Interchannel Coupling Effects Research Articles

Electron-impact ionization of metastable neon

Ionization out of metastable states can dominate the total electron-impact ionization rate for many atomic systems. However, there have been few advanced close-coupling calculations of ionization of metastable atoms. In this paper, we report on an R-matrix with pseudostates (RMPS) calculation of ionization from the 2p53s3P term of neon. Within the experimental systematic uncertainties, our RMPS cross section is in agreement with the measurements by Johnston et al (1996 J. Phys. B: At. Mol. Opt. Phys. 29 531) for ionization from the 3P0 and 3P2 metastable levels. In addition, we have carried out a configuration-average time-dependent close-coupling (TDCC) calculation of ionization from the 2p53s configuration of neon and we find that the TDCC results agree well with RMPS cross sections, further supporting the accuracy of these advanced close-coupling methods. Finally, we have performed a distorted-wave calculation of the ionization cross section of metastable neon, and by comparing it with the RMPS results, we assess the effects of inter-channel coupling on this cross section.

Relativistic and Interchannel Coupling Effects in Photoionization Angular Distributions By Synchrotron Spectrocopy of Laser Cooled Atoms

We investigate the angular distribution of photoionization fragments at low photon energies (12-40 eV) in an open shell atom, by synchrotron radiation recoil ion momentum spectroscopy in a laser cooled and trapped sample. For cesium atoms, for which relativistic effects play an important role and the ion recoil is relatively small, we could determine large and rapid changes of the asymmetry parameter beta from two, observed for s electrons outside resonances and far from the Cooper minimum. They can be explained by relativistic effects and interchannel coupling arising from final state configuration mixing.

Some recently found interchannel coupling effects in atomic photoionization processes

The present article highlights some recently found interchannel coupling effects that must be taken into account to explain an atomic photoabsorption process. The importance of interchannel coupling effects is illustrated in a few select cases of recent interest: (a) sustenance of interchannel coupling effects at photon energies high enough where the independent particle approximation was believed to hold good; (b) demonstration of the generality of a new spin–orbit activated correlation effect as an essentially many-body collective event; (c) recognition of the scope and limitations of the indispensable approximation techniques to account for each ‘kind’ of correlation effect in configuration interactions between ‘discrete’ and ‘continuum’ channels; and (d) the recognition in some recent studies on atomic mercury, that contrary to the earliest expectations, dynamical interchannel coupling between dipole and quadrupole channels is important as found in the studies of photoionization of atomic xenon as well.

Effects of spin–orbit activated interchannel coupling on dipole photoelectron angular distribution asymmetry parameters

The effects of spin–orbit induced interchannel coupling on the dipole photoelectron angular asymmetry parameter β3d for Xe, Cs and Ba are explored using a modified version of the spin-polarized random phase approximation with exchange (SPRPAE) methodology. For Xe, is modified somewhat by the interchannel coupling in the vicinity of the 3d3/2 → ϵf shape resonance, and this effect is significantly more pronounced in Cs where the resonance is larger. In Ba, however, where f-wave orbital collapse has occurred, the shape resonance has moved below threshold and the effect of interchannel coupling on above the 3d3/2 threshold is negligible. But below the 3d3/2 threshold, is dominated by the huge broad 3d3/2 → 4f resonance.

Interchannel-coupling effects in the spin polarization of energetic photoelectrons

Effects of the interchannel coupling on the spin polarization of energetic photoelectrons emitted from atomic Ne valence subshells are examined. Like previously obtained results for cross sections and angular distributions, the photoelectron spin polarization parameters too are found considerably influenced by the coupling. The result completes a series of studies to finally conclude that the independent particle description is inadequate for the {\em entire} range of photoionization dynamics over the {\em full} spectral energy domain

Total cross sections and molecular frame photoelectron angular distributions in the N 1s photoionization of N2: An investigation of electron correlation effects

Multichannel scattering calculations have been performed to study the electron correlation effects in the N 1s photoionization of nitrogen molecules. Both the Hartree–Fock and configuration–interaction methods were applied in the calculations of target states, therefore the role of electron correlation effects in the target states can be investigated. Both single- and multichannel calculations have been performed to study the interchannel coupling effects. With correlated target states and coupled channels in the calculations, our calculated cross sections and asymmetry parameters are in good agreement with the experiments. The shape resonance is very well reproduced in our calculation, and we found weak coupling between channels leading to the 1σg−1 and 1σu−1 states, which agrees well with more recent experimental results by Hergenhahn et al. [J. Phys. Chem. 105, 5704 (2001)], but contradicts the random phase approximation results. The molecular frame photoelectron angular distributions obtained from our calculations are also in agreement with experiments. Our calculations also suggest an autoionization process via a double-excited state of the character of 1σg−11πu−11πg2, which is located near the threshold as observed by the experiments. Dynamics of the shake-up channels are also studied and compared with the experiments.

