The Compton profile produced by the 1s electrons of carbon has been calculated using (i) the impulse approximation, (ii) the hydrogenic approximation, and (iii) a type of Hartree-Fock self-consistent-field (HF-SCF) approximation. Significant deviations between the hydrogenic and the impulse results are found for incident photon energies below 25 keV. Between the hydrogenic and the quasi-HF-SCF results, the main difference is that the Compton defect and the maximum of the profile are much smaller when using the latter approximation. These differences are discussed in terms of simple models.

We report an experimental investigation of the excitation-transfer collision Cs(5$D_{5/2}$)+Cs(6S)→CS ((5$D_{3/2}$)+Cs(6S). The upper 5$D_{5/2}$ state was excited by a cw dye laser tuned to the one-photon, quadrupole-allowed 6S→5$D_{5/2}$ transition. Since the direct 5D→6P fluorescence could not be detected with our apparatus, we monitored instead the cascade 6P→6S fluorescence. The ratio of 6$P_{1/2}$ to 6$P_{3/2}$ fluorescence contains information on the collisional mixing that takes place in the 5D levels but also includes a significant contribution from mixing in the 6P levels. This latter contribution could effectively be subtracted out using the results of a second experiment in which a tunable cw diode laser was used to pump the 6$P_{3/2}$ state, and the same fluorescence ratio monitored. The 5D mixing cross section we obtain, 70 A${̊}^2$, is significantly larger than previous indirect determinations.

We used a multichannel extension of the Schwinger variational principle [K. Takatsuka and V. McKoy, Phys. Rev. A 24, 2473 (1981)] to study the cross sections for excitation of the X ${}_{}{}^1{}_{}{}^{}$ ${\mathrm{\Sigma }}_{\mathrm{g}}^{+}$→b ${}_{}{}^3{}_{}{}^{}\mathrm{\Sigma }_{\mathrm{u}}^{+}{}_{}{}^{}$, a ${}_{}{}^3{}_{}{}^{}\Sigma _g^{+}{}_{}{}^{}$, and $c^3$${\Pi }_u$ transitions in ${\mathrm{H}}_2$ by low-energy electrons. These cross sections were obtained with two open channels for each transition and for energies near threshold to 30 eV. The results for the $b^3$${\Sigma }_u^{+}$ and a ${}_{}{}^3{}_{}{}^{}\Sigma _g^{+}{}_{}{}^{}$ states agree quite well with available experimental data. However, the cross sections for excitation of the c ${}_{}{}^3{}_{}{}^{}\Pi _u^{}{}_{}{}^{}$ state differ significantly from the measured values at the two energies, 20 and 30 eV, where data are available.

Absolute optical-emission cross sections have been measured for the ${\mathrm{O}}_2^{+}$ (A ${}_{}{}^2{}_{}{}^{}\mathrm{\Pi }_{\mathrm{u}}^{}{}_{}{}^{}$,v’→$X^2$${\Pi }_g$,v‘) second-negative-band system for v’=0–13 produced by electron impact with ${\mathrm{O}}_2$ in the ground state (X ${}_{}{}^3{}_{}{}^{}\Sigma _g^{\mathrm{-}}{}_{}{}^{}$, v=0) for incident electron energy from threshold to 500 eV. From the optical measurements of the second-negative-band system with an electron-beam excitation apparatus and with a discharge tube, the apparent cross sections for ionization-excitation to 14 vibrational levels of ${\mathrm{O}}_2^{+}$ ($A^2$${\Pi }_u$,v’) are determined. These apparent cross sections are compared with the direct cross sections predicted by the Franck-Condon principle. Good agreement is found for v’≥3, but the observed apparent cross sections are much larger than the Franck-Condon-model direct cross sections for v’=0 and 1. Comparison with the cross sections for ionization-excitation to the ${\mathrm{O}}_2^{+}$ ($b^4$${\Sigma }_g^{\mathrm{-}}$) state is also discussed.

The photodetachment cross section of the ${\mathrm{H}}^{\mathrm{-}}$ ion has been obtained with the multiconfiguration Hartree-Fock method extended to compute the multiconfiguration Hartree-Fock continuum wave function. The electron correlation and polarization effects that are very important for photodetachment cross-section calculation have been taken into account in an ab initio manner through the configuration-interaction procedure. Our results are compared with other accurate and elaborate theoretical calculations and with experiment. It is found that the present results are in excellent agreement with experiment throughout the range of energies considered. The theoretical results obtained by other accurate methods are also in very close agreement with the present results.

