections for the 3s-photoionization and the 3p-photoionization of the Ar atom and the Ar-like ions K+ and Ca2+ are calculated. The calculation is performed within the configuration-interaction technique using the Pauli-Fock atomic orbitals. The simultaneous analysis of the calculated and measured total and partial cross sections yields the interpretation of the prominent resonances observed in a recent experiment on Ar above the 3s-threshold.

The lifetimes of the ns1 np6 2S1/2 states of Ar II, Kr II and Xe II were determined by measuring the fluorescence decay curves after selective excitation with pulsed monochromatized synchrotron radiation. The lifetimes are Ar = (4.684±0.019) ns, Kr = (30.78±0.17) ns, and Xe = (35.93±0.20) ns. Calculations performed within the configuration interaction approach show the strong dependence of the calculated lifetimes on the configuration interaction. It is proposed to use the precise lifetime values for a test of future calculations of the effective value of the Coulomb interaction of electrons.

The angular asymmetry parameter of the photon-induced fluorescence radiation of fine-structure resolved satellite lines was measured from threshold up to 38.4 eV. It is well known that the cross sections of these valence shell satellites are dominated by the decay of aligned doubly excited states leading to an extremely complicated energy dependence. In contrast, we observe that the parameter becomes less structured with increasing total angular momentum J of the photoion. The measurements can be interpreted by a partial-wave analysis of the photofragmentation products. For the population of the ion state only photoelectrons are emitted on all resonances. For the population of the and the ion states the emission of the parity-unfavoured photoelectrons dominates.

A high resolution electron spectrometry study of the sub-valence 5s-photoelectron correlation satellites of Xe is presented. We report both absolute partial cross sections and angular distributions of several selected satellite lines recorded at photon energies from 40.8 eV up to 150 eV. Our high resolution results are compared to earlier lower resolution measurements and to our calculations which incorporate both configuration interaction and many-body perturbation theory techniques.

A theoretical and experimental study of the behaviour of the correlation satellites arising during 5s photoionization of xenon is presented. Many-body perturbation theory and configuration-interaction techniques have been applied to calculate the wavefunctions of the Xe II ionic states. An expression for the angular distribution parameter of the photoelectrons taking into account final-ionic-state configuration interaction is derived. Photoionization cross sections and angular distribution parameters were calculated for the 5s main line and the majority of the satellite lines and compared with our high-resolution measurements and earlier lower resolution measurements. The differences in the angular distribution parameter dependence on the photon energy for the 5s main line and satellites were analysed in terms of their origin. The most important mechanisms are: interference of several photoionization channels characterizing different orbital momenta of the photoelectron, the mixture of terms with different total orbital momenta in the final ionic state, and the dependence of the photoelectron wavefunctions on the total momentum of the photoelectrons.

Photoionization cross sections of the metastable 3P0 and 3P2 states of the 3p54s configuration of the Ar atom and 4p55s configuration of the Kr atom are calculated in one electron approximation with taking into account the many-electron correlations. It is shown, that many-electron correlations change significantly the calculated one-electron cross sections

Photoionization cross sections, σ, and angular distribution asymmetry parameters, β, for the majority of levels manifested in the subvalence ns-shell photoelectron spectra of the Kr and Xe atoms are calculated. For many levels the results of our calculations are compared with our measured σ and β values and with other works. The main mechanisms giving rise to the complex photon energy dependence of the β-parameters are analyzed.

The photoionization cross sections of the 4s-main line and the corresponding satellite lines of the Kr atom are studied experimentally and theoretically in the near-threshold region. Our measurements have been made using the photon induced fluorescence spectroscopy technique. Many-electron correlations have been accounted for in the calculations. It was shown that the interference of the direct-ionization channel and the channel of the ionization through the decay of the intermediate doubly-excited states explains the experimentally observed complex resonant structure of the 4s-photoabsorption spectrum of the Kr atom.

Cross sections and angular distribution parameters of electrons ejected via two-photon ionization of the hydrogen molecule by linearly and circularly polarized light are computed in the exciting-photon energy range of 8--14 eV, which covers the first four optical resonant states of the molecule. Photoelectron partial waves in the continuous spectrum are obtained within the single center method in precise numerical potential of the molecular ion field. The correlation function technique is used to calculate the two-photon transition matrix element. The presently computed angular-resolved spectra are in good agreement with the theoretical results available for the photon energy of 8.854 eV and can be considered as reliable predictions in the respective energy range.

