Energy spectra of subexcitation electrons for Ar and ${\mathrm{H}}_2$ are studied by using rigorous theory based on the Spencer-Fano equation. The origins of the structure in the energy spectrum and the differences between the two spectra are discussed.

Superelastic scattering of spin-polarized electrons from laser-excited sodium atoms has been measured at incident energies of 2.0, 17.9, and 52.3 eV over the angular range 10°–120°. Circularly polarized excitation of the sodium atoms was used to produce pure 3 ${}_{}{}^2{}_{}{}^{}P_{3/2}^{}{}_{}{}^{}$ (F=3, $M_F$=±3) states, which are deexcited by collisions with spin-polarized electrons. The spin polarization of both the target electron and the incident electron allows the resolution of triplet and singlet contributions to $L_{\perp }$, the angular momentum transferred in the collision perpendicular to the scattering plane, and the measurement of r, the ratio of triplet to singlet cross sections. At low energy, agreement with theory is good over the entire angular range for r, but only at small angles for $L_{\perp }$. At high energy, agreement is excellent over the full angular range.

We present and compare total cross sections for single-electron removal in collisions of electrons, positrons, protons, and antiprotons with atomic hydrogen and helium. These cross sections have been calculated using the classical trajectory Monte Carlo technique in the velocity range of 0.5–7.0 a.u. (6.25–1224 keV/u). The cross sections are compared at equal collision velocities and exhibit differences arising from variations in mass and sign of charge of the projectile. At low and intermediate velocities these differences are large in both the ionization and charge transfer channels. At high velocities the single-ionization cross section for each of these singly charged particles becomes equal. However, the differences in the single-charge-transfer cross sections for positron and proton impact persist to very large velocities. We extend our previous work [Phys. Rev. A 38, 1866 (1988)] to explain these mass and sign of the charge effects in single-electron removal collisions.

Continuum electron emission following 700-keV ${\mathrm{Ar}}^{2+}$-Kr collisions has been investigated by detecting electrons in coincidence with projectile ions scattered through laboratory angles between 5° and 20°. From the analysis of the experimental data in terms of quasimolecular Auger-electron emission, molecular-orbital binding energies at small internuclear distances as well as collision-broadening parameters have been inferred. The results indicate that the electron emission is dominated by the decay of 4sσ and 4pπ vacancies in the highly charged Ar-Kr quasimolecule. The analysis also reveals the presence of an exponentially decreasing high-energy tail of the spectra, which cannot be ascribed to quasimolecular Auger electrons and which has not been taken into account in previous analyses.

We report the calculated cross sections for the removal of ${\mathrm{He}}^{+}$ ions in collisions with hydrogen atoms for the collision energy range between 0.01 meV and 100 eV. Radiative association is the dominant mechanism at the low-energy extreme and for collision energies above 10 meV radiative charge transfer is dominant. Above 8 eV the radiative processes are negligible, and direct charge transfer is the primary removal mechanism. Using a quantum-mechanical method, we obtain the spectrum of the emitted radiation for the radiative charge transfer process at several collision energies. The emission spectrum has a main peak at wavelengths near 113 nm, which corresponds to the energy difference between the A ${}_{}{}^1{}_{}{}^{}\Sigma _{}^{+}{}_{}{}^{}$ and $X^1$${\Sigma }^{+}$ states of the molecular ion ${\mathrm{HeH}}^{+}$ at large internuclear distances. At higher collision energies, significant contributions to the emission spectra occur at shorter wavelengths, and distinct peaks appear. We interpret these peaks as a result of orbiting in the outgoing channel.

Elastic scattering of electrons and positrons by the cadmium atom is studied. A real optical potential is used to calculate the phase shifts of the first few partial waves. Then the differential cross sections (DCS’s) are calculated from the formula given by Nesbet [Phys. Rev. A 20, 58 (1979)] using the calculated phase shifts of the first few partial waves. The calculations are performed at incident projectile energies of 40, 60, 75, 85, 100, and 150 eV, where the experimental DCS measurements for electron scattering are available. The agreement between our calculated DCS and the available experimental DCS is reasonable at lower energies and very good at higher energies. Total cross sections are also obtained, but no other results, theoretical or experimental, are available for comparison with them.

