The scattering of a class of partially coherent electromagnetic fields by a model medium whose dielectric susceptibility fluctuates in space and in time is considered within the accuracy of the first-order Born approximation. It is shown that if the incident field is a linearly polarized, polychromatic plane wave whose spectrum is a line of Gaussian profile, the spectrum of the scattered light is, in some cases, approximately Gaussian and is shifted towards the shorter or the longer wavelengths, depending upon the angle of scattering.

Nonequilibrium charge-state fractions of ${}_{}{}^{35}{}_{}{}^{}\mathrm{Cl}$ ions traversing solid C targets have been measured. Absolute intensities of x rays were measured for transitions between various states of the nearly fully stripped ${}_{}{}^{35}{}_{}{}^{}\mathrm{Cl}$ ions that had an energy of 445 MeV and a velocity of 12.7 MeV/u. An analysis of the data has provided estimates of a number of cross sections for electron capture, excitation, and electron loss of the projectiles. Reasonable agreement has been found between some of these experimental values and theoretical estimates.

A systematic study is made of differential cross sections for the electron-impact excitation of the 2s and 2p states of H-like ions. The cross section is calculated in the Coulomb-Born and the Coulomb-Born-Oppenheimer approximations. A comparison of the resulting cross sections is made along the isoelectronic sequence from Z=1 to ∞ (Z being the nuclear charge) to reveal the role of the Coulomb force in the excitation processes. For the 1s-2p transition, the behavior of the apparent generalized oscillator strength for the ions is found to be different from that for neutral hydrogen.

A very rapid, but accurate, fully relativistic method for calculating the atomic-structure data needed in determining collision strengths for highly charged ions is developed. A more rapid quasirelativistic approximation to this approach is also described. Results for oscillator strengths and energies are compared with those by mostly more elaborate programs for neonlike and nickel-like ions and generally good agreement is obtained.

A rapid fully relativistic distorted-wave method for calculating collision strengths for highly charged ions is described. A more rapid quasirelativistic approximation in which the average over j value -1 is used for the relativistic quantum number κ for the free electrons is also discussed. Very rapid, but accurate, procedures for obtaining results for a given class of transitions for a large portion of an isoelectronic sequence by using fits to Z after making detailed calculations for only a few values of Z are described. Results by the present methods are compared with more elaborate relativistic distorted-wave calculations by other workers, principally for neonlike and nickel-like ions, but also comparisons for He-like, Li-like, and Na-like ions are discussed. In addition, comparisons with a few experimental results for neonlike barium could be made. Generally very good agreement is obtained with all these data.

A simple expression is derived that relates the rate coefficient for dipole-allowed electron-impact excitation of a molecular ion in the Coulomb-Born approximation to the Einstein A coefficient for the corresponding radiative decay. Results are given for several molecular ions of astrophysical interest. A general analytic expression is obtained for the equilibrium rotational level populations in the ground vibrational state of any molecular ion excited by collisions with electrons. The expression depends only upon the electron temperature, the electron density, and the rotational constant of the molecular ion. A similar expression is obtained for neutral polar molecules.

We examine various high-energy approximation methods, with and without boundary corrections for electron-capture processes. From the comparison of the calculated differential cross sections for the proton plus hydrogen-atom system, we show that the boundary correction does not always improve the results. The internuclear interaction must be fully taken into account to explain the experimental results.

The far-infrared pure rotational absorption spectrum of HD and HD-X mixtures was measured at 77 and 195 K. Specifically, the rotational lines R(0)–R(2) were observed for HD-HD, HD-He, HD-Ne, and HD-${\mathrm{H}}_2$ at 77 K and R(0)–R(3) for HD-HD, HD-He, and HD-Ar at 195 K. Refined values of the permanent dipole moment of HD are deduced. Collisional interference in the spectrum is characterized by a parameter proportional to the ratio of the average-induced dipole moment to the permanent dipole moment. Its variation with temperature, particularly a change in sign, is not explainable by current theory. The spectral line-shape parameters of width, frequency shift, and asymmetry are determined and their density and temperature behavior discussed with reference to the impact theory of line broadening.

The carbon and oxygen K-edge near-edge x-ray absorption fine structures (NEXAFS) of a series of molecules, carbon monoxide (CO), carbon dioxide (${\mathrm{CO}}_2$), carbonyl sulfide (OCS), acetone [(${\mathrm{CH}}_3$${)}_2$CO], ethanol (${\mathrm{C}}_2$${\mathrm{H}}_5$OH), diethylether [(${\mathrm{C}}_2$${\mathrm{H}}_5$${)}_2$O], tetrahydrofuran (${\mathrm{C}}_4$${\mathrm{H}}_8$O), and p-dioxane (${\mathrm{C}}_4$${\mathrm{H}}_8$${\mathrm{O}}_2$) have been recorded in the gas phase in transmission, total-ion-yield, and luminescence-yield modes. The results are compared with previous measurements and theoretical predictions, and discussed generally on the basis of molecular structure and bonding. The the NEXAFS parameters are examined in terms of the electron-core-hole interaction, spatial characteristics of the final states, correlation with interatomic distance, and multiple-scattering phenomena.