Sudden interchannel interaction in the Tl 6p ionization above the 5d threshold

The linear magnetic dichroism in the angular distribution of Tl 5d and 6p photoelectrons and their dynamical spin polarization have been measured between hnu = 30 and 50 eV. In contrast to the Xe 5p photoionization at the 4d threshold, our results show that above the Tl 5d threshold strong interchannel coupling effects induce a sudden increase in the asymptotic phase difference of the s and d waves for the Tl 6p ionization. This shows that the valence excitation is different for resonant (Xe 4d) and nonresonant (Tl 5d) excitation from subvalence shells.

Effects of interchannel coupling in a two-dimensional electron gas at high magnetic fields

Effects of interchannel coupling in a two-dimensional electron gas at high magnetic fields

Interchannel coupling effects in the resonant scattering of low-energy electrons from molecular targets

The scattering of low energy electrons from polyatomic targets is analysed in terms of various possible local potentials and the effects of dynamical coupling between single-particle channels on the possible resonant processes are also discussed.

Effects of relaxation and interchannel coupling in inner-shell photoionization of atomic ytterbium.

Photoionization cross sections, branching ratios, and photoelectron angular-distribution asymmetry parameters have been calculated for the Yb (Z=70) 4f, 4d, and 6s subshells using the relativistic random-phase approximation, the truncated relativistic random-phase approximation, and the relativistic random-phase approximation, modified to include relaxation effects. Comparisons are made between the various approximations and experimental data. Important relaxation effects for the 4f and 4d channels are noted and strong effects of interchannel coupling are noted for 6s photoionization in the region near the 4f and 5p thresholds.

Effect of interchannel coupling on the partial and total autoionization widths: Application to the 1s3s3p 4Po and 1s3p2 4P states for Z=2-5, 10.

We have computed the partial and total widths with interchannel coupling for the 1s3s3p $^{4}$${\mathit{P}}^{\mathit{o}}$ and 1s3${\mathit{p}}^{2}$ $^{4}$P states for Z=2--5 and 10, by solving the corresponding complex matrix equations constructed from square-integrable N-electron functions with numerical and analytic real and complex orbitals. The correlated wave functions are computed by the state-specific method. Oscillator strengths for one- and two-electron transitions from the metastable 1s2s2p $^{4}$${\mathit{P}}^{\mathit{o}}$ state suggest that the states are reachable in ${\mathrm{He}}^{\mathrm{\ensuremath{-}}}$ and in Li directly, (1s3${\mathit{p}}^{2}$ $^{4}$P), or via the 1s2${\mathit{p}}^{2}$ $^{4}$P state (1s3s3p $^{4}$${\mathit{P}}^{\mathit{o}}$). The effect of interchannel coupling on the partial widths in the case of the $^{4}$${\mathit{P}}^{\mathit{o}}$ state is considerable. On the contrary, interchannel coupling has no significant effect on the total widths. This computational result is discussed formally.

Triply differential (e,2e) cross sections for He from threshold to 50 eV above

Distored wave calculations of the triply differential cross sections for electron-impact ionization of He are presented for electrons sharing ≤50 eV excess energy (E x ) and having the coplanar, θ 12 =π geometry. The results are in good agreement with absolute experimental measurements of T. Rosel et al. [Phys. Rev. A 46 (1992) 2539-2552] and of A. J. Murray, M. B. Woolf, and F. H. Read [J. Phys. B 25 (1992) 3021-3036]. For final state electrons sharing 2 eV excess energy equally, many-body perturbation theory corrections to the author's first order (e,2e) triply differential cross sections [Phys. Rev. A 45 (1992) 4588-4603] are also presented. These results indicate that singlet-triplet interchannel coupling effects in the final state are very significant; that initial state correlations are significant near the perpendicular plane, i.e., θ 1 =θ 2 =π/2; and that the polarization effects we include are insignificant. Finally, comparison of our first order results with the semiclassical quantal theory results of D. S. F. Crothers [J. Phys. B 19 (1986) 463-483] shows excellent agreeement for excess energies E x ≥2 eV and for θ 1 =θ 2 =π/2; for E x <2 eV, the two results diverge as E x →0, indicating that experimental tests of the Wannier-Peterkop-Rau theory of three-body Coulomb breakup should be carried out in this energy region