It is well known that when a radiator (or absorber) suffers velocity-changing collisions that do not perturb its internal state, the oscillator’s successive Doppler shifts are motionally averaged. This process (Dicke narrowing) gives rise to a homogeneously broadened sub-Doppler resonance, which is often astride a broad, apparently Doppler-width, pedestal. Though it has long been held that this pedestal is an inhomogeneous remnant of Doppler broadening, no evidence has ever been obtained to substantiate this expectation. In the present study we investigate the behavior of Dicke-narrowed line shapes in strong fields in an evacuated wall-coated cell. Our experimental results indicate that the hypothesis of an inhomogeneous Doppler remnant pedestal is invalid. Rather, the pedestal is homogeneously broadened and is best understood in terms of the oscillator’s response to the electromagnetic field’s power spectral density as observed in its rest frame. This conclusion suggests an intimate relationship between the process of Dicke narrowing and the behavior of quantum systems in the presence of stochastic fields. By envisioning the quantum system as a narrow-band antenna that only absorbs a small portion of the field’s energy, we develop a simple intuitive model to account for the interaction of the atom with the nonmonochromatic field; the appeal of this simple theory is that it removes the need for extensive computation. Agreement between this ‘‘atom-as-antenna’’ analogy and experiment is very good.

The radiative and autoionization decay channels of oxygen metastable ionic states have been investigated with beam-foil spectroscopy. Relative branching ratios of the 1s2s2${\mathit{p}}^4$${\mathrm{P}}_{1/2,3/2}^{\mathrm{o}}$ states of ${\mathrm{O}}^{5+}$ to the J=5/2 level have been determined. Radiative and autoionization rates of the J=1/2 and 3/2 fine-structure levels are extracted by using the calculated fluorescence yield of the J=5/2 level of Chen, Crasemann, and Mark [Phys. Rev. A 27, 544 (1983)] and the measured total lifetimes corrected for cascades. The study of the delayed electron emission is extended in nitrogen and carbon ions, where transition rates of the dominating autoionization channel of the 2 ${}_{}{}^4{}_{}{}^{}P_{1/2}^{}{}_{}{}^{}$,${5/2}^{\mathrm{o}}$ states have been determined directly from the experimental lifetimes corrected for cascades.

Auger and radiative transition energies and rates for the states from the 2$p^5$3l3l’ and 2s2$p^6$3l3l’ configurations of the Na-like ions have been calculated for 18 ions with atomic number 18≤Z≤92 using the multiconfiguration Dirac-Fock method. The Z dependence of the transition rates for a few selected states has been analyzed. Numerous irregularities are present in many transitions due to the level-crossing interaction. The effects of relativity and configuration interaction are simultaneously important for medium and heavy ions. These effects can sometimes change the rates for weak transitions by orders of magnitude. Strong configuration interaction between states from the 2$p^5$3$p^2$ and 2$p^5$3s3d configurations has been found. Inclusion of the Breit interaction in calculations of the Auger matrix elements can change the Auger rates by as much as a factor of 2. The spin-orbit mixing and Breit interaction are responsible for the decay of most of the quartet states. In particular, the 2$p^5$3s3$p^4$$D_{7/2}$ metastable state of low-Z ions decays predominantly by Auger-electron emission via the magnetic interaction.

The problem of photodetachment of a negative ion in a Stark field is treated with the account of rescattering of the electron wave reflected by the potential barrier formed by the electric field. In the case of ${\mathrm{H}}^{\mathrm{-}}$ ions the rescattering effect is noticeable and can be observed for the fields above 1 MV/cm. The upper bound for the observation of the effect is about 4 MV/cm, since above this limit the ${\mathrm{H}}^{\mathrm{-}}$ ions would be completely stripped by the Stark field.

A new transition at 55.8 nm (22.2 eV) has been observed in ${\mathrm{N}}_2^{2+}$ arising from multiphoton excitation of ${\mathrm{N}}_2$ with intense subpicosecond 248-nm radiation. The band has an observed width of ∼0.3 nm full width at half maximum, exhibits vibrational splittings, has an estimated radiative lifetime of ∼100 ps, and is attributed to a ${}_{}{}^3{}_{}{}^{}\mathrm{\Pi }_{\mathrm{u}}^{}{}_{}{}^{}$${\to }^3$${\mathrm{\Pi }}_{\mathrm{g}}$ excimerlike transition arising from the direct multiphoton production of molecular levels having a 2${\sigma }_g$ vacancy. The upper level has a (2${\sigma }_g^{\mathrm{-}1}$, 1${\pi }_u^{\mathrm{-}1}$) configuration, an equilibrium internuclear separation of ∼1.2 Å, and results from an entirely new strong-field mode of electronic excitation.