The angular distributions of electrons photodetached from a beam of negative oxygen ions have been measured at photon energies ranging from 1.7 to 2.7 eV. The calculation of asymmetry parameters for negative ions is briefly discussed, and the parameters derived from the measurements are compared with calculations and other measurements.

The cross sections for one-, two-, and three-photon detachment from fluorine negative ions have been measured using light at the fundamental and harmonic frequencies of a 1064-nm pulsed neodymium-doped yttrium aluminum garnet laser. The ions were stored in a Penning ion trap, the depletion due to illumination was measured, and the data were fitted to a model for stored ions illuminated by pulsed light. The values obtained for the detachment cross sections are ${\sigma }^{\mathrm{}\left(1\right)\mathrm{}}$=2.8(5)×${10}^{\mathrm{-}18}$ ${\mathrm{cm}}^2$, ${\sigma }^{\mathrm{}\left(2\right)\mathrm{}}$=2.0(7)×${10}^{\mathrm{-}50}$ ${\mathrm{cm}}^4$ sec, and ${\sigma }^{\mathrm{}\left(3\right)\mathrm{}}$ ${=7.9\mathrm{}}_{\mathrm{-}3.6}^{+6.6\mathrm{}}$×${10}^{\mathrm{-}83}$ ${\mathrm{cm}}^6$ ${\sec }^2$. These values are compared with theoretical calculations and other experimental results.

The time-dependent Schrödinger equation was solved numerically, in one spatial dimension, for an electron bound initially (t<0) by a δ potential, and acted upon by a single-frequency classical electromagnetic field, turned on abruptly at t=0. The dipole approximation was invoked for the laser field, and magnetic interactions were ignored. Characteristic photoionization times were determined for a wide range of scaled laser intensities and scaled laser frequencies. The conditions under which well-defined quiver motion of the ionizing electron appears were clarified, and a spectrum of radiation emitted during the ionization process was determined. Above-threshold-ionization spectra of ionized electrons were also computed.

The pulsed Li atomic beam produced in our experiment is based on controlled transversely-excited-atmospheric ${\mathrm{CO}}_2$ laser–induced ablation of a Li metal target. The atomic beam is propagated in vacuum or in ${\mathrm{CO}}_2$ gas at low pressure. Atoms in the beam are probed by laser-induced fluorescence spectroscopy. This allows the determination of time-of-flight and velocity distributions. Li Rydberg states (n=5–13) are populated in the beam by two-step pulsed-laser excitation. The excited atoms interact with ${\mathrm{CO}}_2$ molecules. l- and n-changing cross sections are deduced from the time evolution of the resonant or collision-induced fluorescence following this selective excitation. l-changing cross sections of the order of ${10}^4$ A${̊}^2$ are measured; they increase with n as opposed to the plateau observed for ${\mathrm{Li}}^{\mathrm{*}}$ colliding with a diatomic molecule. This behavior is qualitatively well explained in the framework of the free-electron model. n→n’ changing processes with large cross sections (10–100 A${̊}^2$) are also observed even in the case of large electronic energy change (Δ$E_{\mathrm{nn}\mathcal{’}}$${>10\mathrm{}}^3$ ${\mathrm{cm}}^{\mathrm{-}1}$). These results can be interpreted in terms of resonant-electronic to vibrational energy transfers between Li Rydberg states and ${\mathrm{CO}}_2$ vibrational modes.

Perturbative-polarization-propagator methods of different orders employing large Gaussian basis sets are used to calculate discrete pseudostate representations of the oscillator strength distribution of the water molecule, from which continuous total and partial photoionization cross sections are then determined by moment-theoretical methods. The cross sections obtained in the first-order and second-order polarization-propagator approximation are in very good agreement with recent photoionization measurements. In particular, the present results agree much better with experiment than previous calculations that use the static-exchange approximation. The main reason for this is that first- and higher-order polarization-propagator approximations obey certain sum rules for the oscillator strength distribution, whereas the static-exchange approximation does not. This makes propagator methods particularly well suited for use in connection with moment theory. Disagreement of previously reported cross sections of the water molecule with experiment, such as the overestimation of the 1$b_2$ partial cross section, have been partially attributed to the neglect of channel coupling in these calculations. Therefore, in the present work, the separated-channel approximation is avoided and all possible couplings between ionizations from all valence-shell molecular orbitals are allowed for.

