We report on a high-resolution study of the Si 2p photoabsorption spectrum of ${\mathrm{SiCl}}_4$ in the gas phase. Characteristic differences in the intrinsic widths of the resonance lines allow a distinction of excitations to valence and Rydberg orbitals, respectively. The Rydberg resonances are found to be narrow and split into vibrational substrates from symmetric-stretching vibrations, providing information—on the basis of a Franck-Condon analysis—on vibrational energies and equilibrium internuclear distances of the core-excited molecule. Thermal population of excited vibrational levels in the molecular ground state has to be taken into account for a proper description of the spectrum.

The electron-sodium system is a prototype of nonrelativistic electron scattering from a quasi-one-electron atomic target and is tractable both experimentally and theoretically. Recently, this system has been studied in a series of sophisticated measurements that together approach complete experiments for elastic (3s→3s) and inelastic (3s→3p) scattering. We apply here the theory of orientation and alignment (OA) in atomic collisions to this system using scattering matrices from coupled channel R-matrix calculations described in the first paper in this series [W. K. Trail et al., Phys. Rev. A 49, 3620 (1994)]. To facilitate the extension of OA theory to other transitions and systems and to clarify its relationship to canonical scattering theory, we present a reformulation in terms of the state spaces identified by a particular scattering event. Following the application of this formulation to paradigmatic OA experiments, we compare our results to those from existing measurements and other theoretical calculations. To contextualize these experiments and aid in identifying promising regions for future measurements, we also present a comprehensive three-dimensional overview of the calculated differential OA parameters for energies from threshold to 8.6 eV.

The time-dependent behavior of subexcitation electrons is studied by a technique that employs the continuous slowing-down approximation, combined with the Spencer-Fano equation via a time-dependent version of the entry spectrum. The method is used first to establish generalities about the time behavior of subexcitation electrons generated by initially-high-energy electrons and then is employed to follow the development of subexcitation electron degradation spectra generated by 1-keV electrons in argon.

We did the Harris-Nesbet algebraic close-coupling calculation for positron-hydrogen scattering at low energies with the (1s,2s)H-(1s,2s)Ps, (1s,2s,2p)H-(1s,2s)Ps, and (1s,2s,2p)H-(1s,2s,2p)Ps coupling schemes. Cross sections of elastic scattering, positronium formation, and Ps(1s)-p elastic scattering were obtained for L=0,1,2,3,4,5,6 partial waves. The integrated cross sections for the relevant processes were then estimated. We compared our six-state close-coupling results with the ones calculated by solving the Lippmann-Schwinger coupled equations in momentum space. Many special features were seen in these cross sections.

Electron capture and direct elastic scattering in collisions of ${\mathrm{H}}^{+}$ ions with ${\mathrm{CH}}_4$ molecules are studied by using a molecular representation within a fully quantum-mechanical approach below 1.5 keV. Calculations are carried out at two different molecular configurations: (i) ${\mathit{C}}_{3\mathit{v}}$ symmetry, in which ${\mathrm{H}}^{+}$ approaches along the direction of a C-H in ${\mathrm{CH}}_4$, and (ii) ${\mathit{C}}_{2\mathit{v}}$ symmetry, in which ${\mathrm{H}}^{+}$ approaches along a bisector of a H-C-H bond angle. We find that electron capture in the ${\mathit{C}}_{2\mathit{v}}$ symmetry configuration takes place preferentially over that in the ${\mathit{C}}_{3\mathit{v}}$ symmetry configuration at scattering angles below 15° at 1.5 keV, and that the situation reverses at larger scattering angles. Hence, interferences arising from these molecular configurations on differential cross sections for electron capture and elastic scattering processes are present but weak, except for angles near the crossing. Accordingly, the total cross section for the ${\mathit{C}}_{2\mathit{v}}$ symmetry is larger by more than an order of magnitude, because in this symmetry ${\mathrm{H}}^{+}$ can penetrate deep inside the electron distribution of the ${\mathrm{CH}}_4$ molecule, causing a strong interaction. In addition, angular dependence in the differential cross section is quite different for the two molecular configurations at all energies studied.

