X-ray absorption and emission measurements in the vicinity of the chlorine K edge of the three chlorofluoromethanes have been made using monochromatic synchrotron radiation as the source of excitation. By selectively tuning the incident radiation to just above the Cl 1s single-electron ionization threshold for each molecule, less complex x-ray-emission spectra are obtained. This reduction in complexity is attributed to the elimination of multielectron transitions in the Cl K shell, which commonly produce satellite features in x-ray emission. The resulting ‘‘satellite-free’’ x-ray-emission spectra exhibit peaks due only to electrons in valence molecular orbitals filling a single Cl 1s vacancy. These simplified emission spectra and the associated x-ray absorption spectra are modeled using straightforward procedures and compared with semiempirical ground-state molecular-orbital calculations. Good agreement is observed between the present experimental and theoretical results for valence-orbital energies and those obtained from ultraviolet photoemission, and between relative radiative yields determined both experimentally and theoretically in this work.

The convergence of the quantum-mechanical (impact-theory) partial-wave series for the shift d and width w of the Lyman-α line due to electron collisions in a plasma is studied. Asymptotically exact expressions for the relevant S-matrix elements are derived, and it is shown that the series for d converges, whereas w diverges logarithmically as in semiclassical theories. The conclusions are verified by numerical calculations. Some numerical results are reported for plasmas with temperatures in the range 1–5 eV and densities 2×${10}^{17}$ and 4×${10}^{17}$ ${\mathrm{cm}}^{\mathrm{-}3}$.

A theory has been constructed for the ${\mathrm{CH}}_3$F molecule’s nuclear-spin conversion induced by the intramolecular magnetic dipole-dipole nuclei interaction. It is shown that for ${}_{}{}^{13}{}_{}{}^{}\mathrm{CH}_3^{}{}_{}{}^{}$F molecules this mechanism leads to the rate of spin conversion (0.71±0.2)×${10}^{\mathrm{-}2}$ ${\mathrm{s}}^{\mathrm{-}1}$/Torr, which is approximately one-half of the experimentally registered value.

A general theory is developed for third-order linear absorption of electromagnetic radiation propagating through a molecular gas. The process is obtained by pairs of molecules that are within a distance smaller than the wavelength. The resulting absorption coefficient is calculated using a statistical model. Numerical calculations of the third-order linear absorption of ${\mathrm{H}}_2$O in the millimeter-wave region are made and the results are compared with the available empirical formula for the water-vapor continuum.

NMR spin echoes were observed under excitation with one continuous source of white noise. The echo is obtained by nonlinear interference of the response with the cube of the excitation in a third-order cross correlator. The position of the echo as well as the time axis for buildup and decay are determined by time delays of the excitation in the correlator. The experiment and the theory are described with reference to pulsed optical and magnetic-resonance echoes. When comparing to 90° and 180° pulses, the excitation power is considerably lower.

We report the discovery of predissociation channels in ${\mathrm{H}}_3$ at energies within a few meV of the lowest ionization threshold. The predissociation affects selected p- and d-Rydberg states with high principal quantum numbers. For the d states, the predissociation process is induced by very weak electric fields, of order less than 1 V/cm. The predissociations are due to interaction with vibrationally excited states of low principal quantum number that exhibit significant coupling to the repulsive ground-state surface of ${\mathrm{H}}_3$, 2${\mathit{p}}^2$E’. We discuss the possible significance of these predissociation channels for decay paths in dissociative recombination of ${\mathrm{H}}_3^{+}$ with low-energy electrons.

By designing transition radiators to emit x rays at the foil material’s K-, L-, or M-shell photoabsorption edge, the x-ray spectrum is narrowed. The source is quasimonochromatic, directional, and intense and uses an electron beam whose energy is considerably lower than that needed for synchrotron sources. Depending upon the selection of foil material, the radiation can be produced wherever there is a photoabsorption edge. In this paper we report the results of the measurement of the x-ray spectrum from a transition radiator composed of 10 foils of 2-μm titanium and exposed to low-current, 90.2-MeV electrons. The measured band of emission was from 3.2 to 5 keV. In addition, a measurment was performed of the total power from a transition radiator composed of 18 foils of 2.0-μm copper exposed to a high-average-current electron beam of 40 μA and at energies of 135, 172, and 200 MeV. The maximum measured power was 4.0 mW. The calculated band of emission was from 4 to 9 keV.