For pt. III see A. Ehresmann et al., ibid., vol., 27, p.1489-96 (1994). The photoionization cross sections for the production of the Kr II 4s state and Kr II satellite states were studied in the 4s ionization threshold region. The interference of direct photoionization and ionization through the autoionization decay of doubly-excited states was considered. In the calculations of doubly-excited state energies, performed by a configuration interaction technique, the 4p spin-orbit interaction and the (Kr II core)-(excited electron) Coulomb interaction were included. The theoretical cross sections are in many cases in good agreement with the measured values. Strong resonant features in the satellite spectra with threshold energies greater than 30 eV are predicted.

Photoionization cross sections of the main 2S term and satellite 2S,2P,2P0,2D terms of Ar and Kr atoms are calculated at the ns-subvalence shell ionization threshold. The calculations performed are in close agreement with the published experimental photoelectron spectra of Ar and are the basis for prediction of the subvalence shell photoelectron spectra of Ar and Kr in the 43–120 eV exciting-photon energy range.

Absolute Kr 4s-electron photoionization cross sections as a function of the exciting-photon energy were measured by photon-induced fluorescence spectroscopy (PIFS) at improved primary-energy resolution. The cross sections were determined from threshold to 33.5 eV and to 90 eV with primary-photon bandwidths of 25 meV and 50 meV, respectively. The measurements were compared with experimental data and selected theoretical calculations for the direct Kr 4s-electron photoionization cross sections.

The Kr 4s-electron photoionization cross section as a function of the exciting-photon energy in the range between 30 eV and 90 eV was calculated using the configuration interaction (CI) technique in intermediate coupling. In the calculations the 4p spin-orbital interaction and corrections due to higher orders of perturbation theory (the so-called Coulomb interaction correlational decrease) were considered. Energies of Kr II states were calculated and agree with spectroscopic data within less than 10 meV. For some of the Kr II states new assignments were suggested on the basis of the largest component among the calculated CI wavefunctions.

The 1s X-ray absorption spectra of atomic calcium, chromium, manganese and copper were determined experimentally and calculated theoretically. The spectra are dominated by a prominent line at threshold which precedes a smooth continuum extending to higher energies. Hartree-Fock calculations taking relativistic effects, relaxation and term dependence into account describe the discrete atomic resonance at threshold very well. The resonances at threshold have no counterpart in the spectra of the metals. The experimental atom to metal core level binding energy shifts agree well with the predictions of the thermodynamical model by Johansson and Martensson (1980).

Absolute cross sections for the transitions of the Kr atom into the 4s1 and 4p4nl states of the Kr+ ion were measured in the 4s-electron threshold region by photon-induced fluorescence spectroscopy (PIFS). The cross sections for the transitions of the Kr atom into the 4s1 and 4p4nl states were also calculated, as well as the 4p4nln'l' doubly excited states, in the frame of LS-coupling many-body technique. The cross sections of the doubly-excited atomic states were used to illustrate the pronounced contributions of the latter to the photoionization process, evident from the measurements. The comparison of theory and experiment led to conclusions about the origin of the main features observed in the experiment.

We have measured the photoabsorption spectra of the Kr-like ions Rb{sup +} and Sr{sup 2+} at photon energies corresponding to the excitation of 4s-np resonances using, the dual laser plasma photoabsorption technique. Dramatic changes in the line profiles, with increasing ionization and also proceeding along the Rydberg series of each ion, are observed and explained by the trends in 4s-transition amplitudes computed within a framework of configuration-interaction Pauli-Fock calculations. Total photoionization cross sections show very good agreement with relative absorption data extracted from the measured spectra.

The radiationless decay of the Xe N4, 5 levels is investigated theoretically. The strong influence of the giant resonance in the 5s5p5(1P)ef channel on the partial and total Auger widths via the interchannel mixing is revealed. The computed widths of the N4 and N5 levels, equal to 105.9 and 109.6 meV, respectively, are in excellent agreement with the measured values 104(3) and 111(3) meV (Jurvansuu et al 2001 Phys. Rev. A 64 012502). The shape of the computed N4, 5OO Auger spectrum is in good agreement with the measured one. Interchannel interaction reduces the probability of the 5p5p − 4deg Auger decay by more than an order of magnitude. This conclusion is supported by the good agreement between computed and measured Auger electron angular distribution parameters.

Many-electron effects in the photoionization of Kr near the 3d9np resonances were investigated theoretically. Cross sections for the population of the 4p4(1D)mp states, their alignment and orientation, and angular distribution of the Auger electrons were computed considering the resonance Auger effect as a coherent process. Probabilities of the radiative cascades following the resonant Auger effect were estimated quantitatively within the transition-array technique. The role of intra- and inter-shell correlations was investigated. Lineshapes computed, taking into account interchannel interaction, are in good agreement with the measured ones. Fourfold excitations of the Kr ground state were proposed to influence the resonance population, alignment and orientation of the 4p4np levels.