We have extended an earlier study of the effect of a laser field on ${\mathrm{He}}^{+}$-He charge-transfer collisions to consider the effect of a finite laser bandwidth and to include the full multiple-photon exchange with the field. The first effect is shown to be negligible so long as the collision time lies within ∼ħ/(bandwidth). As regards the second aspect, we show that in this nonperturbative treatment it can only be studied by discarding the rotating-wave approximation. Charge-transfer cross section after photon exchanges with the field until ‘‘saturation’’ is found to have a nonlinear, oscillatory intensity dependence.

We present a detailed study of the influence of the form and strength of the interatomic potential on the one-dimensional elastic collision kernel W($v_z^{\mathcal{’}}$,$v_z$), a quantity of interest in the study of the effects of velocity-changing collisions on laser spectroscopic line shapes. We find that the absolute magnitudes of collision kernels are very sensitive while normalized collision kernels are moderately sensitive to the potential form used. This indicates the importance of employing realistic interatomic potentials and reliable differential cross sections in the accurate determination of collision kernels. For the case of the Lennard-Jones (12,6) potential, we found a universal semiclassical Lennard-Jones (SCLJ) analytical model function, which is the combination of a semiclassical expression for small to medium scattering angles and a classical expression for large scattering angles, capable of providing correct average quantum-mechanical behaviors of differential cross sections for all scattering angles. This greatly facilitates the (often time-consuming) numerical evaluation of the collision kernel integrals and exhibits the correct collision kernel line profiles. It is found that the SCLJ collision kernel consists of a strongly peaked forward diffractive zone (small-angle scatterings), reflecting the nature of velocity resonance, as well as a broad wing region due to large-angle scatterings. Ambiguities associated with the drawbacks of the hard-sphere model, the small-angle classical long-range model, and the classical Lennard-Jones model are analyzed and clarified. While our analysis is confined to the Na-Ar and Ar-Ar systems, the conclusions derived from this study are general and are expected to be also applicable to other systems where both the long- and short-range interactions play essential roles in velocity-changing collisions.

Differential and total cross sections for proton-impact excitation of helium to the n=2 sublevels are calculated in the distorted-wave Born approximation. Many-parameter correlated wave functions are used to describe the helium atom. A comparison with recent theories and experimental measurements is made. The present calculations for differential cross sections for the n=2 substates yield good agreement with the measured values of Park et al. and Kvale et al. and the multistate eikonal calculations of Flannery and McCann.

Total-cross-section results for electron-impact excitation of ${\mathrm{Li}}^{+}$ from the ground state 1$s^2$ ${\mathrm{}}^1$S to each of the various doubly excited autoionizing states (viz., 2$s^2$ ${\mathrm{}}^1$S, 2s2p ${\mathrm{}}^1$,3P, 2$p^2$ ${\mathrm{}}^1$S, 2$p^2$ ${\mathrm{}}^3$P, 2$p^2$ ${\mathrm{}}^1$D, 2p3p ${\mathrm{}}^1$P, and 2p3d ${\mathrm{}}^1$,3D) are reported. A consistent version of the distorted-wave approximation method, which includes distortion in both the channels due to static, polarization, and exchange effects, is used for the calculation. The contribution arising from the excitations of these autoionizing states to direct ionization by electron impact is also discussed.

We study the collisionally induced excitation and photon emission of a neutral radiator atom surrounded by a gas of neutral foreign perturbers within the binary-collision approximation. We derive an equation of motion for the reduced density matrix of the radiator and obtain the spectrum of the spontaneously emitted photons including components due to collisionally induced radiation and to Rayleigh scattering, which is valid in both impact and quasistatic regimes. In our formulation single and subsequent collisional contributions are taken into account, and various coherent and incoherent corrections are examined.

Using stimulated Raman gain spectroscopy, we have measured the temperature and density dependence of the Raman linewidth and line shift for the Q(1) and Q(0) vibrational transitions in ${\mathrm{H}}_2$ covering the range 77–480 K and 1–20 amagats, and as a function of ortho to para ratio. We conclude that the dominant contribution to the Q(1) linewidth comes from vibrational dephasing collisions, an effect not included in current theories. The line-shift coefficient has a much larger temperature variation than the linewidth coefficient, with the ratio of line shift to linewidth reaching a maximum value of 7.3 at 81 K. These results are important for applications requiring the tuning of the Stokes frequency generated by the stimulated Raman process.

The effects of external electric fields on dielectronic recombination have been studied in the distorted-wave approximation. The eigenvectors for the doubly excited Rydberg states were determined by diagonalizing a Hamiltonian matrix which includes the internal electrostatic and spin-orbit terms, as well as the Stark matrix elements. Calculations were performed for the dielectronic recombination transitions associated with the 2s→2p excitation in the Li-like ions ${\mathrm{B}}^{2+}$, ${\mathrm{C}}^{3+}$, and ${\mathrm{O}}^{5+}$; and the 3s→3p excitation in the Na-like ions ${\mathrm{P}}^{4+}$, ${\mathrm{S}}^{5+}$, and ${\mathrm{Cl}}^{6+}$. The results for the Na-like ions are compared with recent merged-beam measurements, and the differences between the two sequences with respect to the enhancement of dielectronic recombination as a function of electric field strength are discussed.