The angular distributions of fragment ions produced from NO in N K-shell or O K-shell excited states have been measured with a rotatable time-of-flight mass spectrometer. The asymmetry parameter (β) derived from the measured angular distribution is -0.75 at 399.7 eV (N 1s→${\mathrm{\pi }}^{\mathrm{*}}$). The β parameter for transitions of the N 1s electron to Rydberg orbitals changes with the symmetry of the transitions. The β parameter shows a broad maximum around 415 eV due to a ${\mathrm{\sigma }}^{\mathrm{*}}$ shape resonance. The β parameter for O 1s excitation is similar to that for N 1s excitation.

The proton and deuteron spectra following resonant multiphoton ionization of ${\mathrm{H}}_2$ and HD at 193 nm through the E,F state have been studied at power densities of ${10}^{10}$ W/${\mathrm{cm}}^2$. Our results show that the intermediate state induces competition between 3(2+1)-photon ionization, dissociation, and dissociative ionization channels. We find that the competition depends on the specific vibrational and rotational levels excited in the intermediate state. At the same time, the mere presence of this bound intermediate state renders these three-photon fragmentation channels so efficient that higher-order fragmentation processes, such as Coulomb explosions requiring many more photons, are not observed.

Fluorescent emission by an atom near a phase conjugator (PC) based on four-wave mixing is studied from first principles. The Maxwell-Heisenberg equations are solved for the radiation field, and with an asymptotic expansion an expression is derived for the field in the far zone. The total emitted power that can be measured by a detector in this region is evaluated, and it is found that this power acquires three distinct contributions. First, there are photons that are emitted by the atom directly towards the detector, and without any interaction with the medium. Second, there are photons that first travel towards the surface of the PC, and they have a certain probability of being reflected in the specular direction and towards the detector. The third kind of radiation consists of phase-conjugated photons, which are emitted independently of the previous ones. It is shown that the first two processes are a result of simple atomic spontaneous decay, but that the emission of a phase-conjugated fluorescent photon involves a three-photon process. The latter process has a probability proportional to the population of the atomic ground state. It is pointed out that an atom in its ground state polarizes the nonlinear medium of the PC, which subsequently can emit spontaneously two photons. An absorption-emission-absorption process by the atom then produces a fluorescent photon, together with a spontaneous excitation of the atom.

We present a theory of the micromaser including the effect of the arrival-time statistics of the active atoms in the theoretical treatments. Our starting point is an approach that explicitly incorporates the effect of pump statistics into the dynamical description of lasers and masers. We show that the application of standard laser theory to the micromaser is tantamount to the tacit assumption of Poissonian arrival-time statistics. Hence standard laser theory and the theory of the micromaser are, in general, not equivalent, but the latter contains the former, as a limiting case, for completely random arrival times. We show, as an application, that the steady-state value of the average photon number (first moment) is essentially unaffected by the pump statistics since it depends only on the flux of active atoms (average number of atoms entering the cavity per unit time) provided the flux is constant. The intensity fluctuations (second moment), however, exhibit a stronger dependence on the pump statistics. In particular, for sub-Poissonian pump statistics the sub-Poissonian character of the field is enhanced.

We study the angular correlations of photons emitted spontaneously by a set of sources placed at fixed positions with the use of a model of harmonic oscillators coupled to the electromagnetic field. In our previous study of angular correlations, in which the oscillators were treated as one dimensional and their Coulomb interactions were disregarded, we found only positive angular correlations over the whole range of interatomic separations for which the model was applicable. In this paper we use the model of three-dimensional oscillators interacting with the quantized electromagnetic field and also directly among themselves via the Coulomb forces. We solve this model analytically in the dipole approximation, and we calculate the angular correlation coefficient for different separations between the emitters. For separations less than half of the wavelength, we find negative angular correlations (spatial antibunching). This is a purely quantum effect that is not present in the corresponding classical stochastic model.

