Table of contents

Recent experiments on electron capture from elliptic Rydberg states by Ehrenreich et al. (1994) have shown that the total electron capture cross section depends strongly on the eccentricity, e, of the elliptic state and the orbit direction of the electron. By carrying out close-coupling calculations for protons colliding with elliptic H (n=3, 4, 5) atoms we show that the dependence of total capture cross section on e varies weakly with the principal quantum number n. We also present the dependence of the total capture cross section as function of the angle, phi , between the incoming ions and the classical plane of the elliptic orbital. Both the dependence on e and on phi are shown to be consistent with the criteria of velocity matching for the electron capture process.

The three-dimensional exchange integrals in the impact-parameter treatment of heavy particle collisions can be transformed to one-dimensional integrals over finite range. The closed form formula for the integrands of these one-dimensional integrals is derived.

Coupled-state calculations are reported for s-, p- and d-wave scattering by lithium of positrons with momentum k<or=0.45 atomic units (au). Comparison is made with the accurate variational results of Watts and Humberston for s and p waves. Very good convergence towards the variational numbers is found. In the most sophisticated approximation, Ps(1s, 2s, 3s, 4s, 2p, 3p, 4p, 3d, 4d)+Li(2s, 2p, 3s, 3p, 3d), where a bar denotes a pseudostate, resonances are seen in the s- and d-wave cross sections near k=0.30 au and k=0.27 au respectively. Experiments in the range k<or approximately=0.45 au (energies<or approximately=2.75 eV) are strongly urged.

A direct comparison of the ionic levels observed in multiphoton-induced Xe(L) emission from Xe clusters with corresponding charge states experimentally produced with EBIT excitation confirms two new fundamental properties of the multiphoton interaction. They are (1) the dominant role of a coherent multi-electron mechanism in the excitation of the atoms in the cluster and (2) the collaterally imperative presence of channels of ionization involving multi-electron inner-shell ejection. Both characteristics arise because the coherent multi-electron motion induced by the external field simulates the behaviour of a light particle carrying a heavy charge, thereby transforming the fundamental coupling strength for the interaction of electrons with matter from the customary factor alpha ( approximately 1/137) to the strongly enhanced value Z² alpha . The estimated number (Z) of electrons participating in the coherent multi-electron amplitude for excitation of the Xe(L) radiation (2-3 AA) is estimated to be Z approximately=26. Since Z²>10² can be practically attained for the majority of atoms, important general consequences follow for the production of new forms of highly energetic non-equilibrium states of matter.

Individual multiphoton excited clusters can exhibit superradiance in the 1-10 keV spectral region. This process may (1) confer a high level of coherence on the emission of an X-ray amplifier, (2) provide control of the emitted wavelength, and (3) produce X-ray pulses having durations <or approximately=10 fs. For a transition having a lifetime determined by radiative decay, the lower bound on the wavelength of the superradiance for a cluster of n atoms having an interparticle spacing of r₀ is lambda _xr approximately 2.9r₀n^-16/. For a system having a lifetime governed by Auger decay, the corresponding upper bound on the wavelength is lambda _xa=(2 pi )²n¹3/( mu ² tau _a/r₀²h(cross)), with mu representing the transition dipole matrix element and tau _a^-1 denoting the Auger decay rate.

The electromagnetic response of the fullerene C₆₀ in the energy range 0<or=E<or=35 eV, exhausting 84% of the Thomas-Reiche-Kuhn sum rule, has been worked out using the local density and random phase approximations. A detailed correspondence between the calculated pi -plasmons and the peaks determined in optical/UV studies is found. The damping width ( approximately=12 eV) of the Mie resonance observed at approximately=20 eV is well reproduced, and is due to Landau damping. This resonance is found to be determined by sigma to sigma and, to a large extent, also by sigma to pi , delta particle-hole transitions.

Anomalously intense photon emission from He I (n=3 to n=2) has been observed when helium plasmas contact with cold hydrogen molecules. This enhancement of photon emission can be understood to be due to accidental resonant double electron capture by alpha particles, He²⁺, when molecular hydrogen gases are injected into helium plasmas: He²⁺+H₂ to He*(1s3l)+H₂²⁺. The present observation has also confirmed theoretical prediction of strongly selective double electron capture by alpha particles in He²⁺+H₂ collisions.