We consider the excitation of atomic hydrogen in its ground state by an intense radiation field tuned close to the ionization threshold. Results are presented for the ac Stark shift of the 1s and 2s states, the width of the 1s state, and the photoelectron energy spectrum. In the first instance, our nonperturbative approach includes the 1s state, the p series of excited states, and the p continuum: we demonstrate that the p Rydberg states generate a rich structure in the photoelectron energy spectrum through the action of the counter-rotating wave dynamics when the laser frequency is tuned below the ionization threshold. We then go on to analyze in detail how the interaction of the p states with the other Rydberg series and continua increases the width of the p states; it is shown, however, that at the field intensities considered here (≊${10}^{13}$ W ${\mathrm{cm}}^{\mathrm{-}2}$), this effect is not sufficient to smear out the structure in the photoelectron energy spectrum.

Calculations of photoionization parameters are presented for the 4d shell of atomic Ba. A sequence of calculations was performed in the framework of the relativistic random-phase approximation: (i) including correlations within the 4d shell only; (ii) including correlations arising from coupling with 5s, 5p, and 6s shells; and (iii) including relaxation of the 4d shell. These results are compared with some other theoretical calculations and with recent experimental data.

The effect of laser intensity on the branching ratios for the vibrational distribution of a ${\mathrm{H}}_2^{+}$ ion and the corresponding photoelectron angular distribution for the (3+1)-photon ionization of a ${\mathrm{H}}_2$ molecule via the B’ ${}_{}{}^1{}_{}{}^{}\Sigma _u^{}{}_{}{}^{}$ state have been studied. The branching ratios at the electron scattering angle θ=0° and the asymmetry parameters for photoelectron angular distribution have been calculated in the range of ${10}^8$–${10}^{12}$-W/${\mathrm{cm}}^2$ laser intensity. For ionization via different vibrational levels ($v_i$=0,1,2) of the B’ ${}_{}{}^1{}_{}{}^{}\Sigma _u^{}{}_{}{}^{}$ state, the laser-intensity dependence of the branching ratios is prominent in the range ${10}^{10}$–${10}^{12}$ W/${\mathrm{cm}}^2$, although the peak of the distribution remains always at Δv=0, i.e., $v_i$=$v_I$ ($v_I$ is the vibrational level of the ion). Our results have been compared with experimental data of Pratt, Dehmer, and Dehmer [J. Chem. Phys. 85, 3379 (1986); 86, 1727 (1987)].

Vibrational transitions in collisions of two polyatomic systems are described in terms of a Hamiltonian bilinear in momentum and position operators, for several degrees of freedom. The relative motion is assumed to be classical and leads to time-dependent coefficients in the Hamiltonian. The present treatment employs a simple procedure that does not require the construction of the time-evolution operator and leads to recursion relations for transition amplitudes, suitable for numerical applications. As examples, we consider the special cases of a single degree of freedom, and of the linearly driven harmonic oscillator.

A quantum-mechanical treatment is developed to describe the hydrogen atom in a uniform magnetic field. The method starts from a Born-Oppenheimer-type separation in cylindrical coordinates, with z the slow coordinate. In this adiabatic representation, each quasi-Landau-level, with (3/2${\omega }_c$ spacing near the ionization threshold, emerges as &(z). This representation suggests a simple interpretation of the narrowness of individual lines in quasi-Landau-spectra above the ionization threshold, as observed in recent high-resolution measurements.

A general, nonrelativistic, multichannel theory for angle- and spin-resolved photoelectron spectroscopy of polyatomic molecules in the electric dipole approximation has been developed. Expressions for angular distribution of electrons ejected with defined spin polarization, both in molecule- and laboratory-fixed frames of reference from molecular targets with fixed as well as with random orientation in space, are derived. The transformation properties of the molecular point symmetry group are used to their full advantage to reduce these expressions to their simplest possible forms. The ionization amplitude is thus shown to decompose into a sum of transitions each involving the final-state wave function belonging to an irreducible representation of the point group of the target molecule. The formulas obtained herein can therefore be used to study spin-resolved photoionization in both linear and nonlinear molecules of any symmetry corresponding to one of the 32 point groups, either in their gaseous phase or oriented on liquid and solid surfaces. In order to study both the angular distribution as well as the degree of spin polarization of photoelectrons in terms of alternative contributions of the angular momentum transfer ${(\mathrm{j}}_t$) from photon to the ejected electron, a new set of ionization amplitudes is introduced whose contributions to the angular distribution add incoherently. This new approach not only simplifies the resulting expressions, but also eliminates the interference terms $j_t$≠$j_t^{\mathcal{’}}$ which were present in an earlier expression for angular distribution of spin-polarized atomic photoelectrons. The theory developed in this paper has been used in the accompanying article to analyze photoionization in a nonlinear molecule whose point symmetry group is $T_d$.