Calculations of photoionization accompanied by excitation are reported for Ca using the R-matrix approach. Cross sections for leaving the ${\mathrm{Ca}}^{+}$ ion in the states 3$p^6$4s, 3$p^6$3d, and 3$p^6$4p are presented from the 4p threshold to a photon energy of 26 eV. Extensive configuration-interaction wave functions are used to describe both the initial and final states, and coupling between continua based on different thresholds is also included. The results differ considerably from previous calculations where coupling between the continua has been neglected. We show that in particular the 4$s^2$→4sεp cross section is very sensitive to a number of factors. It is predicted that the 3d photoelectron satellite will be stronger than the main line (4s) from below the 4p threshold up to approximately 12 eV. No experimental measurements exist at the moment below 21.22 eV. At this energy there is reasonable agreement between the different calculations and with experiment.

Resonant three-photon ionization of barium, via five closely spaced excited bound states—labeled 6s8s ${}_{}{}^1{}_{}{}^{}S_0^{}{}_{}{}^{}$, 6$p^2$ ${}_{}{}^3{}_{}{}^{}P_0^{}{}_{}{}^{}$, 6$p^2$ ${}_{}{}^3{}_{}{}^{}P_1^{}{}_{}{}^{}$,6s7d ${}_{}{}^1{}_{}{}^{}D_2^{}{}_{}{}^{}$, and 6$p^2$ ${}_{}{}^3{}_{}{}^{}P_2^{}{}_{}{}^{}$—has yielded both angular distributions of energy-resolved photoelectrons and branching ratios to the 6s, 5d, and 6p configurations of ${\mathrm{Ba}}^{+}$. In contrast to earlier ab initio and semiempirical theoretical analyses which attributed nearly pure configurations to these five bound states, our experimental findings are consistent with a significant degree of 6$p^2$ character in the 6s8s ${}_{}{}^1{}_{}{}^{}S_0^{}{}_{}{}^{}$ and 6s7d ${}_{}{}^1{}_{}{}^{}D_2^{}{}_{}{}^{}$ levels, more in accord with their original configuration labels in the tables of Moore. Branching to the 6p states of the ion predominates in each case, and the angular distributions of the corresponding photoelectrons from the 6$p^2$ ${}_{}{}^3{}_{}{}^{}P_2^{}{}_{}{}^{}$ and 6s7d ${}_{}{}^1{}_{}{}^{}D_2^{}{}_{}{}^{}$ levels are described well by a parametric theoretical model approximating these states as pure 6$p^2$ configurations. For the 6s8s ${}_{}{}^1{}_{}{}^{}S_0^{}{}_{}{}^{}$ level, inferences based on these observations must be made cautiously because of the possibility of an ‘‘anomalously’’ low photoionization amplitude for the 6s8s component of the state. Available oscillator strengths suggest the possibility of configuration interaction via spin-orbit coupling. Overall, strong configuration mixing and/or mislabeling of the configurations is implied; we suggest that the choice of configurational labels for these states be reexamined.

Partial cross sections σ and angular distribution parameters β were measured for the 5d subshell of atomic lead between 25 and 110 eV, and for the major components of the 6s and 6p subshells between 25 and 55 eV. Data are compared with Dirac-Slater (DS) calculations. Good agreement is found for the β values of both the 5d and 6p subshells. However, as characteristic for single-particle models, the 5d delayed cross-section maximum is seen to be too high. The theoretical DS cross section σ(6$p_{1/2}$) follows our data satisfactorily, but the DS σ(6s) and the DS β(6s) do not. Earlier experimental data for Pb are in satisfactory agreement with the present 5d and 6p data. A comparison of our Pb data with experimental data for Hg and relativistic random-phase approximation results for the 5d and 6s subshells of Hg shows a generally satisfactory accord.

Calculations of the photoionization cross sections of excited nl states of cesium for n≤9 and l≤3 have been carried out within the framework of the central-field model. In addition, oscillator strengths for discrete transitions among these excited states have been obtained. The oscillator-strength distributions are presented and analyzed and the prominent zeros in the matrix elements for excited states are discussed in detail. Procedures and pitfalls in interpolation and extrapolation of discrete oscillator strengths are discussed, and the photoelectron angular distributions from these excited states are also presented.

An ‘‘amplitude model’’ of the laser-rf double-resonance process has been quantitatively verified, providing a complete explanation of the anomalous line shapes observed in studies of ${\mathrm{Na}}_2$ hyperfine structure. We find excellent agreement with new laser-rf double-resonance measurements in low-J’’ rotational levels of the $X^1$${\Sigma }_g^{+}$(v’’=0) ground state of ${\mathrm{Na}}_2$ as well as with earlier data for J’’=28. These results confirm the idea that absorption and stimulated-emission Rabi cycles in the presence of the laser field create coherence between the ground-state hyperfine levels when these levels are optically unresolved. A novel ground-state ‘‘quantum beat’’ experiment is proposed.