The Dirac Coulomb Green function in the representation of the annihilation and creation operators is established and on the basis of the application of this new operator representation an algebraic method of calculation of atomic characteristics is briefly discussed. The relativistic polarizability of hydrogen-like atoms is calculated and its simple analytical formula is obtained.

The triply differential cross section (TDCS) for double photoionization of helium is calculated using a three-body Coulomb correlated final state wavefunction which treats the three Coulomb particles on an equal basis. Complementary to preceding publications by Maulbetsch and Briggs (1992, 1993) emphasis is given to the case of unequal sharing of the available excess energy. The breakdown of selection rules which apply in the equal energy sharing case leads to a greater variety of angular patterns. Excellent agreement is obtained with the first experimental results for unequal energy sharing.

The systematics of resonance parameters in the photoionization of He-like ions above the N=2 threshold is discussed along the isoelectronic series. We show that a simple Z scaling of the resonance parameters is only achieved for large nuclear charges due to the unexpected cancellation of the Fano interferences near Z=5.

The Ge^- photodetachment cross sections from the outer 4p, 4s and inner 3d subshells are calculated within the spin-polarized version of the random phase approximation with exchange accounting for the intershell interactions and relaxation effects. It is revealed that rearrangement of the residual atomic core changes the positions and profiles of the window-type resonances in the outer-subshell cross sections, which occur due to transitions of the 4s electron to vacant states in the 4p subshell. Similar transitions take place for photodetachment from the inner 3d subshell, for which qualitative changes in the near threshold 3d cross section behaviour, due to relaxation effects, have been found.

The relative cross sections for single and double photoionization of the refractory element platinum have been measured in the range of the 4f and 5p excitations using synchrotron radiation, an atomic beam produced by thermal evaporation, and a pulsed field time-of-flight technique. The total photoionization cross section has been calculated with a relativistic Hartree-Fock method. The result is compared to the total ion yield and shows good agreement. A tentative assignment of the transitions giving the main contributions to the cross section is made. The ratio of the direct (5p+4f) and (5d+6s) ionization is calculated and compared to the relative amount of double ionization.

We have studied high Rydberg states of krypton which are excitable from the 5p[³/₂] states with J=1 and 2, using one-photon resonant, two-photon ionization beginning with the metastable 5s[³/₂]₂ state. An analysis of the ns and nd Rydberg state energies has allowed us to determine the ionization potential of Kr to an accuracy which is better than previous work, and resolves a discrepancy between previous laser-based and conventional spectroscopic results. The use of these intermediate states for the excitation of Rydberg states makes it possible to observe high members of series such as (n+2)s[³/₂]₁ and nd[¹/₂]₁ which cannot be efficiently excited from the 5p' intermediate states which have been used in previous works.

In a quasiclassical approximation analytical solutions of the Schrodinger equation are found for the electron affected by both Coulomb and light fields under an assumption about slow angular motion. The applicability conditions of these solutions and approximations are evaluated. The solutions are shown to be valid in a range of light field strengths that includes both weak- and strong-field regions. The quasiclassical Coulomb-Volkov wavefunctions found are used to describe strong-field ionization from Rydberg levels. The theory predicts a qualitatively new behaviour of the ionization rate and of the strong-field atomic quasienergies in their dependence on the light field strength amplitude epsilon ₀. In particular, multiple stabilization and destabilization regions following each other are predicted.

We derive symmetry selection rules for polarized resonance inelastic X-ray scattering (RIXS) in randomly oriented molecules belonging to any of the 32 crystallographic point groups or to the two groups of linear molecules. Compact formulae are derived for cross sections of RIXS processes in randomly oriented molecules for photons of arbitrary polarization; linear, circular or elliptical polarization. It is shown that a specification of polarization ratios of RIXS cross sections yields sufficient information to permit an unequivocal identification of the symmetry species of the occupied and unoccupied molecular orbitals (MOS). The given tabulation of RIXS symmetry selection rules can be used as a quick guide to assign symmetry properties of molecules of experimental interest.

