Energy transfer in atom-dimer collisions is analyzed using a model of loosely bound hard spheres. The results are compared with calculations for a real system. It is found that large energy transfer at a particular scattering angle is sensitive to the initial internal state of the dimer. A quantum theory of atom-dimer collisions is developed and it is found that resonances in the dissociation channel of the dimer are expected. The final rotational states of the dimer are also investigated. It is found that large energy transfer occurs with no change in the rotational state of the dimer.

Cross sections are determined for stripping (charge transfer and ionization) of the αμ ion in collisions with a number of elements. The calculations are done with the classical-trajectory Monte Carlo method using the Thomas-Fermi model for the neutral-atom target. The total stripping cross sections are shown to differ only slightly from those for collisions with the bare nuclei. A simple analytic formula is given for evaluation of the stripping cross section for any target element. Finally, the results are applied to stopping and stripping of αμ ions in several metallic foils.

Absolute total cross sections for single-electron capture have been measured for collisions of ${\mathrm{O}}^{2+}$ ions with He, Ne, Ar, ${\mathrm{H}}_2$, ${\mathrm{D}}_2$, ${\mathrm{N}}_2$, and ${\mathrm{O}}_2$. The relative collision energies of these experiments range from approximately 3 to 400 eV. The electron-capture cross section for ${\mathrm{O}}^{2+}$+He collisions is found to agree well with previous distorted-wave calculations. Capture cross sections are generally larger for molecular targets than for the atomic targets.

A time-resolved study of the Sr(5 ${}_{}{}^3{}_{}{}^{}°_J^{}{}_{}{}^{}$) intramultiplet collisional mixing, following optical excitation of the ${}_{}{}^3{}_{}{}^{}\mathrm{°}_1^{}{}_{}{}^{}$ state, is presented. The degeneracy-averaged rate coefficients of Sr(5 ${}_{}{}^3{}_{}{}^{}°_J^{}{}_{}{}^{}$→5 ${}_{}{}^3{}_{}{}^{}°_{J\mathcal{’}}^{}{}_{}{}^{}$) due to collisions with each of the noble gases in a quasiequilibrium cell are derived from least-squares fittings of the triexponential time behavior of 5 ${}_{}{}^3{}_{}{}^{}°_1^{}{}_{}{}^{}$ fluorescence and $5^3$P${\mathrm{°}}_2$, 0→6 ${}_{}{}^3{}_{}{}^{}\mathrm{S}_1^{}{}_{}{}^{}$ absorption data. A 5 ${}_{}{}^3{}_{}{}^{}°_1^{}{}_{}{}^{}$ radiative rate of 22±0.5 μs is also obtained. The dependence of the rates upon buffer pressure demonstrated the presence of both linear and quadratic terms, due to two- and three-body reactions, respectively. The behavior of the binary rate coefficients with varying noble-gas species and temperature is qualitatively consistent with Stückelberg’s model for noncrossing levels.

A theoretical study is presented of the process of photodetachment of a negative ion by sub-threshold-frequency radiation in the presence of a simultaneous collision. Calculations are carried out for the ${\mathrm{H}}^{\mathrm{-}}$-He case and the resulting cross section is compared with other competing processes, such as two-photon photodetachment and nonradiative collisional detachment.

Approximate amplitudes for excitation and electron capture in ion-atom collisions frequently employ plane waves of relative motion, even though it is known that higher-order terms in the formal expansion are frequently divergent when the fragments in the final state interact through long-range Coulomb interactions. The Coulomb-Born expansion is free of such divergences, but the higher-order terms are more difficult to evaluate than the corresponding terms with plane waves. We show how to use plane waves of relative motion without encountering divergent terms in higher order even with Coulomb potentials present. The procedures apply to several common expansions, but encounter conceptual difficulties when applied to multiple-scattering expansions.

${\mathrm{Ar}}^{+}$ and ${\mathrm{Na}}^{+}$ ion beams bombarding Na ns and np Rydberg-state targets at impact velocities near the Bohr-orbital velocity of the target atoms (i.e., ṽ≊1) induce transitions to (n-1)l states (l≥2) with larger-than-geometric cross sections. Depopulation of ns states proceeds directly into the full n-1, l≥2 manifold rather than populating the np or (n-1)p states as the first of a sequence of dipole-allowed steps. Depopulation of np states leads to a distribution of final states that is dominated by the nearer or high-l part of the n-1 manifold. n dependences of the cross section for ns depopulation are given at several energies for n=32–41 and at a single energy for np, n=26–32. The absolute cross section for Na(36s) depopulation falls gradually but steadily for reduced velocities ṽ increasing from 0.3 to 1.35. Bombardment of Na 39s and 39p states in applied fields 0–18.5 V/cm reveals that the final-state distributions become narrower as manifold states are Stark shifted close to the initial states. No theoretical results are available for direct comparison with these experiments, but the method of coupled channels seems to be the most suitable candidate.