Interchannel coupling and ground state correlation effects in the photoionization of CO

We describe a general procedure for applying the complex Kohn variational method to the calculation of molecular photoionization cross sections and asymmetry parameters. In this initial application of the method, we examine the effects of interchannel coupling and ground state correlation on the X 2Σ+(5σ−1), A 2Π(1π−1), and B 2Σ+(4σ−1) partial photoionization cross sections and asymmetry parameters for the CO molecule. We find that the dominant effect of interchannel coupling is to remove a spurious π→π* resonance feature from the continuum that appears at the frozen-core Hartree–Fock level. We also find that it appears to be important to combine the effects of final channel coupling with a correlated initial target state to achieve quantitatively correct cross sections.

Resonances and the effects of interchannel coupling in the photoionization of CS2

The photoionization cross sections of CS2 leading to the X 2Πg, A 2Πu, B 2Σu+, and C 2Σg+ states of CS2+ have been computed including the effects of interchannel coupling in the multichannel frozen core Hartree–Fock approximation. The results were obtained using the iterative Schwinger variational method using a partial wave expansion up to l=120. The photoionization cross sections are obtained in the mixed dipole representation which ensures that the Thomas–Reiche–Kuhn sum rule is satisfied. We have found shape resonances in the kπg and kπu continuum channels. In particular, the kπg resonance is seen to affect dramatically both total cross sections and photoelectron asymmetry parameters in all other channels when interchannel coupling is included. The form of the resonant wave function indicates that this resonance is caused by low lying virtual d orbitals on the sulfur atoms. Using a Schwinger-type iterative method we have found the position of the corresponding poles of the single channel T matrix in the complex plane. We have determined that the position of the pole is relatively insensitive to the interatomic bond lengths. This is indicative of the atomic nature of the resonant state. We have also determined cross sections and asymmetry parameters in the region of autoionization in the channel leading to the X 2Πg state of CS2+.

Effects of interchannel coupling on the photoionization cross sections of carbon dioxide

The photoionization cross sections of CO2 leading to the first four electronic states of CO+2 have been computed including the effects of interchannel coupling. The results were obtained in the Tamm–Dancoff approximation using the Padé-approximant C̃-functional method to solve the resulting scattering equations. All of the required matrix elements have been computed using single-center expansions and numerical integration of the resulting radial functions. An alternative approach for computing products of single-center-expanded functions is presented where the functions are transformed into a coordinate representation, then the product is computed, and finally the product is transformed back into the angular momentum representation. The computational effort required in this approach depends on the second power of the number of partial waves in contrast to the third power dependence found in methods used previously. The photoionization cross sections are obtained in the mixed dipole representation which ensures that the Thomas–Reiche–Kuhn sum rule is satisfied. In the coupled-channel approximation, the shape resonance in the 4σg→kσu channel is found to remain at the same energy and have the same width as was found in earlier single-channel calculations. Both the total cross section in the (4σg)−1 channel and the photoelectron asymmetry parameter are somewhat less affected by the resonance than in the single-channel approximation, but there is still a substantial disagreement with experimental data. The kσu shape resonance is found to modify the cross sections and asymmetry parameters in the other channels, with the largest effect being in the (3σu)−1 ionization channel. The full coupled-channel results, which include coupling among the 1πg→kπu,1πu→kπg,3σu→kσg, and 4σg→kσu channels, are found to significantly modify the cross section in the 1πg→kπu channel leading to good agreement between theory and experiment for the total ionization cross section in the (1πg)−1 channel. However, this coupling is found to significantly perturb the other channels, and in the case of the asymmetry parameters in the (3σu)−1 channel, this leads to relatively poor agreement with experimental data.