In the present study we have shown that the dissociative recombination of Ne₂⁺ ions which gives rise to non-thermalized Ne atoms may be regarded as the main process responsible for the distortion of the profiles of the 753.5 and 754.4 nm neon lines. The distorted profiles were analysed as a superposition of the Ballik profile for thermalized atoms and the modified Ballik profile for non-thermalized atoms and the Lorentzian and Gaussian widths as well as the relative density of non-thermalized and thermalized atoms have been determined. We have found for the dissociation energy of the neon molecular ion Ne₂⁺ in the ² Sigma _u⁺ state the value D=(1.20+or-0.08) eV. We have also found for the 3d₆ excited neon atoms of speed 1275 m s^-1 the excitation transfer cross section Q_3d6 with a value Q_3d6=(39+or-15)*10^-16 cm².

We have investigated the collisional excitation of hydrogen atoms by protons and multiply charged ions at intermediate velocities applying the optical method. The population of the np levels (n=2,...,6) has been determined for a variety of projectiles with charge states q ranging from 1+ to 11+ and scaled velocities nu / square root (q between 0.7 and 5.6 au. The cross sections are shown to fulfil the scaling relation sigma /q=f( nu ²/q) with respect to the projectile charge g and velocity nu for q>or=3. The relevance of scaled intermediate velocities for a classification of excitation mechanism is pointed out.

Our previous forced impulse method calculations of single and double ionization of helium by protons and antiprotons have been improved by including d orbitals in the target centred basis. The calculations are in good agreement with experimental measurements of the ratio R of double to single ionization, without the 1.35 scaling factor we applied to our previous results. We also compare the separate single and double ionization cross sections to experiment and find good agreement. Experimental cross sections differential in projectile scattering angle at large angle (greater than 2.5 mrad) are compared to our impact parameter dependent ionization probabilities at small impact parameter, for the double to single ratio. The agreement is good, except at the lowest energy we have considered, 0.3 MeV.

The L-shell X-ray production cross sections of ₄₉In, ₅₀Sn, ₅₂Te, ₅₅Cs, ₅₇La, ₅₉Pr, ₆₀Nd and ₆₂Sm for protons of energy between 2 and 6 MeV have been measured. From these measurements, the individual X-ray production cross sections for the L_alpha , L_{beta 1.3.4}, L_{beta 2,15}, L_{gamma 1.5}, L_{L gamma 2.3}, L_{gamma 4.4'} and L_l lines, together with the cross section ratios L_alpha /L_beta and L_alpha /L_gamma , are calculated and reported. The results are compared with the experimental data published by other authors and with the predictions of the ECPSSR theory.

A high resolution electron spectrometry of the (3lnl') Rydberg series populated in N⁷⁺+He and Ne¹⁰⁺+He collisions at 10 q keV, 10 degrees allows us to observe, for the first time by this method, two post-collisional effects. First, it is found with nitrogen ions that, when n increases from n=4 to 9, the L-distribution peaks more and more on the high angular momentum states. This is qualitatively understood as a Stark deformation of the Rydberg orbit by the Coulomb field of the receding ion. Also, in the n range where the double capture process populates symmetrical 4l4l' states (n>9), an enhancement of the intensities of the 3lnl' Rydberg lines is observed for both collisional systems. This is thought to be a signature of the so-called auto transfer to Rydberg states effect. The transfer of population from the 4l4l' to the 3lnl' states is found to be favoured against a direct autoionization of these 4l4l' states into the n=2 continuum. These experimental findings together with preliminary spectroscopic calculations concerning the configuration interaction of the Ne⁸⁺(4l4l') states with the Ne⁸⁺(3lnl') Rydberg series are also discussed within the context of the electron stabilization which follows a double capture.

Qualitatively different features are predicted for the radiative tail in the electron spectrum due to bremsstrahlung in electron scattering in comparing the cases of slightly ionized and highly ionized atoms. For slightly ionized atoms there are minima and maxima in the electron angular distribution at fixed energies when a soft photon is radiated and in the electron energy spectrum at fixed small angle. For highly ionized atoms these distributions and spectra are monotonic. Our analytic results are obtained in the framework of the Born approximation using a simple analytical potential; the more realistic Hartree-Fock potential also exhibits these features.