Angular distributions of the projectile products have been measured for electron capture from He by C, N, O, F, and Ne ions in charge states between 5+ and 8+ at energies ranging from 7.4 to 16 keV. The angular distributions depend in character more on the incident charge state than on ionic species. A strong forward peaking is found for the 5+ and 7+ incident ions, whereas the 6+ ions show a maximum at an angle outside the half-Coulomb scattering angle. Oscillatory structure is seen for ${\mathrm{O}}^{6+}$ on He. The general features of the distributions can be understood on the basis of classical deflection functions for the heavy-particle motion.

A multichannel study of the photodetachment of a negative ion such as ${\mathrm{S}}^{\mathrm{-}}$ in the presence of a constant electric field is presented. The analysis is carried out as a series of frame transformations for the outgoing photoelectron. An earlier analysis in the absence of the external field is now augmented by the further transformations from spherical to cylindrical symmetry that are required. The interplay between the various thresholds due to fine structure in the negative ion and residual atom and the effects of the external field are analytically treated. The net effect is to eliminate the thresholds, to induce modulations in the detachment cross section, and to introduce field-assisted-tunneling contributions to the photodetachment. Numerical illustrations are provided. Our treatment closely parallels similar treatments of photoionization of neutral atoms in an electric field and of photodetachment in a magnetic field.

The spectral widths of the Cu K${\alpha }_1$ and Cu K${\alpha }_2$ x-ray lines have been determined from a series of x-ray diffraction profiles. All profiles were obtained using a two-crystal Ge (111) monochrocollimator and a single (100) GaAs study crystal. The dispersion of the monochrocollimator was determined from the measured separation of the K${\alpha }_1$ and K${\alpha }_2$ peaks. The known dispersion was then utilized to calculate the natural widths of the Cu x-ray lines from the observed diffraction profile widths. The Cu K${\alpha }_1$ and K${\alpha }_2$ lines have full widths at half maximum of 0.000 461(9) and 0.000 61(4) Å, respectively.

The simplest atoms having nonspherical symmetry are those with a single p electron in a valence shell. Of these, the group IIIB elements are excellent examples. As such, they form test cases for photoionization from open-shell systems. Through photoelectron-spectroscopy techniques, we have examined both partial cross sections and angular-distribution parameters for autoionization corresponding to promotion of a 3d electron to the 4p shell of gallium. The resulting ${\mathrm{dp}}^2$ configuration gives rise to a complicated multiplet structure across which the angular-distribution parameter varies considerably. We have also looked at the simpler structure resulting from promotion of one s electron to an np level, n=5,6. For these cases, the multiplet structure is simpler, but the influence of the resonance on the cross section and the angular distribution is pronounced. For the 4s4p${(}^3$P)5p resonance we find a value of β=-1 at the cross-section minimum. No calculations have been performed for this system, so we attempt a qualitative interpretation of our results based on an angular-momentum-transfer analysis.

We have performed a set of numerical calculations of the one-dimensional, time-dependent Schrödinger equation in order to study the interaction of an intense, electromagnetic field with a model atom. We approximate the atom by a local, square-well potential and the electromagnetic wave by an oscillating electric field. We investigate the multiphoton-absorption process that leads to the ‘‘above-threshold’’ ionization (ATI) phenomenon. We not only examine the ionization probability and electron-energy distributions within the field but also the effects of spatial and temporal boundaries that represent the finite dimensions of a laser pulse. We observe all the main features of the ATI behavior from the shifting of the peaks to their disappearance. In addition, we find differences in the positions of the peaks according to how the electron exits the field region. We also explore resonant ionization as well as Volkov wave packets and the recombination process.

Theory is developed for photodetachment of an electron from a potential well in a magnetic field. The cross section has a maximum as a function of photon energy just above each Landau-level threshold. If the final-state interaction is effectively negative, there is in addition a Feshbach resonance below each of these thresholds, except the first. However, none of these maxima occurs if the final-state wave function is antisymmetric with respect to reflection in a plane perpendicular to the magnetic field direction. The oscillations observed experimentally are attributed to the maxima located above the thresholds, not the Feshbach resonances.

Fluorescence of Li excited by laser light and simultaneous thermal ${\mathrm{Sr}}^{\mathrm{*}}$ collisions in the reaction Sr(5p)+Li(2s)+ħω (λ=669.8 nm)→Sr(5$s^2$)+Li(4d) shows an asymmetric excitation spectrum of the energy transfer versus λ. The peak variation with light intensity demonstrates the strong-field regime and verifies current models. A change in the slope of the quasistatic wing agrees with the result of a recent three-level model.

