Inner-shell Photoionization Research Articles

Trends in near-threshold photoionization of off-the-center endohedral atoms

Calculations of the photoelectron angular distribution from atoms confined in a fullerene cage have been performed using a recently developed theory of inner-shell near-threshold molecular photoionization. Trends uncovered in the modification of near-threshold photoelectron angular distribution of the A-C{sub 60} molecules as a function of the displacement of the confined atom from the center of the C{sub 60} cage are demonstrated. Specific results for Li 1s from Li-C{sub 60}, and Ar 1s, 2s, and 3s from Ar-C{sub 60} molecules are presented. In addition, the situation where the encapsulated atom lies in the vicinity of the center of one of the carbon hexagons of C{sub 60} and also when it is displaced beyond the fullerene cage, has been studied. It has been found that the photoelectron spectra are extremely sensitive, both qualitatively and quantitatively, to even a small displacement of the confined atom from the center of the cage. The existence of the C{sub 60} molecule even affects the atom's photoelectron angular distribution when it is a significant distance away.

Atomic data for the K-vacancy states of Fe XXIV

As part of a project to compute improved atomic data for the spectral modeling of iron K lines, we report extensive calculations and comparisons of atomic data for K-vacancy states in Fe XXIV. The data sets include: (i) energy levels, line wavelengths, radiative and Auger rates; (ii) inner-shell electron impact excitation rates and (iii) fine structure inner-shell photoionization cross sections. The calculations of energy levels and radiative and Auger rates have involved a detailed study of orbital representations, core relaxation, configuration interaction, relativistic corrections, cancellation effects and semi-empirical corrections. It is shown that a formal treatment of the Breit interaction is essential to render the important magnetic correlations that take part in the decay pathways of this ion. As a result, the accuracy of the present A-values is firmly ranked at better than 10% while that of the Auger rates at only 15%. The calculations of collisional excitation and photoionization cross sections take into account the effects of radiation and spectator Auger dampings. In the former, these effects cause significant attenuation of resonances leading to a good agreement with a simpler method where resonances are excluded. In the latter, resonances converging to the K threshold display symmetric profiles of constant width that causes edge smearing.

Ionic pathway studies in fragmentation of the core-excited (CH 3) 3SiBr molecule involving the Br(3d) and Si(2p) inner-shells in the range of 60–133 eV

Dissociative multiple photoionization of bromotrimethylsilane [(CH 3) 3SiBr] has been probed in the valence, the Br(3d), and the Si(2p) core-level photoexcitation/photoionization regions by time-of-flight (TOF) mass spectrometry and photoion–photoion coincidence (PIPICO) method together with linearly polarized synchrotron radiation. The inner-shell photoionization spectra are dominated by a huge resonance attributed to the diffusion of the outgoing photoelectrons by the centrifugal barrier for the Br(3d 9 ϵf ) outgoing channel. In addition to the giant resonance, discrete photoabsorption bands below the Br(3d 5/2) and Si(2p 3/2) thresholds have been observed in both the total photoion and PIPICO yield spectra. In the valence excitation/ionization region, single ionization leading to the formation of C n H p SiBr +( n=0–3), C n H p Si +( n=0–3), and CH 3 + prevails. Above the Br(3d) threshold including the Si(2p) region, however, destruction of the three or more bonds could account for the dominant PIPICO channels leading to the formation of H +–CH p +( p=1–3) and CH n +–H p Si +( p=0,1) ion pairs. The exclusive formation of only the singly charged ions over the entire energy range indicates that charge redistribution is much faster than the bond cleavages.

Relaxation effects in inner-shell photoionization of Mg, Ca, and Sr

Total and partial photoionization cross sections have been calculated for all inner subshells of atomic magnesium, calcium, and strontium in the relativistic random-phase approximation, the relativistic random-phase approximation modified to include relaxation effects, and the relativistic random-phase approximation modified to include relaxation and Auger decay. Results of the three models are compared with Dirac-Hartree-Fock calculations and with semiempirical results so that the import of various many-body effects can be assessed. Branching ratios and photoelectron angular-distribution asymmetry parameters are also presented for np subshells. Substantial relaxation effects are noted for photoionization of $1s$ and $2s$ subshells.

