Table of contents

We demonstrate the feasibility of tackling continuum states with grid techniques. Our method consists of using a Lagrange mesh within the finite R-matrix sphere. We test it on a simple model scattering problem. In contrast with previous proposals, it is capable of handling anisotropic interactions, an essential property to consider realistic applications.

We demonstrate the first observation of X-ray fluorescence resulting from inner-shell photoionization by a subpicosecond laser-produced plasma X-ray source. 8 keV electrons produced by a 100 fs laser at 3*10¹⁶ W cm^-2 intensity create K-shell vacancies in a Ti layer. This generates a 4.51 keV K alpha X-ray pulse which is used to photoionize a Ca sample. Fluorescence of Ca at 3.69 keV (K alpha ) and 4.01 keV (K beta ) is observed as a result of Ti K alpha photoionization.

Recent experimental investigations of the high-order above-threshold ionization peaks near the onset of the plateau have exhibited anomalous angular distributions of the emitted photoelectrons with pronounced side lobes surrounding emission in the direction of the laser electric field. It is shown that the existence and angular position of these side lobes are consequences of the classical kinematics of electrons in laser fields.

The fluorescence spectra observed after the excitation of the rare gases (Rg) by ionizing radiation consist of several continua. Using a Rittner potential the higher continua are assigned to the radiant decay of the species Rg²₂+ and Rg₃²⁺ into the final states Rg⁺(p₃2/)+Rg⁺(p₃2/, P₁2/) and Rg⁺₂(² Sigma , ² Pi )+Rg⁺(p₃2/, P₁2/), respectively.

We study double excitation of helium by 2 MeV amu^-1 protons and antiprotons (or electrons) and the strong effect on the 2s²¹S^e and 2p²¹D^e resonance structures in the doubly differential ionization cross sections associated with a change of the projectile charge sign. This charge-sign effect is entirely due to an interference between different partial waves that disappears in angle integrated cross sections. We show that this peculiar behaviour results from a lack of interference between the contribution to the double excitation amplitude from the first and second order of the Born series. Following the analysis of Stolterfoht (1993), our results suggest a loss of time ordering in the second-order contribution to the excitation of the 2s²¹S^e and 2p²¹D^e resonances, i.e. excitation in two independent steps.

Elastic differential cross sections for electron scattering from argon as a function of electron energy from 20-110 eV are reported at angles of 60, 90 and 120 degrees . Comparisons are made with corresponding positron scattering studies exhibiting structure at 55-60 eV. The absence of structure in the electron scattering results limits the possible mechanisms which can give rise to the positron structures.

Here we study the single-particle and the electron-pair densities for the ground state of two-electron atomic systems using the wavefunctions which provide the best values for the energy known up to now. Special attention is paid to the convergence of some properties of the densities when we increase the dimension of the basis used. With these functions we have increased significantly the precision in the determination of the single-particle and the electron-pair densities at the origin and, therefore, of Kato's cusp conditions.

The Hartree-Fock ground state potential energy curve for the carbon monoxide molecule is calculated using both the finite difference and the finite basis set methods. The results of calculations performed at ten internuclear separations are reported and a comparision is made of the calculated potential energy curves over the range of internuclear separations, 1.970-2.330 a₀, around the experimental equilibrium value. Potential energy curve constants and spectroscopic constants are determined.

Atomic information entropies are computed from configuration interaction wavefunctions for members of the lithium isoelectronic series. For each member of the series, a sequence of wavefunctions built from increasingly larger basis sets is generated which satisfies a density convergence criterion, and exhibits a monotonic behaviour with respect to the density generated from the wavefunctions. Studies of the behaviour of the entropies along a sequence for a particular atom includes trends which have been shown to be present at the Hartree-Fock level. Results suggest that the position and momentum entropies are sensitive to small changes in density and thus can be useful indicators of the quality of the density. The sum of the entropies is also shown to be a useful indicator when density differences are larger. The nature of the information entropy-energy relationship is examined.

Analytical expressions for the spherically averaged molecular charge distributions and radial moments in position and momentum space have been formulated. Calculations of radial moments with both self-consistent-field and configuration interaction wavefunctions have been carried out for 22 molecules in their ground states. The applications of the current results in density functional theory are discussed.

Magnetic dipole transitions between the fine structure components of atomic Se and Sn in their ground ³P states have been observed by the technique of carbon monoxide laser magnetic resonance. The fine structure intervals have been measured much more accurately than previously. For Se, the ³P₁-³P₂ transition was detected. Measurements were made for ⁷⁷Se through the nuclear hyperfine splitting (I= 1/2 ) and for ⁷⁸Se and ⁸⁰Se by the observation of Lamb (saturation) dips. The J=1-2 interval was measured as 1989.49812(75), 1989.49798(36) and 1989.49831(36) cm^-1 for ⁷⁷Se, ⁷⁸Se and ⁸⁰Se respectively. The ⁸⁰Se-⁷⁸Se isotopic shift has been measured very accurately from saturation dips. The g_J-factor for the ³P₂ component was estimated to be 1.49129(14) from Zeeman splittings. The parameters which describe the hyperfine splitting for ⁷⁷Se, were determined to be A₁=-137(88) MHz and A₂=544(45) MHz. For Sn, both J=1 from 0 and 2 from 1 transitions were detected. Hyperfine structure was observed, but not resolved, for ¹¹⁷Sn and ¹¹⁹Sn, both of which have I= 1/2 . The J=1-0 interval was measured to be 1691.807 65(75) and 1691.807 10(75) cm^-1 for the unresolved pairs ¹¹⁷119/Sn and ¹¹⁸120/Sn respectively. The J=2-1 interval for the unresolved pair ¹¹⁸120/Sn was determined to be 1735.864 05(75) cm^-1.