The resonance condition is used to describe theoretically the three main kinds of standing waves with E⊥B (linear) and E?B (helicoidal and circular polarizations) that can be produced by the same laser and their nonlinear interaction with an atomic medium. It is shown that the saturation behavior of a single-mode gas laser is drastically modified by the nature of the operating standing wave. The predictions are checked experimentally with a laser from which each kind of standing wave can be generated with the same losses. The anisotropic structure of the saturation and in particular of the Lamb dip is isolated in each case. Very good agreement is observed between experiments and theory.

We investigate the atomic inversion dynamics when a multilevel atom interacts with a two-mode quantized radiation field in a Raman-type process. By utilizing the rotating-wave approximation and adiabatically eliminating all but two of the atomic levels, we derive a nonlinear-interaction Hamiltonian for this system. Working in the Schrödinger picture, we obtain numerical results that depend on the width and average photon number of the field statistics. Varying these photon statistics allows us to uncover secondary revivals that arise due to the complicated double sum over the two field modes that appear in the solutions. The origin of this phenomenon is described analytically.

Coherent subharmonic anti-Stokes Raman scattering in six-wave mixing is analyzed in strongly driven four-level systems. Fractional-Rabi-frequency Stark splittings are predicted and studied as a function of the laser-beam intensities and frequency detunings. New resonances around the fractional Raman resonance are also predicted.

We present results of Floquet calculations of shifts and widths of the 1s and 2s energy levels of atomic hydrogen irradiated by intense linearly or circularly polarized light whose frequency ω is above the (weak-field) threshold ${\mathrm{\omega }}_{\mathrm{thr}}^{\left(\mathit{i}\right)}$ for one-photon ioinzation from state i. We have studied the dependence of the shifts and widths on ω and on the intensity I. Where possible, we compare our results with those obtained from a high-frequency theory [M. Pont and M. Gavrila, Phys. Rev. Lett. 65, 2362 (1990)] that yields shifts that depend only on ${\mathrm{\alpha }}_0$ (∝ √I /${\mathrm{\omega }}^2$), the excursion amplitude of a free electron, rather than on I and ω separately. As I increases, with ω fixed, the width reaches a maximum value ${\mathrm{\Gamma }}_{\max }$ at an intensity ${\mathit{I}}_{\max }$ for which √(ħω/2P) ≊1, where P==2πI/μc${\mathrm{\omega }}^2$, the ponderomotive shift. As I increases beyond ${\mathit{I}}_{\max }$, the width decreases toward zero, in accord with the high-frequency theory, and the shift approaches the result of that theory. (For different fixed ω, the shifts first cross the ω=∞ curve as ${\mathrm{\alpha }}_0$ increases, and they intersect, almost at a common value of ${\mathrm{\alpha }}_0$, before approaching the ω=∞ curve.) As ω increases, ${\mathit{I}}_{\max }$ increases as roughly ${\mathrm{\omega }}^3$, and ${\mathrm{\Gamma }}_{\max }$ decreases. If ω is sufficiently large, we find that (2π/ω)${\mathrm{\Gamma }}_{\max }$/ħ<1, so that, for very high frequencies, ionization takes place over more than one cycle even at large intensities. At frequencies below ${\mathrm{\omega }}_{\mathrm{thr}}^{\left(\mathit{i}\right)}$, we detect states that emerge from ‘‘shadow’’ states; these states allow for a continuous variation of the shift and width across the threshold. Furthermore, we conjecture that the rise and fall of the width, in the vicinity of ${\mathit{I}}_{\max }$, occurs through an interaction between shadow and real (or ‘‘dominant’’) states.

Quantum-noise properties of a semiconductor laser under constant-voltage operation, where the laser-field amplitude is regulated through nonlinear gain, have been studied using quantum-mechanical Langevin equations. In the ideal constant-voltage-drift case, the linewidth enhancement from the gain–refractive-index coupling, as well as the relaxation oscillations, disappear, but for realistic nonlinear gains the amplitude noise will increase. The requirement on the laser’s series resistance for constant-voltage operation is, however, very strict, and seems difficult to meet in practice.