Inner-shell near-threshold photoionization ofA@C60endohedral atoms

Photoelectron angular distributions and total photoionization cross sections of near-threshold $1s$ photoionization of Li from the oriented-in-space endohedral $\mathrm{Li}@{\mathrm{C}}_{60}$ fullerene are investigated within the framework of our recently developed photoionization theory of multicenter formations. Both at-the-center and off-the-center endohedral Li are considered, and off-the-center effects are shown to be of extreme importance, in contrast to the conventional wisdom. Multiple-scattering effects on the outgoing photoelectron are included and found to influence the photoionization spectra strongly, e.g., decreasing the cross section by more than an order of magnitude in certain cases, as compared to the situation when such effects are neglected. Diffraction resonances in the photoionization cross section of the endohedral Li atom are found as a result of the multicenter nature of the ${\mathrm{C}}_{60}$ surrounding cage, and these are compared with the results of other empirical and ab initio theories. Since there is nothing particularly special about Li $1s$ with respect to photoionization, the trends uncovered in this paper should qualitatively apply to near-threshold inner-shell $\mathrm{ns}$ photoionization spectra of virtually any endohedral atom A from any $A@{\mathrm{C}}_{60}$ endohedral fullerene formation.

Inner-shell photoionization of O III

A 25-state R-matrix calculation is performed to obtain photoionization cross-sections for transitions from the 1s22s22p23P ground state of the O III ion. Results are obtained for a range of photon energies, including those at which K-shell photoionization processes take place. We compare our results with those from previous calculations. Excellent agreement is obtained. We also consider resonances owing to transitions of a 1s electron excited into the 2p orbital and compare with a recent calculation.

Resonance analysis in the photoionization of excited lithium below the1s2land1s3lionization thresholds of the Li atom

Calculations of $1s$ electron inner-shell photoionization from the ${1s}^{2}2p{}^{2}{P}^{o}$ Li excited state were previously reported using the R-matrix code [Zhou et al., Phys. Rev. A 59, 462 (1999)]. These calculations included important resonances in the partial and total photoionization cross sections for incident photon energies (i) lower than 74 eV and due to the ${1snln}^{\ensuremath{'}}{l}^{\ensuremath{'}}{}^{2}{L}^{e}$ autoionizing states for $n=2$ and 3 and $L=S,$ $P,$ and $D,$ and (ii) larger than 140 eV and due to the ${2ln}^{\ensuremath{'}}{l}^{\ensuremath{'}}{n}^{\ensuremath{''}}{l}^{\ensuremath{''}}{}^{2}{L}^{e}$ autoionizing states. The latter resonances corresponding to ${2ln}^{\ensuremath{'}}{l}^{\ensuremath{'}}{n}^{\ensuremath{''}}{l}^{\ensuremath{''}}$ were recently analyzed by Zhou et al. [Phys. Rev. A 62, 012707 (2000)], using an eigenphase derivative technique along with quantum defect theory. The present paper proposes an analysis of the former resonances in the 58--74 eV incident photon energy range. The determination of the resonant positions for each series converging to a $1s2l{}^{1,3}L$ or $1s3l{}^{1,3}L$ ionization threshold of Li is obtained using the same method as in Zhou et al. [Phys. Rev. A 62, 012707 (2000)]. The identifications are compared with theoretical results of Chung [Phys. Rev. A 57, 3518 (1998)] using the saddle-point method for energies below the ${\mathrm{Li}}^{+}1s2s{}^{1}S$ threshold. Agreement is excellent in most cases and some discrepancies are discussed.