The cross sections for single-electron capture, including transfer ionization, and for double-electron capture, have been measured for ${\mathrm{Ar}}^{\mathit{q}+}$+${\mathrm{H}}_2$ for q=6, 7, 8, 9, and 11 with projectile energies from q eV to q keV. Theoretically, the cross sections for ${\mathrm{Ar}}^{6+}$ and ${\mathrm{Ar}}^{8+}$ impact were calculated using a molecular-orbital expansion method in the energy region from 240 eV to 80 keV, and are in good agreement with experiment. The single-electron-capture cross sections were found to be more than one order of magnitude larger than those for double capture in both experiment and theory. The single-electron-capture cross sections were also compared to the Langevin cross section, a scaling law developed by Müller and Salzborn [Phys. Lett. 62A, 391 (1977)], and the absorbing sphere model.

The continuum-distorted-wave eikonal initial-state (CDWEIS) model is used to study the ionization of hydrogen by protons of energy 10–500 keV. Ejected electron spectra are presented and discussed in the context of the saddle-point mechanism. The behavior of the ejected electron spectrum as the charge of the incident projectile is varied is also considered. It is concluded that within its range of validity, CDWEIS does not provide any evidence of the existence of saddle-point electrons.

The distribution over the n states populated by double-electron capture in ${\mathrm{Ne}}^{10+}$-He collisions has been determined in the energy range of 0.9–9 keV/u by analyzing the radiative decays of projectile Rydberg transitions. It has been found to be in fair agreement with predictions of the autotransfer-to-Rydberg-states model of Roncin et al. [J. Phys. B 26, 4181 (1993)]. Double-capture stabilization ratio ${\mathit{R}}_{\mathit{S}}$, defined as the ratio of true double capture ${\mathrm{\sigma }}_{\mathrm{TDC}}$ to the sum of ${\mathrm{\sigma }}_{\mathrm{TDC}}$ and autoionizing double capture ${\mathrm{\sigma }}_{\mathrm{ADC}}$, has been studied at various energies. A slight enhancement of ${\mathit{R}}_{\mathit{S}}$ (35–50 %) and an increase by a factor of 5 of the cross sections for emission of optical transitions from (3,n) Rydberg states have been found as the projectile velocity decreases from 0.6 to 0.19 a.u. We propose a tentative explanation following a model of postcollisional satellization of Rydberg electrons.

A substantial enhancement of intensity in the high-energy part of the ${\mathrm{N}}_2$ KLL Auger spectrum has been observed at photon energies a few tens of eV above the N 1s threshold. By monitoring the corresponding 1s photoelectron spectrum, we show that the effect is due to the emergence of strong conjugate shake-up satellites. As shown in previous e-2e studies for CO, such Auger spectra are useful in determining the intensity behavior of shake-up satellites in the immediate threshold regions.

We have carried out a comprehensive theoretical and experimental study of electron scattering from molecular nitrogen at energies below 10.0 eV. In the theoretical component of this project we have generated differential and integral cross sections for elastic scattering and vibrational excitation in converged vibrational close-coupling calculations. In the experiments, we have measured differential cross sections for these processes at scattering angles from 20° to 130° in a crossed-beam experiment at a large number of energies between 0.55 and 10 eV and, in a complementary time-of-flight experiment, total cross sections at energies between 0.08 and 10.0 eV. The measured angular distributions have been extrapolated to 0° and 180° using a procedure based on a nonlinear least-squares fit constrained by known physical properties of the e-${\mathrm{N}}_2$ scattering matrix; numerical integration of the resulting extrapolated distributions yields integrated cross sections with almost no error beyond that inherent in the measured angular data. We find generally good agreement between the present experimental and theoretical cross section, particularly at energies near the ${\mathrm{\Pi }}_{\mathit{g}}$ resonance near 2.39 eV. In previous studies of scattering in this region, such comparisons have been made problematical by the difficulty of ascertaining the appropriate theoretical scattering energy. We recommend here a protocol for resolving this problem for both elastic scattering and vibrational excitation.