This work investigates theoretically the triply differential cross sections for electron-impact ionization of He in doubly symmetric geometry, i.e. equal energies and equal emission angles for the two electrons in the final state. Dominant ionization mechanisms are identified and the contribution of different scattering amplitudes are discussed. It turns out that the cross section in doubly symmetric geometry is strongly influenced by interference of these amplitudes. In particular, the theory predicts zero cross sections due to interference under certain kinematical conditions. This explains the appearance of deep minima in the measured differential cross sections.

The R-matrix method is used to obtain collision strengths for the electron impact excitation of the oxygen-like Ca XIII ion for all transitions between the 16 target states of even and odd parity. The wavefunctions for these states are generated using the configuration interaction method. The calculated values of the target state energies agree with the experimental values within 7%. The initial and final state wavefunctions for the composite (electron+ion) system are obtained by retaining these states in the R-matrix basis function expansion. In the low partial wave region (total angular momentum L<or=9) the standard full exchange R-matrix method is used for the scattering calculation. Beyond this, a no-exchange R-matrix method is applied to calculate collision strengths for a sufficiently large number of partial waves (up to total angular momentum L=30). The Burgess sum rule is used to determine the high partial wave (L>30) contribution to the cross sections for dipole allowed transitions. However, it is seen that collision strengths for almost all the transitions, including the dipole allowed, converge below L=30. The calculations are carried out in LS coupling and the results are compared with those obtained in the distorted-wave approximation along with other results, where available.

Absolute differential cross sections (DCS) for elastic electron scattering and for electron impact excitation of xenon to the states 6s(³/₂)_1,2 were measured with the static-gas-target technique. In the energy range from 40 to 100 eV (elastic) and from 15 to 100 eV (inelastic) angles were varied from 3 degrees (elastic) and 6 degrees (inelastic) to 135 degrees . The integral cross section for elastic scattering was calculated by subjecting the differential cross sections to a phaseshift analysis. A representation of the inelastic DCS in terms of the generalized oscillator strength was used to determine the optical oscillator strength of the excitation to the state 6s(³/₂)₂ and the integral electron impact excitation cross section. Our data were compared with recent theoretical results.

The complete valence shell electron separation energy spectra and momentum distributions are measured for cyclopropane (C₃H₆) by high momentum resolution electron momentum spectroscopy (EMS) at a total energy of 1500 eV. Many-body calculations of the separation energies and spectroscopic factors using Green function methods are carried out and compared with the experimental data. The measured momentum distributions are compared with those calculated in the plane wave impulse approximation (PWIA) formalism using the double zeta quality molecular wavefunction of Snyder and Basch (1972). The agreement between the measured momentum distributions and the PWIA-SCF orbital momentum distributions is, in general, fair, although for the outer valence 2e' and 1e" states the SCF wavefunction underestimates the density at low momentum. The inner valence 1e' and 1a₁^' orbitals are found to be split by final state correlation effects. The agreement between the measured and calculated spectroscopic factors and separation energies is quite good, although the measured separation energy spectra contain significant strength up to 40 eV, this strength being of 1e' and 1a₁^' origin. The controversy with respect to the assignment of the 1a₂^" and 2a₁^' valence orbitals is examined in detail with the present EMS results and third-order algebraic diagrammatic construction ADC(3) Green function calculations seemingly leading to a diametrically opposite conclusion.

Resonant R-matrix theory is used to describe inelastic electron-methylchloride scattering in a semiempirical way. Parameters of the theory are obtained by fitting differential vibrational excitation cross sections to experimental ones. Some specific features of the vibrational excitation cross sections are discussed, particularly the competition between the dipole and resonant contributions. The obtained parameters are used for calculations of the dissociative attachment from CH₃Cl and CD₃Cl with production of the Cl^- negative ions. The results for a low-energy peak agree satisfactorily with the swarm analysis of Datskos et al. (1990) at temperatures of about 700-800 K. At lower temperatures the experimental cross sections in all measurements strongly exceed the theoretical ones.