The ionization spectrum of the He atom under double-resonance irradiation is studied theoretically. A previously developed theory is extended and adapted to treat this process. The first source (synchrotron) is tuned around the transition from the ground state to the 23sp– ${}_{}{}^1{}_{}{}^{}P_{}^o{}_{}{}^{}$ autoionizing state, and the second source (laser) is put in resonance with the transition from 23sp– ${}_{}{}^1{}_{}{}^{}P_{}^o{}_{}{}^{}$ to the higher-lying quasibound state 2p3p– ${}_{}{}^1{}_{}{}^{}P_{}^e{}_{}{}^{}$ (or 2p4p, 2p5p). Accurate ab initio calculations have been performed to obtain all the necessary parameters. It is found that the single-field ionization spectrum is strongly modified, exhibiting coherent trapping and ac Stark splitting. These results are in consonance with previous model calculations as well as with other ab initio calculations on a different zone of the He continuum. The results show that for the resonances considered these strong effects can be observed at megawatt laser intensities.

The photodetachment of a particle bound in a one-dimensional square well and subject to a static electric field is considered. This cross section is shown to have a rich structure that can be analyzed in relatively simple terms. The appearance of regularly spaced zeros in the cross section, as a function of photon energy, is one such feature. The spacing of the intervening peaks is either identical to or similar to that found in other studies. The peak asymmetry, the oscillatory amplitude, and another type of zero in the oscillator strength, which are related to the target’s binding potential, are properties of this model not found in other studies. A connecting formula between the finite- and zero-field oscillator strengths is derived and found to be useful for field strengths between zero and that for which tunneling from the initial state becomes a competing mechanism for ionization. An approximate final-state function provides a particularly simple relationship between the two cases and it displays a part of the structure found in the full calculation. The possible connections between this model and experiment are discussed.

Recently, results of photoemission measurements for xenon were reported in the photon energy range 160 to 270 eV. These time-of-flight studies performed by Lindle et al. [Phys. Rev. A 37, 3808 (1988)] at the Stanford Synchrotron Radiation Laboratory suggested that photoemission from the Xe 4p subshell is remarkably influenced by photoionization channels from the 4d subshell. In the present study, calculations in the relativistic random-phase approximation have been made in which we have included interchannel coupling between dipole photoionization channels from the 4p and 4d subshells. The effect of this interchannel coupling is perceptible in the results for 4p photoionization.

Previous work on quantum correlations has focused attention on the influence of the correlation on observables. In this paper we introduce an observable-independent index of correlation based on the information content, or entropy, of that correlation. We analyze a pair of spins and verify that they are maximally correlated when the index of correlation is maximized. The index of correlation suggests that two optical-field modes are most strongly correlated when prepared in a two-mode squeezed state.

The fluctuation behavior of the mode amplitudes in a multimode dye laser near threshold is investigated numerically. The laser is modeled by semiclassical, third-order laser theory that includes a stochastic Langevin force to simulate quantum noise. We find two different limiting types of behavior. Just above the lasing threshold the mode fluctuations are found to be dominated by quantum noise, while above a certain pump power the fluctuations are found to be essentially deterministic. The effect of pump fluctuations is also simulated. We find that a multimode laser acts as a low-pass filter on the pump fluctuations, as is found for a single-mode laser.

The quantum and classical dynamics of the nonlinear oscillator are contrasted by comparing the evolution of the quantum Q function with that of a similar classical probability distribution. The quantum nonlinear rotator is shown to generate a superposition of two distinct coherent states from a coherent-state input. Measurements of the angular-momentum components and the signature of a superposition state are discussed. The effects of a continual measurement of one angular-momentum component are introduced into the model, and its effects on quantum coherences are shown to degrade the quantum coherence effects. We present a quantum optical model that obeys the nonlinear rotator dynamics and can generate superpositions of SU(2) coherent states of the two-mode electromagnetic field.

A semiclassical treatment of four-wave mixing is presented for the case for which a two-frequency pump wave of arbitrary intensity and a weak probe wave interact with a three-level system. Numerical, as well as analytical, results for the emission spectra are obtained from the steady-state response. At high pump power, the spectra are strongly modified by the ac Stark effect.

When a coherent beam of light of arbitrary polarization travels in a medium composed of randomly oriented chiral (parity-broken) particles, it is predicted to induce a constant magnetization parallel or antiparallel to the direction of propagation. Under normal laboratory conditions the induced magnetization should be very weak, yet detectable by modern means of measurement. This nonlinear-optical effect, which we name ‘‘inverse magnetochiral birefringence,’’ has the opposite sign for enantiomers. It is a difference-frequency effect related to optical rectification or to the inverse Faraday effect.

An assessment is made of the accuracy of a direct variational method for obtaining approximate analytical solutions of the nonlinear Schrödinger equation. The method involves trial functions and a Rayleigh-Ritz optimization procedure. The accuracy of the approximation scheme is inferred from the ability of the optimized solution to preserve the invariants of the nonlinear Schrödinger equation.