This paper is concerned with the spectral profile of spontaneously emitted radiation from Λ and cascade models of driven three-level atoms, and with the absorption spectra of a weak probe. The atoms are excited by a pair of coherent external fields that are resonant or nearly resonant with the two dipole-allowed transitions of each of these two models. The main aim of this work is to extend earlier studies of the V-model configuration of three-level atoms and to present a comprehensive survey of the emission and absorption features of these systems. In addition to a derivation of exact formulas for the spectra and explicit analytic approximations in the high-intensity limit, we provide an explanation for the existence of simultaneous stationary population inversion between pairs of atomic levels and explore the effect of Doppler broadening on the absorption profile of the weak probe. In view of the latter analysis, in particular, we conclude that gain features persist even in the presence of inhomogeneous broadening. This suggests the possibility of experimental tests in a cell rather than an atomic-beam environment.

Motivated by the possibility of developing new computational techniques for studying multiphoton ionization of atoms by monochromatic radiation, we have analyzed, in a nonrigorous fashion, the behavior of the ac quasienergy as a function of the complex field strength F. We elaborate upon earlier work of Manakov and Fainshtein [Theor. Math. Phys. 48, 815 (1981)] and conjecture that the ac quasienergy is a multivalued analytic function ${\mathit{E}}_{\mathrm{ac}}$(F) whose branches originate from different unperturbed (real or ‘‘shadow’’) atomic levels for real values of F. We further conjecture that the branch points of ${\mathit{E}}_{\mathrm{ac}}$(F) are of the square-root type. These branch points occur at complex values of F where two branches coalesce, and, as F sweeps along the real axis, the passage past a branch point coincides with the passage past either an intermediate multiphoton resonance (R) or a multiphoton ionization threshold (T). Branch points of type-R group into quadruplets, while branch points of type-T group into pairs. The two branches that intersect at a type-R branch point are both physically accessible, and originate from real levels, while only one of the two branches that intersect at a type-T branch point is physically accessible—the unphysical type-T branch is a ‘‘shadow’’ eigenvalue, which corresponds to a state with unphysical boundary conditions. We discuss the probability for the atom to make a transition from one branch to another when F is a slowly varying function of time. Normally, a type-R branch point enhances the ionization signal, while a type-T branch point diminishes the signal. In partial support of some of our conjectures, we present results of an accurate numerical study of the quasienergy (and its perturbation expansion) for the ground state of both the hydrogen atom and a model atom (an electron bound to a zero-range potential).

The beam-wave interaction in a Čerenkov traveling-wave tube (TWT) is considered theoretically. The system is assumed to operate at 8.75 GHz and the injected electrons have an average kinetic energy of 0.85 MeV. We initially consider the case corresponding to a prebunched beam. In order to avoid saturation, two kinds of tapering are considered: one that is adaptable, in the sense that the phase velocity follows exactly the particle’s velocity, whereas the other is characterized by a dielectric coefficient that varies linearly in space. When the electrons are in antiphase with the wave (for ${\mathit{E}}_0$=5 MV/m, I=1 A, and d=0.8 m), the adaptable tapering causes the accelerated bunch of electrons to achieve an energy that is twice that obtained with a linear tapering. For the same parameters there was practically no acceleration in the case with a uniform dielectric coefficient. The other case considered corresponds to a beam whose electrons have a uniform distribution of phases relative to the wave. The development of the bunching is illustrated as well as the strong Compton regime, where the exponential gain occurs. The latter is in reasonable agreement with the predictions of conventional TWT wave theory; however, the bunching takes more of the interaction region to develop than predicted by Pierce’s theory. We show that by tapering both gain and efficiency increase. When operated as an amplifier (${\mathit{E}}_0$=1.0 MV/m, I=450 A, and d=0.3 m) with an optimal linear variation of the dielectric coefficient, the device’s efficiency is improved to about 30% from 5% in the uniform device.