Merged-beam studies have shown that the recombination of ${\mathrm{HeH}}^{+}$ with electrons is not slow as previously believed. The initial (ground state) of the ions has been identified by examining thresholds for dissociative excitation leading to H-atom formation. The recombination of and excitation from quasimetastable excited states of ${\mathrm{HeH}}^{+}$ have also been measured.

The x-ray radiation emitted by a charged particle as it penetrates a perfect crystal is studied. We find that in addition to the already known Čerenkov radiation peak, there is a second peak which is due to the dynamical diffraction of the electromagnetic field associated with the electron. The theory developed here, which is an extension of the dynamical theory of x-ray diffraction, allows us to treat situations in which the direction of incidence of the electron, as well as the orientation of the crystal surface relative to the crystal planes, are kept arbitrary. Our equations apply also to situations in which the Bragg angle is close to π/2, where several of the approximations usually made in the dynamical theory are known to fail. Various features such as intensities, direction, frequency, and the spectral and angular widths of both the Čerenkov and the ‘‘transition-diffracted’’ radiation are studied. A geometrical construction of the dispersion surfaces has been found to be very useful in visualizing the content of the theory. It is possible that these two processes of emission may find application as a tunable source of monochromatic and very directional x rays.

Energies and oscillator strengths for electric dipole transitions between states with principal quantum numbers 3 and 2 of singly excited oxygenlike ions have been calculated for 32 ions with atomic numbers 10≤Z≤79 using the multiconfiguration Dirac-Fock method. The calculations include finite nuclear size, Breit interaction, and quantum electrodynamic corrections. The Z dependence of the oscillator strengths throughout the sequence is analyzed for some selected transitions. Effects of both relativity and configuration interaction are seen to be very important for medium and highly charged ions. Numerous irregularities and some sharp discontinuities in the Z dependence of the oscillator strengths are found for many transitions due to the effect of level crossings. A rapid interpolation based on results obtained for a few ionization stages can therefore lead to serious errors.

Excitation spectra of ${\mathrm{Cd}}_2$ van der Waals molecules produced in a molecular beam employing free-jet supersonic expansion, crossed with a pulsed dye-laser beam, were studied using He, Ne, and Ar as carrier gases. Well-resolved structures, arising from transitions between the vibrational levels of the ${\mathrm{XO}}_g^{+}$${(}^1$${\Sigma }_g^{+}$) and $O_u^{+}$${(}^3$${\Pi }_u$) states, were recorded at various translational temperatures. An analysis of the spectra yielded the vibrational constants and bond strengths of the molecule in the two electronic states. The lifetimes of the Cd(5 ${}_{}{}^3{}_{}{}^{}P_1^{}{}_{}{}^{}$) atoms and ${\mathrm{Cd}}_2$(${\mathrm{O}}_{\mathrm{u}}^{+}$) molecules in various vibrational states were also determined.

A projection-operator formalism is employed in studying photorecombination at energies close to an autoionizing resonance. A resonance profile is defined, and is presented without approximation for a model system consisting of one electron continuum, one autoionizing state, and one final radiatively stabilized atomic state. Effective line-profile parameters ɛ^ and q^ are defined, and the photorecombination resonance profile is shown to be the same as the photoionization resonance profile (for the process in which initial and final atomic states are interchanged from the photorecombination process), provided one-photon spontaneous radiative decay effects are included. The effective line-profile parameters are examined, and are shown to reduce to the standard autoionization line-profile parameters in the limit of no spontaneous radiative decay. Effective line-profile parameters are also defined in the pole approximation for two model systems featuring an isolated autoionizing state. Comparison is made with effective line-profile parameters that have been defined elsewhere in the context of photoionization into coupled electron-continuum channels. The similarity in mathematical formalism for systems supporting multiple continua, regardless of whether these continua are electron or photon continua, is discussed.

Most of the lithiumlike-ion resonance-line wavelengths have been remeasured relative to well-established lines of He ii and C v, C vi in the Tokamak Fusion Test Reactor and to these are added the newly measured lines of Kr xxxiv and Mo xl from the Joint European Torus, which allow a systematic evaluation of differences from calculated one-electron Lamb shifts.