Vibrational elastic, rotationally summed cross sections for electron collisions with ${\mathrm{CH}}_4$ are calculated with ab initio static-exchange interactions and using a symmetry-adapted, single-center-expansion representation for the close-coupling equations. The correlation forces are included through density-functional theory and via a near-Hartree-Fock self-consistent-field description of the target wave function. Integral and differential cross sections are calculated over a broad range of collision energies, from the shape resonance region up to 50 eV. Comparisons with experiments and with previous calculations show that the present results indeed exhibit very good overall accord with measurements at these collision energies and describe very efficiently the electron angular distributions as given by a very broad range of measurements.

Rotational excitation in electron–diatomic-molecule collisions at intermediate collision energies (∼${10}^2$ eV) is discussed within a quantum-mechanical two-center model using a Coulomb-scattering approximation. Rotational rainbow effects are discussed, in particular for heteronuclear diatomic molecules. The model is applied to collisional excitation of HCl and CO as well as to the homonuclear cases ${\mathrm{N}}_2$ and ${\mathrm{Cl}}_2$. The model predictions are compared with recent experimental results as well as with elaborate numerical computations for the homonuclear systems.

We investigate the effect of an intense optical field on the energies of electrons ejected in an Auger process. To this end, part of a subpicosecond, infrared laser beam is focused onto a metal target to produce subpicosecond, broadband x radiation. Electrons resulting from the x-ray-induced LMM Auger transition in argon are detected in a time-of-flight electron spectrometer. The other part of the infrared laser beam is directly focused onto the argon atoms. When the x-ray-induced Auger transition takes place in the presence of the intense dressing beam, sidebands appear around the Auger peaks in the electron spectrum, corresponding to absorption or emission of photons from the dressing field. The magnitude of the sidebands is studied as a function of dressing field intensity. Good quantitative agreement with quantum-mechanical theory is reported.

We present the convergent close-coupling theory for the calculation of electron-helium scattering. We demonstrate its applicability at a range of projectile energies of 1.5 to 500 eV to scattering from the ground state to n≤3 states. Generally, good agreement with experiment is obtained with the available differential, integrated, ionization, and total cross sections, as well as with the electron-impact coherence parameters up to and including the 3 ${}_{}{}^3{}_{}{}^{}$D-state excitation. This agreement is shown to be overall the best of the currently used electron-helium scattering theories. On occasion, some significant discrepancies with experiment are observed, particularly for the triplet-state excitations.

Fragment angular distributions resulting from photodissociation induced by intense, short, linearly polarized laser pulses are investigated using a two-dimensional time-dependent split operator wave-packet propagation technique. The rotational excitation of the molecule leads, in general, to an alignment of the photofragments with respect to the field polarization vector but, unexpectedly, the reverse situation may also happen for some specific wavelengths for which fragmentation mainly occurs in a plane nearly orthogonal to the polarization axis. A detailed and comprehensive interpretation is provided referring to two typical wavelength regimes corresponding either to the blue or to the red wing of the single photon absorption line shape. The relevant strong field mechanisms upon which this interpretation rests are, respectively, the bond softening and the vibrational trapping of the molecule by the radiation which induces substantial modifications on the internal force field, viewed in the radiatively dressed adiabatic frame. An illustrative example, concerning the photodissociation of an isotropic distribution of ortho-${\mathrm{H}}_2^{+}$ in the ${}_{}{}^2{}_{}{}^{}$${\mathcal{J}}_{\mathit{g}}^{+}$(J=1/2, N=1,v=0) initial state, pertaining to Hund’s (b) case, reveals that, at λ=1600 Å, an efficient alignment is obtained with femtosecond pulses delivering an intensity in the range ${10}^{13}$–${10}^{14}$ W/${\mathrm{cm}}^2$, as a consequence of the bond softening mechanism. More surprising is the possibility of observing fragments in a near orthogonal direction to the laser polarization, predicted by the trapping mechanism and confirmed by quantum calculations at shorter wavelengths, such as λ=800 Å.