Using a potential model method, we calculate the Stark evolution of the diamagnetic manifolds of Rydberg caesium. Results show that general features appearing in hydrogen lithium and helium are found in caesium. But significant modifications characteristic of atomic species and magnetic field strength are also evident and are attributed to core effects. We also find numerical manifestations of the fine structure effect of Rydberg states in strong external fields which have been long neglected.

The processes with two electrons in the final continuum state are considered for small energy excess E above the threshold. The distribution of the total energy E between the electrons is calculated within the quantum Wannier theory where the quadratic approximation for the Hamiltonian is employed in the vicinity of the Wannier ridge. The problem is cast in terms of the wavepacket propagation. The Green function formulation is employed and its physical implications are analysed. The final distribution is shown to be uniform and essentially independent of the boundary condition at the border of the inner Wannier zone. The extended model is developed which does not resort to the quadratic approximation but accounts for the attractive Coulomb singularities of the potential. It is solved numerically using the grid method. The manifestations of the competing process are discussed (single ionization with the other electron in a high Rydberg state).

An alkali-like ion interaction with inner electrons of an alkali-like ion leads to a significant increase in the photoionization cross section of the outer s electron. This occurs not only for ground state ions with one s electron in the outer shell, but also when the outer s electron is in an excited state. The reason for this amplification, in addition to coherent enhancement in summing of the correlation amplitudes, is that the zero in the direct amplitude occurs below threshold. This leads to a constructive interference with the correlation amplitude above the photoionization threshold, in contrast to a destructive interference in the case of a neutral atom with the same electronic configuration, for which the zero occurs above threshold.

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.

Spectra emitted from N₂- and O₂-doped solid krypton and xenon have for the first time been studied using energetic alpha and proton beams generated by a cyclotron. The studies were made in the NIR, visible and UV wavelength regions. With nitrogen gas doping, the Vegard-Kaplan bands of N₂ show strong spectral features. With oxygen gas doping, the most prominent feature is the atmospheric absorption system of O₂ in the IR and red wavelength regions. Vibrational constants for the transitions are given. Time-resolved spectroscopy has been used to study the non-radiative depopulation of the vibrational states. The results are consistent with the vibrational relaxation process in steps of two vibrational quanta. Pure krypton and xenon solids show broad band structures in agreement with published photographic recordings. It is concluded that heavy energetic particles give similar excitation conditions for the dopants in solid rare gas matrices as other non-selective matrix excitation methods. The studies are motivated by the search for new media for the detection of ionizing radiation by laser amplified scintillation.

The vanadium K_beta /K_alpha intensity ratios of some simple vanadium compounds, VO, V₂O₃, VO₂, V₆O₁₃, V₂O₅, VN and VC have been measured by using a Si(Li) X-ray spectrometer. Strong covalent compounds in general have large vanadium K_beta /K_alpha intensity ratios which can be attributed to the non-diagram lines due to the band mixing between the metal and the ligands. For relatively weak covalent compounds the contribution from the change of the diagram line may have to be included.

Single-centred finite Hilbert basis set calculations, using bases with a substantial number of orbitals, have been shown to accurately reproduce excitation and electron removal cross sections for the p+H(1s) and p+H(n=2) systems. The present paper extends the method to the p+He⁺(1s) and p+He⁺(n=2) collision systems at incident projectile energies of 25 keV and above. Cross sections for 1s to 2l', 3l' and 4l', 2s and 2p to 3l' and 4l' excitation and for electron removal from the 1s, 2s and 2p are presented and compared to experiment and to other theoretical calculations.

Atomic excitation and ionization processes affect the shape of yield curves of narrow nuclear resonances obtained with high beam energy resolution. Yield curves for the narrow resonances in the ²¹Ne+p reaction at E_p=126, 272 and 291 keV and for the E_p=309 keV resonance in ²³Na+p were measured with very low target densities (i.e., with thin targets fulfilling atomic single collision conditions). The ionization probabilities of K and L electron shells were determined at an impact parameter close to zero. Collision spectra were measured which provide information on the energy loss integrated over all impact parameters in a single collision. From an analytical description for the thin-target yield curve and the experimental collision spectra, the experimental thick-target yield curves with an intense Lewis peak could be reproduced with a Monte Carlo computer simulation.