Detailed multiconfiguration Dirac-Fock calculations through the proposed alternative special average-level version with the inclusion of the transverse (Breit) interaction, self-energy, and vacuum polarization corrections have been carried out on molybdenum, palladium, and holmium to elucidate the structure of the Kα$L^0$$M^r$ lines in their x-ray spectra and to explain reliably the influence of additional holes in the M shell on the shapes and positions of Kα$L^0$ bands. The structure of Kα$L^0$$M^r$ lines has been shown to be very complex. Because the distances between the neighboring Kα$L^0$$M^r$ lines are very small, the effects of multiple M-shell ionization are manifested in the spectra as an asymmetric broadening and net shift of K${\alpha }_1$$L^0$ and K${\alpha }_2$$L^0$ bands. The results obtained in the present work corroborate some of my previous conclusions [Polasik, Phys. Rev. A 39, 616 (1989)]: that removing a 3ρ electron is more effective than a 3s or 3d electron in producing a Kα$L^0$ energy shift and that the shift effects remarkably increase with atomic number and are strongly nonadditive. The results of this work can be used to construct different shapes of theoretical Kα$L^0$ bands for molybdenum, palladium, and holmium, satisfactorily reproducing the shapes and positions of various experimental Kα$L^0$ bands generated by different inducing projectiles.

The electron-impact excited high-resolution $L_2$,3VV Auger-electron spectra of HCl and DCl are presented. Four narrow bands are discerned that show a rich fine structure. Comparison between the HCl and DCl spectra shows this fine structure to be due to transitions to vibrational sublevels in the (2${\pi }^{\mathrm{-}2}$${)}^3$${\Sigma }^{\mathrm{-}}$, (2${\pi }^{\mathrm{-}2}$${)}^1$Δ, and (2${\pi }^{\mathrm{-}2}$${)}^1$${\Sigma }^{+}$ states of the doubly ionized molecule. By a Franck-Condon analysis the potential curves for all these doubly ionized states in the 2${\pi }^{\mathrm{-}2}$ configuration have been determined. The adiabatic double-ionization energies for the (2${\pi }^{\mathrm{-}2}$${)}^3$${\Sigma }^{\mathrm{-}}$, (2${\pi }^{\mathrm{-}2}$${)}^1$Δ, and (2${\pi }^{\mathrm{-}2}$${)}^1$${\Sigma }^{+}$ states were determined to be 35.68(5), 37.30(9), and 38.45(10) eV, respectively. The potential well is shallower than 1 eV for all these states, and dissociation occurs via a Coulomb-potential mechanism. The double-ionization energies are used to assign a number of lines in the x-ray-excited photoelectron spectrum of HCl.

The ratio d${\sigma }_K$(θ)/d${\sigma }_{\mathrm{free}(\mathrm{\theta }}$) of the differential cross sections for Compton scattering by K-shell electrons of lead to Compton scattering by free electrons has been measured for an incident photon energy of 662 keV and scattering angles between 15° and 150°. The experimental methodology differed in several aspects from those employed by previous investigators. High-resolution Ge detectors were used for both γ-ray and x-ray detection, and the critical determination of true coincident background events was carried out in a different way. We observed a cross-section ratio that is smaller than unity for large scattering angles and goes through a slight maximum at intermediate angles, in agreement with relativistic calculations.

The photoelectron energy spectra resulting from a multiphoton process in ${\mathrm{H}}_2$ with high laser intensities is, in general, determined by the character of the intermediate resonance state. Above-threshold ionization (ATI) is observed in the ionization of the molecule ${\mathrm{H}}_2$ as well as of its excited atomic fragments, and even of atomic hydrogen in the ground state. ATI is observed with five, three, and two extra photons for three different wavelengths λ=532 nm, λ=355 nm, and λ=266 nm, respectively. In the λ=532 nm spectra radiative coupling between dressed states is observed. A shift to higher values of the center of the vibrational distribution of the ion by seven vibrational quanta is observed between ATI peaks which differ by four photon energies. A calculation, taking into account a coupling of the ${\mathit{B}}^1$${\mathrm{\Sigma }}_{\mathit{u}}^{+}$ state to the dressed ${}_{}{}^1{}_{}{}^{}\mathrm{\Sigma }_{\mathit{g}}^{+}{}_{}{}^{}$(2p${\mathrm{\sigma }}_{\mathit{u}}$${)}^2$ state, and of the X ${}_{}{}^2{}_{}{}^{}\mathrm{\Sigma }_{\mathit{g}}^{+}{}_{}{}^{}$ state by the dressed 2p${\mathrm{\sigma }}_{\mathit{u}}$ state, shows the trend of the observed ionization spectrum. The calculation is performed to describe the effect on the average of the vibrational distribution and not on the distribution itself. The structure of the ATI spectrum in the λ=355 nm and λ=266 nm is nearly a reproduction of the normal ionization spectrum, repeated several times at higher electron energies. In the λ=266 nm spectra both ATI of the molecule, as well as ATI of the atomic fragments, are observed.