The ionization of low-Rydberg-state He atoms in the n ${}_{}{}^1{}_{}{}^{}P$ state (n=14) in collisions with the polar molecules ${\mathrm{NH}}_3$, ${\mathrm{SO}}_2$, and acetone (${\mathrm{C}}_3$${\mathrm{H}}_6$O) is carried out in a crossed-beam experiment at thermal kinetic energies. The dependence of the experimental ionization cross sections ${\sigma }_i^{\mathrm{expt}}$ on the relative velocity of the reactants is obtained by a time-of-flight method. We observe that it varies mainly as 1/$v_r^2$ in a large part of the studied velocity range, while a 1/$v_r$ velocity dependence is predicted by the free-electron model for the higher Rydberg levels.

The absolute values of cross sections for the collisional ionization of low-Rydberg-state He atoms in the n ${}_{}{}^1{}_{}{}^{}P$ state (n=14) by the polar molecules ${\mathrm{NH}}_3$, ${\mathrm{SO}}_2$, and acetone (${\mathrm{C}}_3$${\mathrm{H}}_6$O) are measured in a crossed-beam experiment at thermal kinetic energies, for a well-defined relative velocity. Simultaneous Penning ionization of ${\mathrm{NH}}_3$ by metastable He${(2\mathrm{}}^3$S) atoms is carried out in situ for calibration. Experimental collisional-ionization cross sections ${\sigma }_i^{\mathrm{expt}\left(v_r\right)}$, for $v_r$=2400 m/s, as large as 9×${10}^{\mathrm{-}15}$ ${\mathrm{cm}}^2$ for ${\mathrm{NH}}_3$, 5×${10}^{\mathrm{-}15}$ ${\mathrm{cm}}^2$ for ${\mathrm{SO}}_2$, and ${10}^{\mathrm{-}14}$ ${\mathrm{cm}}^2$ for ${\mathrm{C}}_3$${\mathrm{H}}_6$O at 300 K, are obtained. These large cross sections cannot be explained by the ‘‘free-electron model’’ usually used in Rydberg-atom–molecule collisions: This model predicts no ionization of Rydberg levels as low as n=14, since the amount of energy available from rotational ΔJ=-1 ΔK=0 relaxation transitions of the molecule is insufficient. Our hypothesis of large ‖ΔJ‖ rotational transitions is strengthened by comparison of the results for the three studied molecules.

Simple expressions that include configuration and intermediate coupling mixing effects are given for the cross sections and rate coefficients for populating the n=4 excited levels of nickel-like ions by the process of inner-shell ionization of copperlike ions. Some illustrative numerical results for the rate coefficients are given for gadolinium (Z=64) and compared with the relativistic distorted-wave rate coefficients of Hagelstein [Phys. Rev. A 34, 874 (1986)] for populating the same excited level of Ni-like gadolinium by the alternative process of excitation from the ground level.

Fine-structure intervals separating the 10H(L=5), 10I(L=6), 10K(L=7), and 10L(L=8) states of helium have been measured precisely, using a fast-beam microwave–laser resonance technique. The results: 10H-10I, 157.0508(26) MHz; 10I-10K, 60.8152(18) MHz; and 10K-10L, 27.1835(63) MHz agree well with theoretical predictions which include contributions due to ‘‘retardation’’ or ‘‘Casimir’’ forces.

Two-photon transition probabilities in atomic inner shells are calculated relativistically with self-consistent-field wave functions, including contributions from all multipoles, angular distributions, and the effect of resonances. Results are evaluated numerically for Mo, Ag, and Xe; they agree well with available experimental data.

We formulate a computationally feasible method for calculating multiphoton ionization rates for atoms exposed to intense fields in the intensity regime where perturbation theory ceases to apply. The method is based on the time-independent picture of ionization, which starts with the Floquet ansatz. The question of the gauge of the radiation field is discussed in some detail. Various expressions for the ionization amplitude are derived from a variational principle, with the radiation field expressed in the velocity gauge. We consider in particular an approximation in which the wave vector developing from the initial state is replaced by a trial vector that is the Floquet expansion truncated just below the threshold for ionization, and in which the wave vector developing into the final state is replaced by a trial vector that is just the wave vector appearing in the Kroll-Watson low-frequency approximation in scattering theory. We have applied this approximation to hydrogen and we present some results for both nonresonant and resonant multiphoton ionization. We argue that the experimentally observed resonance structure in the above-threshold peaks of the ionization signal occurs through the electron jumping from one dressed-state energy-eigenvalue curve to another.

A polycrystalline metal surface develops a nonuniform surface charge distribution to bring different crystal faces into thermodynamic equilibrium, thus generating electrostatic fields outside the metal. We calculate the statistical properties of these fields and their effect on time-of-flight energy spectroscopy in the classical limit. These analyses are done in one-dimensional and in cylindrical geometries. Calculated quantities include the autocovariance function of the potential, the rms deviation of the electric field, corrections to the free-particle time of flight, and the mean minimum energy required to transit the random potential. Numerical simulations agree well with theory in the limits considered and indicate a wide regime of validity.