Quantitative ion yields from photoionization of gaseous CO have been measured with time-of-flight mass spectrometry and photoion-photoion coincidence techniques using synchrotron radiation from 270 to 360 eV and from 515 to 615 eV, in the region of C 1s and O 1s excitation and ionization. Dissociative multiple-ionization efficiencies, ion-branching ratios, and the relative yields of all possible ionization reactions are derived. The distributions of kinetic energies released to the (${\mathrm{C}}^{+}$+${\mathrm{O}}^{+}$) and (${\mathrm{C}}^{2+}$+${\mathrm{O}}^{+}$) pairs are measured. Very large contributions of symmetric dissociative double ionization producing ${\mathrm{C}}^{+}$ and ${\mathrm{O}}^{+}$ are found, even in the region of the 1s→${\pi }^{\mathrm{*}}$ resonances. The results are discussed in the context of the autoionization and Auger electron spectroscopy of core-excited CO and with the aid of recent calculations of the potential curves of ${\mathrm{CO}}^{2+}$. The relationship between the present results and those for photon-stimulated desorption following core excitation and ionization of solid and chemisorbed CO is explored.

The interference between different values of the total angular momentum of photofragmentation, and its behavior in the high-J limit, are discussed using the semiclassical angular momentum theory of G. Ponzano and T. Regge [in Spectroscopic and Group Theoretical Methods in Physics, edited by F. Bloch et al. (North-Holland, Amsterdam, 1968)]. Taking the angular momentum of an unobserved part as a quantization axis reveals an interesting aspect of the interference in the high-J limit. The result indicates that a high-J limit does not necessarily correspond to a ‘‘classical’’ limit. The relation between the representation of angular momentum transfer and of total angular momentum is also obtained in the high-J limit in the same way. This relation completes the quantum derivation of classical results for the polarization of fluorescence excited by photodissociation.

Results of Raman scattering experiments carried out on fluid n-${\mathrm{H}}_2$ at room temperature at pressures up to 4.6 GPa are reported. Observed frequency shifts of rotational and vibrational lines are consistent with those observed at lower densities. Line broadening of the $S_0$(0) and $S_0$(1) rotational lines and the $Q_1$(1) and $Q_1$(2) vibrational lines exhibits a nonlinear density dependence. The linewidth of the $Q_1$(1) line saturates at around 2.2 GPa, the pressure at which frequency degeneracy of the $Q_1$(1) and the $Q_1$(2) lines occurs. Results are compared to the impact theory of Fiutak and Van Kranendonk.

The problem of a two-level atom and M degenerate modes of radiation interacting with the atom is solved exactly within the rotating-wave approximation. The results are applied for obtaining the evolution of energy distribution in the atom and the various modes resonant to the atom in an ideal cavity. For a small number of modes the atomic oscillation and the total radiant energy exhibit irregularities in their evolution patterns, while considerable amounts of energy are being transferred from one mode to the other. As the number of modes increases, regular patterns emerge in the atomic oscillation and the total radiation, while only small movements of energy appear among the modes.

The intensity-noise characteristics of a nearly single-longitudinal-mode semiconductor laser are analyzed by adopting a two-mode model. An approximate solution of the resulting Langevin rate equations is used to obtain an analytic expression for the enhancement of the main-mode intensity noise caused by mode partition. We also obtain analytic expressions for the autocorrelation and cross-correlation functions which are used to discuss noise variances in the two modes. In particular, the cross-correlation function is related to the mode-partition coefficient, a measure of anticorrelation between the two modes that is useful to analyze the system performance. Our results show explicitly how the intensity correlation is affected by the mode-suppression ratio and other device parameters.

The effect of current fluctuations on the linewidth of semiconductor lasers is analyzed using the single-mode rate equations. Since the time scale of such fluctuations can generally be longer than the intrinsic time scale of relaxation oscillations, current fluctuations are modeled using a non-Markovian random force in the rate equation governing the carrier dynamics. In the absence of nonlinear-gain effects, the contribution of current fluctuations to the linewidth is negligible at all power levels. However, when the gain saturation resulting from spectral hole burning is included, current fluctuations are found to give rise to a power-independent contribution to the linewidth. At low operating powers, this contribution is small compared with the spontaneous-emission contribution. For InGaAsP lasers, the power-independent contribution is estimated to be ∼1 MHz/μA and can significantly affect the intrinsic linewidth at high power levels (>10 mW). Furthermore, the line shape is not strictly Lorentzian and tends towards Gaussian with increasing power. We have discussed the dependence of the line shape and linewidth on various device parameters.