A final-state wavefunction, involving three appropriate confluent hypergeometric functions, which satisfies the correct asymptotic condition is used to study positron-impact ionization of atomic hydrogen. Results are reported for the double as well as triple differential cross sections and compared with the available theoretical findings. The double differential cross section curves for different ejection angles show the presence of a cusp-like structure at the region of equal energy sharing by the outgoing positron and the ejected electron. This is in conformity with the findings of Mandal et al. (1986) at zero ejection angle but unlike the structure exhibited by the curves of Schultz and Reinhold (1990).

Excitation and ionization cross sections of He, Ne and Ar atoms by positron impact have been presented in the energy range from the respective thresholds up to 100 eV using a time-of-flight (TOF) method. These data were compared with the theoretical and experimental data. For excitation, the present data for Ar in the whole range and for Ne in the range 50-100 eV coincide roughly with the electron impact data of de Heer and co-workers. However, for He the present data are much smaller than the electron data above 30 eV, but in good agreement with the theoretical data of Varracchio below 35 eV. The present data for the ionization cross section for He coincide well with the experimental data of Fromme et al. in the range less than 100 eV.

Laser spectroscopy of a thermal atomic beam of natural lanthanum has been performed. An active filtering technique has been used to remove spurious cavity modes from the laser output, leading to unambiguous identification of hyperfine structure associated with ¹³⁸La. Hyperfine structure coefficients and isotope shifts for several transitions of the isotopes ¹³⁸La and ¹³⁹La are presented.

Studies of multiphoton-induced X-ray generation in Kr and Xe clusters give direct information concerning (1) the atom-specific energy transfer rate, (2) the dependence of the X-ray yield on the strength of the intra-cluster inelastic electron scattering cross section, and (3) the threshold intensity for X-ray generation. Measurements of these three classes of observables with subpicosecond ( approximately 300 fs) ultraviolet (248 nm) radiation at a maximum intensity of approximately 10¹⁹ W cm^-2 all indicate that the non-linear coupling to the cluster has an anomalous strength with respect to that derivable from conventional single-particle interactions. Five cases have been examined (Kr(M), Kr(L), Xe(N), Xe(M) and Xe(L)), spectrally spanning the range from approximately 80 eV to approximately 5 keV. In order to reconcile theoretical estimates with these experimental findings, three generalizations of the original formulation of the interaction were necessary. The most important involves an enhancement in the coupling arising from the coherent motion of the (Z) field-ionized electrons induced by the external driving field. The coherently energized electrons act like a quasi-particle possessing a charge Ze and a mass Zm, thereby presenting a sharply augmented coupling resembling that associated with energetic ion-atom collisions. The second involves the process of multiple electron ejection from an inner-shell, a mechanism that was found imperative to interpret the high level of ionization observed in the Xe M-shell. The third modification concerns multiple transits of the driven electrons in the cluster. The inclusion of these considerations consistently brings the theoretical analysis into agreement with the three measured properties. These results also indicate that energy deposition rates exceeding approximately 1 W/atom are feasible in appropriately designed molecules incorporating heavy atoms. The limiting magnitude of the excitation energy Delta E_max characteristic of the coherent coupling is estimated to be in the range Zamc²<or= Delta E_max<or=Z²amc², a bound that can considerably exceed the K-shell binding energy of the heaviest atoms.

The existence of coherent electronic motions induced by the presence of a strong external optical field enables the process of direct multi-electron ejection from an inner-shell to occur with high probability. Experimental evidence for this mechanism is present in the observed spectra of five cases studied (Kr(M), Kr(L), Xe(N), Xe(M) and Xe(L)). Analysis shows that the multiphoton interaction can enter a regime of strong-coupling (Z²a>or=1) in which the rate of multiple electron ejection can become comparable to the removal of a single electron. The enhanced coupling arises from the coherent motion of the (Z) field-ionization electrons whose interactions constructively interfere, thereby behaving like a quasi-particle with a charge Ze. This mechanism consistently explains (1) the unusually high values of Xe M-shell ionization observed in both Xe(M) (Xe³⁶⁺) and Xe(L) (Xe⁴⁵⁺) emissions and (2) the anomalous double 2p vacancy production seen in the Xe(L) spectrum. Clusters of heavy atoms appear as exceptionally favourable systems for the copious production of multiple vacancy hollow atom configurations.