Nonhydrogenic photoabsorption cross sections near threshold are well approximated by radial dipole-matrix elements between phase-shift-modulated regular and irregular Coulomb continuum and quantum-defect bound-state wave functions. Results based upon numerical quadratures are inaccurate for high-Rydberg transitions, while previous analytical formulas are difficult to evaluate near threshold. A simple analytic algorithm for the near-threshold region based on the analytic properties of the Coulomb functions in the complex energy plane is presented.

We report a series of measurements of the nearly degenerate four-wave-mixing response in atomic sodium in the presence of a helium buffer gas. The results show the line shape associated with both the ground-state and excited-state dynamics. The line shape evolves as a function of pressure from a dip at a pressure of 0 Torr to a spike at a few Torr due to the presence of collisions. The width of the spike is determined by residual Doppler broadening. At higher pressure (≥10 Torr), the width of the spike is further narrowed below the residual Doppler width. The additional reduction is due to a form of Dicke narrowing associated with the dynamics of the lower- and upper-level excitations. The results are in qualitative agreement with a perturbation solution of the density-matrix equations which include the effects of velocity-changing collisions and state-changing collisions.

An optical ring cavity is numerically simulated in which the propagation of an envelope pulse is described by a particular form (the ‘‘linear plus smooth step’’ model) of the generalized nonlinear Schrödinger equation which admits bistable-soliton solutions. This cavity is synchronously pumped by a train of identical pulses. Some examples of the influence of the bistable solitons on the bifurcation sequence, i.e., the ‘‘route to chaos,’’ are given. The period-doubling bifurcations and chaotic behavior which occur as the pump amplitude A is increased are examined in detail, and a comparison made with known results for the pumped cubic nonlinear Schrödinger equation. In the cubic case chaos sets in at a relatively low A value and completely predominates for larger amplitudes (very narrow windows of periodicity can occur but are difficult to find). In contrast, the dominant behavior for the ‘‘linear plus smooth step’’ model is one of periodicity interspersed with bands of chaos at intermediate A values with a return to period 1 occurring at large A. This latter behavior is to be expected from any model of the intensity-dependent refractive index which saturates with increasing intensity.

We derive stochastic density-matrix equations for an atom strongly driven by finite-bandwidth squeezed light. The quantum properties of the light are accounted for by a doubling of dimensions of the stochastic process for c-number electric field amplitudes. Saturation properties and the weak-field absorption spectrum of a two-level atom embedded in finite-bandwidth squeezed light and driven by a coherent field are calculated. We discuss the effect of finite bandwidth of the squeezed light in obtaining subnatural linewidth in the atomic absorption spectra, based on nonperturbative solutions of the stochastic optical Bloch equations.

The temperature dependence of vibronic quantum beats in dye-doped polymer films has been investigated using femtosecond accumulated photon echoes. It has been found that the vibronic quantum beat is not discernible above liquid-nitrogen temperature. It makes a striking contrast with the previous observation of the quantum beat in transient pump-probe experiments performed at room temperature. Based on the density matrix incorporating the fluctuation of the transition frequency, it is pointed out that there is no contribution of sublevel coherence to the accumulated photon echo, and the line broadening of the vibronic state or phonon sideband obscures the quantum beat at higher temperature. On the other hand, in the transient pump-probe experiment, the quantum beat is caused by the sublevel coherence, as was previously pointed out by Mitsunaga and Tang [Phys. Rev. A 35, 1720 (1987)]. Hence it is concluded that the influence of the frequency fluctuation on the sublevel coherence is relatively small despite the serious line broadening of the vibronic state.

This paper analyzes the interaction of a strong wave and one or two probe waves with an intrinsic semiconductor medium which is partially inverted by an injection current. The many-body interactions of the electron-hole excitations are described using recently developed generalized Bloch equations for semiconductors. Coupled-mode equations for the different light fields are derived and solved, and predict that the gain and the coupling coefficients experience asymmetric dips. These dips are generated by scattering of the strong field off carrier-density pulsations induced by the pump-probe interference.

A perturbative density-matrix theory of quantum beats developed previously has been extended to include the case of a three-level system with ground-state splitting. It is shown that quantum beats should be observable in a transmission-correlation experiment when the optical pulses are resonant or nonresonant with the excited state. The beats may be of magnitude comparable to those from a split excited state, and are insensitive to the amount of homogeneous or inhomogeneous broadening of the optical transitions.

Switching events are studied by means of a parametrically operated, fast transition from a monostable to a bistable potential in the continuous presence of colored noise. The problem is thus the decay of an unstable state with random initial conditions. We calculate, using contemporary colored-noise theory, and measure by analog simulation, the relaxation time to cross a reference boundary, and we contrast this with the strictly defined mean first passage time.