An analytical theory is described for the vibrationally induced photodetachment of electrons from negative molecular ions with large dipole moments. The approach is based on the use of rotationally adiabatic potential curves which are obtained by diagonalizing a simple charge-dipole interaction potential. The mechanism of the vibrationally induced electron detachment is related to the crossing of an attractive rotationally adiabatic potential-energy curve for the vibrationally excited state with repulsive curves for the vibrational ground state of the parent molecule. A simple Franck-Condon theory is used to treat the curve crossing, and a formula for the linewidths in the photodetachment spectra is derived. An interesting prediction is that, for large values of the total angular momentum J, the linewidths decrease with increasing J. This finding is in agreement with recent experimental results for the photodetachment of electrons in ${\mathrm{PtN}}^{\mathrm{-}}$.

Analytic solutions are developed for the time-dependent intensity that results from the three-wave mixing of multimode fields. These differ dramatically from the solutions for single-mode (time-independent) fields. New frequencies in the output spectrum are created as input intensities increase. Under conditions of strong nonlinear coupling the output spectrum broadens, ultimately reaching the phase-match bandwidth limit, and the dynamics can become chaotic. A new technique for enhancing mixing efficiencies is presented.

We study the interaction of two optical fields in a laser operating on two coupled lines. A semiclassical model of such a system is developed. We focus our attention on the intensity correlation between the two fields, referred to as cross correlation. This two-photon correlation affects populations of lasing levels as well as the polarization of the medium. A set of field equations derived within this model describes the behavior of the intensities and their correlation along the laser cavity. Numerical solutions of these equations show a substantial increase of the degree of intensity cross correlation in the course of propagation. The cross correlation evolves in a way significantly different from that of the product of intensities. Theoretical results are in good qualitative agreement with the experiment carried out on a helium-neon laser, which is also reported here.

We investigate the resonance fluorescence spectrum of a bulk semiconductor subject to an injection current and an arbitrarily intense electromagnetic wave. We assume that the electron-hole system is in quasiequilibrium due to the rapid carrier-carrier intraband scattering. The analysis is based on the generalized Bloch equations for semiconductors. Although the fast carrier scattering heavily damps the electronic coherence, it nevertheless allows the total carrier density to adiabatically follow the relatively slowly varying field fluctuations. As a consequence, the resonance fluorescence spectra reveal an asymmetric peak generated by pump scattering from carrier-density pulsations induced by the interference between the pump and a vacuum mode. Similar to the two-level fluorescence and probe absorption, the semiconductor-probe absorption coefficient equals the difference between the resonance fluorescence and reabsorption coefficients. The results are important for nonlinear semiconductor spectroscopy and semiconductor laser instabilities.

Photon-echo and stimulated-photon-echo experiments in ${\mathrm{Pr}}^{3+}$:${\mathrm{LaF}}_3$ on the three transitions, ${}_{}{}^3{}_{}{}^{}\mathrm{P}_0^{}{}_{}{}^{}$-${(}^3$${\mathrm{H}}_5$${)}_1$, ${}_{}{}^3{}_{}{}^{}\mathrm{P}_0^{}{}_{}{}^{}$-${(}^3$${\mathrm{H}}_5$${)}_2$, and ${}_{}{}^3{}_{}{}^{}\mathrm{P}_0^{}{}_{}{}^{}$-${(}^3$${\mathrm{H}}_6$${)}_1$ [none of which, therefore, involves the ${(}^3$${\mathrm{H}}_4$${)}_1$ ground state] at liquid-He temperatures have revealed the presence of anomalously large residual homogeneous linewidths. The subscripts (e.g., 1,2) denote the crystal-field-split levels of the ${\mathrm{Pr}}^{3+}$ ion states. Amplitude-modulated trilevel echoes were observed involving the ${}_{}{}^3{}_{}{}^{}\mathrm{P}_0^{}{}_{}{}^{}$-${(}^3$${\mathrm{H}}_5$${)}_1$ and ${}_{}{}^3{}_{}{}^{}\mathrm{P}_0^{}{}_{}{}^{}$-${(}^3$${\mathrm{H}}_5$${)}_2$ transitions while no trilevel echoes were observed involving the ${}_{}{}^3{}_{}{}^{}\mathrm{P}_0^{}{}_{}{}^{}$-${(}^3$${\mathrm{H}}_4$${)}_1$ and ${}_{}{}^3{}_{}{}^{}\mathrm{P}_0^{}{}_{}{}^{}$-${(}^3$${\mathrm{H}}_6$${)}_1$ transitions. Photon echoes were observed on many of the satellite lines of the principal resonance ${}_{}{}^3{}_{}{}^{}\mathrm{P}_0^{}{}_{}{}^{}$-${(}^3$${\mathrm{H}}_4$${)}_1$ transitions.