Theory of inner-shell photoionization of fixed-in-space molecules

A theory is presented for the calculation of molecular inner-shell photoionization near thresholds and electron\char21{}molecular-ion elastic scattering. The interaction of the incoming/outgoing electron with each atom of the formation (except for the ionized atom) is accounted for by suitable boundary conditions imposed on the electron wave function at nuclei of these atoms in the system. The theory is applied to calculations of the form of the photoelectron angular distribution for $1s$ photoionization of the C atom in the CO diatomic molecule, as well as for the photoelectron angular distribution due to photodetachment from a quasimolecular negative ion consisting of a ${\mathrm{C}}^{\ensuremath{-}}$ ion located near a neutral O atom.

Photoionization of O III low-lyingstates: autoionization resonance energies and widths ofsome 1s-2p excited states

Twenty-four-state close-coupling calculations are performed onthe inner-shell photoionization of the terms belongingto the configurations 2s22p2 and 2s2p3of the O III ion. The resonance energies andwidths of the transitions of a 1s electron excited intothe 2p orbital are determined by analysing the resonancestructures of the photoionization cross sections. Thecalculations show that the autoionization resonancewidths of these inner-shell transitions are larger thanthe corresponding Doppler full width at half maximum(FWHM) and Stark FWHM by a factor of two or three undertypical plasma conditions (such as a temperature of10 eV and a density of 0.0016 g cm-3). Theautoionization resonance broadening is obviously themain broadening mechanism and should be considered whenanalysing the plasma spectrum. Only when the electrondensity is larger than5×1021 cm-3 may the Stark FWHM be largerthan the autoionization resonance FWHM.

A photoelectron–photoion coincidence method for the investigation of decay probabilities after innershell photoionization

The photoelectron–photoion method presented is shown to be very successful for the quantitative investigation of decay probabilities after innershell photoionization. Especially for the determination of K-shell fluorescence yields of light elements, where only scarce data exist up to now, this method opens up new ways. For all elements we present a general approach to determine fluorescence yields. As an example the investigation of the 1 s decay of free sodium atoms is presented.

Ultrafast inner shell photo-ionization pumped X-ray emission in C

A new model employing five energy levels and a fit time-varying linear photo-pumping rate of x-ray source is proposed to analyze the gain of inner-shell photo-ionization (ISPI) x-ray laser. The deduced analytical expressions for the time of the onset of the peak gain and its peak value reveal the effects of Auger decay, electron impact ionization, the density of lasing medium and the ratio of statistical weights on the duration and peak value of the laser gain, also the requirements on the intensity of the driving USP laser can be estimated.

Inner-shell photoionization of the 3dx ground state Fe ions

The total photoionization cross section for the 3dx ground stateFe ions, Fe III through Fe VIII, is calculated from theionization threshold to the K edge including resonance features. TheR-matrix method is used, the work being undertaken as part of theinternational RmaX(`R-matrix for x-ray processes') network. The dynamics of thephotoionization process is illustrated by examining isonuclear trends andby quantifying some of the main resonance features due to inner-shellprocesses, particularly the large resonances arising fromphotoexcitation of the core (np53dx + 1), and the Kedge. We also show that to calculate the cross section correctly through adeep inner-shell edge, requires the inclusion of states with inner-shellvacancies in all the preceding subshells.

Approximately analytical model for inner-shell photoionization x-ray lasers in low-Zelements

Approximately analytical model for inner-shell photoionization x-ray lasers in low-<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi>Z</mml:mi></mml:math>elements

Measurement of angular dependence of photon-induced differential cross-sections of M X-rays from Pt, Au and Hg at 5.96 keV