We have measured oscillator strengths for the transitions 6 ${}_{}{}^1{}_{}{}^{}S$ to (5$d^{10}$6s np) n ${}_{}{}^1{}_{}{}^{}$,3${\mathrm{P}}_1$ for n=6 to 13 and to (5$d^9$6$s^2$6p) 6p’ ${}_{}{}^1{}_{}{}^{}P$ for atomic Hg. Two new techniques for measuring dispersion based on nonlinear-optical sum-frequency mixing are used to achieve 10% accuracy for the stronger transitions. One method uses crossed beams in a Hg heat-pipe cell to map dispersion curves in regions of anomalous dispersion. The second method uses input beams of three separate frequencies and maps regions of normal and anomalous dispersion by tuning one of the input frequencies to the red and blue of the 6 ${}_{}{}^3{}_{}{}^{}P_1^{}{}_{}{}^{}$ level. Because the dispersion of the refractive index near a resonance is determined over a large wavelength range, the oscillator strength of that resonance is determined independent of the strengths of other transitions.

Both active- and passive-cavity interferometers are considered at present for displacement sensing in broadband gravitational-radiation detectors. In spite of an apparent difference between the noise sources that limit their performance, we show that active- and passive-cavity interferometers are of the same strain (or displacement) sensitivity if identical resonators and stored fields of equal intensity are assumed.

We develop the formalism for studying transient four-wave-mixing phenomena in a Raman medium, and apply it to the study of solitary-wave propagation. We consider the problems of second-Stokes generation, anti-Stokes scattering, and two-pump Raman scattering.

Čerenkov free-electron-laser gain has been calculated in a number of ways by many authors. Often, the method employed depends upon the operational regime, either single-particle or collective. A technique is presented here which is largely regime independent and offers an intuitive picture of the stimulated-emission mechanism. The technique employs the ‘‘averaged Lagrangian,’’ and the reader is referred to Whitham for a more thorough discussion of that method.

By closely following the work of Reid and Walls, a fully quantum-mechanical treatment of an atomic radiation-field interaction is developed with the aim of deriving basic equations for a nondegenerate four-wave mixer. The resultant equations are examined in forward and cavity geometries for their implications regarding squeezed states. Good agreement between our theory and the experiments of Slusher et al. is demonstrated.

A general theory of transient stimulated Raman scattering and the phase-matching diagrams are developed based on an interaction Hamiltonian which includes all the possible competing interaction channels. In a Raman system pumped by multiple crossing beams with multiple Stokes and anti-Stokes components, these channels are defined by the interactions among fields at different frequencies and/or k vectors of the pump pairs and Stokes and anti-Stokes fields. An ensemble-averaged equation of motion for the active phonon mode is derived within the Markovian approximation in which the phonon-phonon coupling (pressure effect) is reduced to an energy relaxation and a phase fluctuation. A complete set of transient coupled equations including phase modulation, Stark shift, medium (anharmonic) saturation, and all the possible four-wave-mixing channels is presented. In the transient regime, it is found that the effects of the Stark shift on the Raman efficiency are mainly governed by the input pump energy in contrast to the steady-state theory where the pump intensity plays the main role. Both analytic and numerical methods are developed in describing the nonlinear effects due to the energy feedback mechanism in the strong pump-depletion regime. Twelve possible four-wave-mixing channels are analyzed for nonlinear phase diagrams characterized by the angles among pump pairs, and Stokes and anti-Stokes fields. Finally, numerical examples are shown for the effects of the coherence lengths of high-order Stokes fields and pump pairs on the generation of low-order Stokes fields.

We describe a simple nonlinear optical device which transforms an ingoing cw light beam into a periodically modulated one. The mechanism of operation is due to a spin precession in the ground state of optically pumped atoms with a J=(1/2)→J’=(1/2) transition in the presence of a static transverse magnetic field. With optical feedback from a resonator, this magnetically induced spin precession can be self-sustained and then can give rise to a self-pulsing of the transmitted light at roughly the Larmor frequency. A detailed theoretical description of the system is presented and stability criteria are considered. In contrast to earlier work, our calculations take into account the resonator round-trip time, an optical detuning from the atomic resonance, and absorptive losses within the resonator. New signal features are predicted: These include a complicated structure of the initial transient as well as a precipitation to a stationary state. Our theoretical model is confirmed by measurements which are performed by means of a Fabry-Perot resonator containing sodium vapor. The behavior of the device is studied for a wide range of experimental parameters; threshold powers for oscillation (≳5 mW) and the oscillation frequency and its tuning range (140 kHz–13 MHz) are investigated as well as the dynamics of the system following a step input of light.

The interference of scalar optical Floquet-Bloch waves in periodically stratified media is discussed in detail. Two main types of interference are identified and fully analyzed: normal and exchange interference, producing, respectively, real and virtual spatial fringes. In the paper considerable use is made of a reciprocal-space representation of the Floquet-Bloch waves (the wave-vector diagram), first introduced in the dynamical theory of x-ray diffraction. Experimental observations of the interference of integrated versions of the Floquet-Bloch waves, made in corrugated planar ${\mathrm{Ta}}_2$${\mathrm{O}}_5$ waveguides, are included by way of illustration.