Xenon 4p photoionization near the 4d Cooper minimum: Interchannel coupling effects.

Recently, results of photoemission measurements for xenon were reported in the photon energy range 160 to 270 eV. These time-of-flight studies performed by Lindle et al. [Phys. Rev. A 37, 3808 (1988)] at the Stanford Synchrotron Radiation Laboratory suggested that photoemission from the Xe 4p subshell is remarkably influenced by photoionization channels from the 4d subshell. In the present study, calculations in the relativistic random-phase approximation have been made in which we have included interchannel coupling between dipole photoionization channels from the 4p and 4d subshells. The effect of this interchannel coupling is perceptible in the results for 4p photoionization.

Photoemission from Xe in the vicinity of the 4d Cooper minimum.

Partial photoionization cross sections and angular-distribution asymmetry parameters have been determined for the Xe 4d and ``4p'' subshells in the photon-energy region of the 4d Cooper minimum (160 eV to 270 eV, to 520 eV for the 4d asymmetry parameter). The Cooper minimum is observed as a distinct cross-section minimum in the 4d photoionization channel. The 4d angular-distribution results are in excellent agreement with Dirac-Fock and relativistic random-phase-approximation calculations and with previous measurements. Effects of interchannel coupling are evident in the ``4p'' results. Both the partial cross section and the angular distribution for the ``4p'' photoelectron peak track the 4d cross section and angular distribution as functions of photon energy. A similar result is observed for the summed intensity and the angular distribution of the 4${d}^{\mathrm{\ensuremath{-}}1}$5${p}^{\mathrm{\ensuremath{-}}1}$np satellites of the 4d main line, which is probably due to the effects of electron correlation between the 4d and the satellite photoionization channels.

PHOTOIONIZATION QF XENON WITH SOFT X-RAYS

Partial cross sections and angular-distribution asymmetry parameters were measured for Xe 4d and 3d subshell photoionization using photon energies of 250 to 1000 eV. Effects of interchannel coupling are observed at the onset of the 3d cross section. Outside this energy region our results agree qualitatively with the predictions of the Hartree-Fock theory if one takes into account al1 photoelectrons associated with a given subshell including multielectron processes.

Satellite structure in atomic spectra. IV. The L23-MM Auger spectrum of argon

For pt.III see ibid., vol.15, no.12, p.1811 (1982). The L23-MM Auger spectrum of argon has been calculated using correlated initial- and final-state wavefunctions based upon a frozen-core model developed previously. The energies and intensities of the 3P satellite lines have been investigated as well as the diagram lines. Strong interactions between (3s3p) and (3p3(2D0))nd states, and between (3s2) and (3s3p2(2D))nd states are evident. The effect of these interactions is to shift the diagram line energies downwards by as much as 9.3 eV for the (3s2)1S line and decrease the diagram line intensities. The intensity lost from the diagram lines appears in intense satellite structure due to the interacting final states. On the basis of the calculations, the assignments of the (3p3(2D0))3d3P0 and (3p3(2P0))3d 3P0 optical levels have been interchanged. Some of the final-state satellite doublets have been assigned. Most of the assignments agree with those made earlier by McGuire (1975). From the diagram line intensity calculations it is concluded that interchannel coupling effects are important in redistributing the intensities especially for L23-M23M23 lines. Intensity calculations for final-state 'shake-up', initial-state and spectator satellites indicate that only spectator satellites are of any importance. The total spectator satellite intensities reflect the initial-state population.

Angular Distribution ofXe 5s→εpPhotoelectrons: Direct Evidence for Anisotropic Final-State Interaction

The angular distribution of Xe $\mathrm{Xe} 5s\ensuremath{\rightarrow}\ensuremath{\epsilon}p$ photoelectrons has been measured yielding an asymmetry parameter $\ensuremath{\beta}=1.4\ifmmode\pm\else\textpm\fi{}0.1$ at 304 \ifmmode \breve{A}\else \u{A}\fi{}. This deviation from the intuitively expected result $\ensuremath{\beta}=2$ (for a pure ${cos}^{2}\ensuremath{\theta}$ distribution) for photoionization of an $s$ subshell of a $^{1}s_{0}$ target is a direct manifestation of anisotropic interactions experienced by the electron during the ejection process. The combined effect of interchannel and spin-orbit coupling is proposed as the likely interaction mechanism.