The resonance fluorescence spectrum of linear and nonlinear oscillators driven by a monochromatic field of frequency ${\omega }_0$, near the resonant frequency ω, is calculated both classically and quantum mechanically. The result for the linear oscillator is the same in both calculations: a δ function at ${\omega }_0$. For the nonlinear oscillator, however, the classical and quantum-mechanical spectra near resonance are qualitatively different: Classically, a δ function at ${\omega }_0$ is obtained; quantum mechanically, a weak-field calculation leads to a δ function at ${\omega }_0$ together with finite-width peaks nearby. An intuitive explanation for the difference is suggested by the examination of the quantum-mechanical energy-correlation function for the linear oscillator. This function describes energy oscillations with frequencies within a band centered at ${\omega }_0$-ω. The oscillations can be interpreted as the beating of the driven oscillation with those produced by the vacuum fluctuations (the latter having a frequency spread of the order of the relaxation constant). Because of the nonlinearity, the energy oscillation modulates the effect of the driving field to produce sidebands. Higher-order processes produce additional peaks. A heuristic picture of vacuum fluctuations is developed.

We have studied laser optical pumping of Rb at densities up to 4×${10}^{14}$ atoms/${\mathrm{cm}}^3$ and produced polarized ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ by spin exchange with Rb. Numerical calculations show that for a high-density Rb vapor GaAlAs diode laser arrays can provide results similar to those obtained with a broadband ${\mathrm{Kr}}^{+}$-ion–dye-laser system. We have confirmed these calculations with measurements using a 40-GHz full width at half maximum bandwidth dye laser and 100- and 500-mW GaAlAs diode laser arrays (Spectra Diode Labs Array Nos. SDL-2410C and SDL-2450C). We have optically pumped Rb of density [Rb]=4×${10}^{14}$ atoms/${\mathrm{cm}}^3$, and we have measured up to 70% ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ polarization in a 4-${\mathrm{cm}}^3$ volume with 600 mW from the dye laser. With the diode laser arrays, we have pumped with 30 and 300 mW with results comparable to those obtained with the dye laser. With a 1-W diode laser array, we anticipate 70–80 % ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ polarization in a 4-${\mathrm{cm}}^3$ volume.

We present a linear stability analysis for coupled nonlinear solitonlike pulses propagating in optical guiding structures supporting two linearly and/or nonlinearly coupled modes. The analytical predictions are well confirmed by numerical experiments.

In this paper we describe the effects of slippage on the single-pass high-gain free-electron laser (FEL) amplifier. We use a one-dimensional computational code to show the existence of two new dynamical regimes characterized by a dimensionless parameter K, which is a measure of the slippage in one gain length. We define the long-pulse limit to be when K≪1 or the electron pulse length $L_e$ is much greater than a properly defined ‘‘cooperation length’’ $L_c$ ($L_e$≫$L_c$). In this case we find that only the leading region of the propagating radiation pulse exhibits the usual steady-state behavior, with peak power proportional to $n_e^{4/3}$ (where $n_e$ is the electron-beam density). The trailing (slippage) region exhibits a spiking behavior with peak intensities reaching many times the saturated intensity predicted by steady-state theory. We define the short-pulse regime to be when K≳1 ($L_e$≲$L_c$). In this regime the peak power emitted by the electrons does not scale as $n_e^{4/3}$, as predicted by steady-state theory, but scales as $n_e^2$, which is typical of superradiant behavior. Fur- thermore, energy is extracted from the electrons in a continuous way, with no steady-state synchro- tron oscillatory-type behavior.

We have investigated the Jaynes-Cummings model for amplitude-squared squeezing. It exists when the atom is initially either in its excited or ground state. It is noted that it occurs during the collapse period.