L-shell X-ray production cross sections by 0.5 to 5.0 MeV He⁺ and 5.5 to 8.0 MeV He²⁺ ions are reported for elements with X-rays between 0.70 keV and 1.19 keV. Thin targets of ₂₆Fe, ₂₈Ni, ₂₉Cu, ₃₀Zn, ₃₁Ga and ₃₂Ge were manufactured using a cleaning process that reduced the level of light element impurities. The X-rays were measured with a windowless Si(Li) detector, whose efficiency was determined by the atomic-field bremsstrahlung method. The data are compared to the predictions of the first-order Born and the ECPSSR theories using the single- and multiple-hole fluorescence yields. The ECPSSR theory is clearly superior to the first-order Born approximation, although the data fall-when single-hole fluorescence yields are used-on the average about 5% below the ECPSSR. At the lowest velocity, however, on the average this theory underestimates our measurements by 15% when single-hole fluorescence yields are employed. When multiple-hole fluorescence yields are used, the theory is within 3% of the averaged data at the lowest ion velocity. Coupled-state calculations that account for intrashell transitions and recently proposed modifications to the ECPSSR treatment of the binding effect give larger cross sections than the ECPSSR and hence-when multiple-hole fluorescence yields are used-they are in larger disagreement with our measurements.

By considering the asymptotic forms of continuum distorted-waves (CDWs) we present a new variational CDW ansatz which uniquely lifts the degeneracy between outgoing and incoming waves. When the target and projectile charges are equal, this ansatz has the important property that the wavefunctions form a time-symmetric basis set. We perform a two-state calculation employing this new time-symmetric ansatz for the specific case of charge transfer between protons and atomic hydrogen in the ground state. We calculate total cross sections in the energy range 10-1000 keV and show that the results at low and intermediate energies are significantly improved over previous two-state coupled channel continuum distorted-wave theories.

Excitation rate coefficients for transitions among the lowest 12 excited states namely (1s²2s²2p⁶) 3s²¹S, 3s3p ^1.3P degrees , 3P²¹D, ³P, ¹S, 3s4s^1.3S, 3s3d^1.3D and 3s4p ^1.3P degrees , of Al II are reported in a wide electron temperature range of 5000 to 100000 K. These are based on collision strengths obtained from the R-matrix program using configuration interaction wavefunctions. On comparison, the earlier available excitation rates are found to be in error by more than an order of magnitude for some of the transitions. These differences are discussed and the accuracy of the present rates is assessed.

Absolute cross sections for the single ionization of Al⁺ and Cd⁺ by energy-resolved electrons have been measured from their thresholds to 115 eV and 95 eV respectively. The Al⁺ cross section was similar in magnitude to those reported by Montague and Harrison (1983) and Belie et al (1987) but differed in detail. Contributions from excitation-autoionization and the states that may give rise to it are discussed. The Cd⁺ cross section was similar to that of Belie et al except that considerable step-like structure due to indirect ionization processes was resolved for electron energies above 20 eV.

A new R-matrix approach for calculating cross sections and rate coefficients for electron-impact excitation of complex atoms and ions is described. This approach, based on an expansion of the total wavefunction in target configurations rather than in individual target states and taking advantage of the special status of the scattered electron in the collisional wavefunction, enables the angular integrals to be performed very much more efficiently than hitherto. It also enables electron correlation effects in the target and in the electron-target collision complex to be treated consistently, eliminating pseudo-resonances which have caused serious difficulties in some earlier work. A major new program package RMATRX II has been written that implements this approach and, as an example, electron-impact excitation of Fe²⁺ is considered where the four target configurations 3d⁶, 3d⁵4s, 3d⁵4p and 3d⁵4d are retained in the expansion of the total wavefunction. RMATRX II is compared with the standard R-matrix program package and is found to be much more efficient showing that accurate electron scattering calculations involving complex targets, such as the astrophysically important low ionization stages of iron-peak elements, are now possible.

We consider the scattering of transversely polarized electrons by spatially oriented and rotating oxygen molecules, and present results for polarization fractions and differential cross sections. We show that the polarization fraction can deviate significantly from unity, unlike the randomly oriented case, for some molecular orientations and rotational transitions. These deviations occur when the differential cross sections are small and their contribution to the randomly oriented case is small.

Absolute elastic cross sections for e-C₂F₆ collisions have been measured for impact energies of 2 to 100 eV and scattering angles from 10 to 130 degrees . From these, the integrated and momentum transfer cross sections were obtained by extrapolation and numerical integration. Energy loss spectra were measured for vibrational excitation. The vibrational excitation functions show resonances at 4.3 eV and 8.5 eV. By decomposition of the loss spectra and symmetry analysis according to angular correlation theory, the resonances have been assigned to a temporary trapping of the electrons in the a_2u (4.3 eV) and e_u (8.5 eV) antibonding orbitals of the molecule.

We report experimental measurements of the differential cross sections for scattering of electrons in the range 8-20 eV from helium and argon through scattering angles of 10 degrees -140 degrees in the presence of a high intensity ( approximately 10⁸ W cm^-2) carbon dioxide laser. The majority of the results reported are scattering from argon under conditions for which the Kroll-Watson approximation, usually applied to these types of measurements, predicts zero cross sections. The cross sections reported range up to approximately 12% of the appropriate field-free elastic scattering cross sections in strong disagreement with the theoretical predictions.