The differential cross-section for the emission of M shell fluorescence X-rays from Pt, Au and Hg by 5.96 keV photons at seven angles ranging from 50° to 110° have been measured. The differential cross-section is found to decrease with increase in the emission angle, showing an anisotropic spatial distribution of M shell fluorescence X-rays. The present results contradict the predictions of Cooper and Zare (Cooper, J., Zare, R.N., 1969. Atomic Collisions Processes, vol. XIC. Gordon and Breach, New York. p.317) that after photoionization of inner shells, the vacancy state has equal population of magnetic substates and that the subsequent X-ray emission is isotropic, but confirm the predictions of the calculations of Flugge, Mehlhorn and Schmidt that the atomic inner-shell vacancies produced after photoionization are aligned and X-ray emission from the filling of vacancies in states with J>1/2 is anisotropic. The vacancies were created by the 5.96 keV γ-rays from an 55Fe source and the M X-rays were measured using a Si(Li) detector.

Energy levels and optical oscillator strengths of inner-shell excited states and photoionizations of the ground and first excited states of C IV ion

Extensive configuration interaction wave functions are determined to calculate the energies of the inner-shell excited states and the oscillator strengths of the optically allowed inner-shell transitions of C IV ion. Photoionization cross-sections of the ground and the first excited states of C IV ion are also obtained by using the R-matrix method. The positions of some inner-shell excited states are redetermined more accurately by analyzing the resonance structures of the photoionization processes. Some of the results are compared with other available theories and experiments.

K-shell photoionization cross-sections of N V and O VI

An 11-state R-matrix calculation is performed to obtain cross-sections for the photoionization of the 1s22s 2S ground states of N v and O vi. For both ions at low photon energies, corresponding solely to the 2s photoionization cross-section, excellent agreement is found with previous calculations. For higher photon energies, permitting inner-shell photoionization of the 1s electron, excellent accord is found with the recent calculation of Nahar for O vi, whilst for N v resonance structure is found which previous calculations omitted.

Inner-shell photoionization with excitation from excited atomic Na

Many-body perturbation theory is applied to the calculation of inner-shell photoionization (2p kl) and photoionization with excitation (2p,3p)(4p,kl) and (4s,kl) from atomic Na excited into the 2p63p state. We have calculated the cross sections for leaving the positive ions in various final states, corresponding to the main line and to the satellite lines. The theoretical results are in good agreement with experiment.

Energy levels and optical oscillator strengths of inner-shell excited states and photoionizations of the ground and first excited states of C IV ion

Energy levels and optical oscillator strengths of inner-shell excited states and photoionizations of the ground and first excited states of C IV ion

Inner-shell photoionization at relativistic energies

At relativistic energies the cross section for the atomic photoelectric effect drops off as does the cross section for liberating any bound electron through Compton scattering. However, when the photon energy exceeds twice the rest mass of the electron, ionization may proceed via electron-positron pair creation. The cross section for this channel saturates at several times the threshold energy, and hence the most probable way of photoionizing an atom at high energy is to make the vacuum spark. We present estimates for the cross section of this ``vacuum-assisted photoionization'' which proceeds either via direct pair creation on the bound electron or via pair creation on the atomic nucleus binding the inner-shell electron followed by an electron-electron or positron-electron encounter.

Inner-shell photoionization of excited lithium

Calculations of inner-shell photoionization of ${1s}^{2}2p{}^{2}{P}^{o}$ and ${1s}^{2}3p{}^{2}{P}^{o}$ Li excited states have been performed using recent developments of the R-matrix code with a 29-term target representation for incident photon energies up to 165 eV, with particular emphasis on multiple electron processes. Partial and total cross sections are given for a number of excitation processes including the lower members of 19 hollow atom resonance series. The results are compared with recent experimental measurements of the lowest ${2s2p}^{2}{}^{2}{L}^{e}$ resonance of even-parity hollow lithium states produced by photoexcitation of laser-excited lithium atoms. Experiment and theory are in excellent agreement on a relative scale. The enhancement of the shake-up process increases with initial state excitation, up to 500% over that of the ground state for ${1s}^{2}3p{}^{2}{P}^{o}.$ Good agreement is also obtained for K-shell photoionization when comparing branching ratios with available experimental results.