We present results of calculations for ${\mathit{e}}^{\mathrm{-}}$+${\mathrm{H}}_2^{+}$ scattering in the energy regime below the first excited state for resonance symmetries Σ and Π. We employ three distinct and independent methods: close-coupling linear algebraic, effective optical potential linear algebraic, and R matrix. We report extended calculations on the ${}_{}{}^1{}_{}{}^{}\mathrm{\Pi }_{\mathit{g}}^{}{}_{}{}^{}$ resonance, important to dissociative recombination. We show binding of the ${}_{}{}^1{}_{}{}^{}\mathrm{\Sigma }_{\mathit{g}}^{}{}_{}{}^{}$ state resonance between 2.6 and 2.7 bohrs. Our ${}_{}{}^1{}_{}{}^{}\mathrm{\Sigma }_{\mathit{u}}^{}{}_{}{}^{}$ state results agree very well with previous calculations and reside a factor of 2 below a recent experiment.

The close-coupling approximation for electron-atom scattering is extended to include the effect of one-photon radiation damping at a level nonperturbative in the wave function. The complex potential is derived and introduced directly into the set of integrodifferential equations used to calculate the S matrix. The formulation is ideal for inelastic scattering and photoionization with radiation damping and can be used to calculate photorecombination cross sections. The numerical solution of the resulting differential equation is accomplished through a combination of R-matrix, perturbation theory, and analytic techniques. Some of the implications of this method are discussed. The connection to previous theoretical approaches are discussed.

The eigenchannel R-matrix method is used to describe a set of two-electron escape processes. This method is applied to calculate both bound state excitation and ionization cross sections for a simplified model of electron-hydrogen scattering and electron-${\mathrm{He}}^{+}$ scattering, in which only zero angular momentum states are considered. The results shown are calculated for incident electron energies ranging from 13.6–50 eV for electron-hydrogen scattering and from 54.4–200 eV for electron-${\mathrm{He}}^{+}$ scattering. A frame transformation technique is introduced to improve the stability of calculated ionization cross sections. The R-matrix results are compared with cross sections obtained from a convergent close-coupling approach and with other results.

We introduce a multistep cascade model for the deexcitation of highly charged ions in the presence of a solid. We apply this model to analyze the yield of the KLL-Auger peak as measured in angular resolved electron energy spectra. The calculation makes use of three probability densities, one for the time distribution of the Auger decay, one for the transport of the ions through the first atomic layers of the target, and the last one for the transport of the electrons from the source to the detector. Each of these probabilities is discussed and modeled. The calculations are compared to measurements of KLL-Auger electrons from low-energy ${\mathrm{Ne}}^{9+}$ ions impinging on solid Al. The results support previous estimations that the L-shell sidefeeding of the projectile depends on the incident velocity. For ion energies greater than 1000 eV, the mean path length needed to capture an electron is found to be approximately equal to the next-neighbor distance in the Al lattice. We estimate the yield of KLL electrons emitted from Ne ions with 2,3,...,8 electrons in the L shell. The mean depth of emission of the KLL electron is determined.

We report experimental evidence for the loss of neutral ${\mathrm{C}}_2$ from photoexcited ${\mathrm{C}}_{60}$ and ${\mathrm{C}}_{70}$. These species are detected by postionization with xuv radiation. Interestingly, C, ${\mathrm{C}}_3$, and ${\mathrm{C}}_4$ also are products in the nascent distribution. An upper limit to the translational energy release into C atoms has been obtained using a resonant autoionization transition. The detection of these products gives insight into the fragmentation mechanism of fullerenes and may also have an impact on the field of fullerene formation.