The photoionization cross section and angular-distribution asymmetry parameter of the 4$d^{10}$ subshell of atomic xenon have been calculated using many-body techniques. Our main purpose is to determine whether relaxation effects can account for some of the discrepancy between a recent 4d photoemission experiment and total-absorption experiments. Coupling between many final-state channels has been included. Relaxation effects have been included by evaluating many-body diagrams and also by calculating excited orbitals in the field of the relaxed ionic core. In the latter case, overlap integrals between initial- and final-state orbitals of the spectator electrons were included. Results are compared with experiments and other calculations.

The radiative lifetimes of excited ${\mathrm{Hg}}^{+}$ 5$d^{10}$nl states are reported. They are measured by the multichannel delayed-coincidence method and for some of them Hartree-Fock calculations are made. These data are compared with other authors’ theoretical and experimental results. Radiative lifetimes of n ${}_{}{}^2{}_{}{}^{}S$, n ${}_{}{}^2{}_{}{}^{}D$, and n ${}_{}{}^2{}_{}{}^{}G$ states are discussed as functions of the principal quantum number.

Excitation of an atomic system subjected to a slowly varying external electromagnetic field is studied using a two-level model. Time evolution of the system is found using an approach which is nonperturbative in the field strength. There is no constraint to small values of the applied field, that is, the field (in appropriate energy units) need not be small compared to the difference in energies of the two levels. Rather, we use the fact that the situation of interest to us is where the frequency of the exciting field is small compared to the frequency associated with the level difference. Transition probabilities and resonance conditions are found which circumscribe both the large- and small-field limits. In the weak-field limit the previous results of high-order perturbation theory are readily recovered. For a monochromatic field the characteristic features of resonance excitation at high harmonic number of the applied field are (a) extremely narrow resonance widths and (b) shifts in resonance positions which are strong functions of field intensity. Because of this sensitivity, we are able to demonstrate that when slow temporal evolution of the field amplitude is taken into account (e.g., due to finite pulse duration) the appropriate mean excitation rate is that due to the uncorrelated contribution of many resonances. The results of this analysis are used to estimate excitation rates in a specific atomic system, ${\mathrm{Cd}}^{12+}$, which are then compared to multiphoton-ionization rates. Our calculations suggest that the ionization rate exceeds the excitation rate by several orders of magnitude.

The resonantly enhanced multiphoton ionization of atomic hydrogen is studied for the case of four-photon ionization (three-photon resonant). The time-dependent probabilities, the shape of the resonance profile at different intensities, and the nonlinear index are computed for coherent light at frequencies around the resonance (365 nm). The systematic dependence of the cross section as a function of the principal quantum number of the resonant state (as this tends towards the Rydberg states) is investigated at zero detuning. The probabilities of absorption of one photon more than necessary for the four-photon ionization process (resonantly enhanced above-threshold ionization) is computed and the relevant branching ratios are examined. Then it is shown that the effect of the spatio-temporal structures of the laser pulse is to red shift the maximum of the resonance profile, whereas that of an ideal chaotic light is to blue shift and broaden the resonance peak. The shape, shift, and width of the profile resulting from both these effects are in very fair agreement with the experimental ones.

The ionization of hydrogen can be treated by classical theory when the initial quantum number is large and the photon energy is small. Classically, the electron motion is stochastic for high intensities and the resulting diffusion can lead to ionization. However, Casati et al. [Phys. Rev. Lett. 57, 823 (1986)] have found that the ionization threshold is often higher than the threshold for classical stochasticity. We present here a heuristic explanation: classical stochasticity will be suppressed when the phase-space area escaping through classical cantori each period of the electric field is small compared to Planck’s constant. We obtain a scaling law which agrees remarkably well with the numerical results of Casati et al.

The results of an experimental study of the Rydberg spectrum of rubidium in crossed electric and magnetic fields are reported and compared with theoretical predictions. The present investigations have been conducted in the low-field regime in which the paramagnetic and linear Stark interactions are perturbations to the Coulomb energy. When quantum-defect corrections are negligible, the SO(4) symmetry of the Coulomb interaction organizes the hydrogenic energy spectrum according to the law ${\mathit{E}}_{\mathit{n}}$,k=-R/${\mathit{n}}^2$+ħk(${\mathrm{\omega }}_{\mathit{L}}^2$+${\mathrm{\omega }}_{\mathit{S}}^2$${)}^{1/2}$, where k is an integer such that -(n-1)≤k≤(n-1), and ${\mathrm{\omega }}_{\mathit{L}}$,${\mathrm{\omega }}_{\mathit{S}}$, are, respectively, the Larmor and linear Stark frequencies. The experimental evidence for such a quantization is presented in rubidium Rydberg series using Doppler-free two-photon spectroscopy. The role of quantum defects is incorporated into the theory and experimentally demonstrated.

Calculations of photoionization cross sections, angular-distribution asymmetry parameters, and spin-polarization parameters for photoelectrons ejected from the 3p and 3d subshells of krypton have been performed in the relativistic random-phase approximation. The six relativistic dipole channels originating in the spin-orbit-split 3d levels and the five channels originating in the 3p levels were coupled to incorporate the more important intrashell correlations. In the sensitive region above the 3p threshold, both intrashell and intershell coupling were included in order to minimize truncation errors. The 3p cross section is seen to go through a dip at ≃17 eV above the threshold. A plausible physical basis for this feature is discussed. The 3d cross section shows a delayed onset due to the centrifugal barrier faced by the continuum f waves. The spin-polarization parameters for the photoelectrons in the threshold regions are moderately influenced by rapidly varying strengths in the various dipole channels.

The high-energy excited (1487 eV photon energy) valence x-ray photoemission spectra of ${\mathrm{C}}_2$${\mathrm{H}}_2$ and ${\mathrm{C}}_2$${\mathrm{D}}_2$ are presented. Several new satellites are detected in these spectra. Small but significant differences due to isotope effects are observed. Using simple arguments it is shown that the differences in linewidths for the 2${\sigma }_g$, 2${\sigma }_u$, and 3${\sigma }_g$ main lines can be explained as originating mostly from differences in the ground states. For the satellite at 26.6 eV binding energy a shift of ∼0.1 eV between the ${\mathrm{C}}_2$${\mathrm{H}}_2$ and ${\mathrm{C}}_2$${\mathrm{D}}_2$ spectra is detected.

We consider a system in which a ground state is coupled to a general quasicontinuum of levels and to a true continuum. In the long-time limit, the true continuum spectrum can exhibit zeros, the positions of which depend on the energies of the quasicontinuum levels. The initial conditions determine the existence of these zeros. We describe this phenomenon by studying dressed states for the complete system and in particular derive dressed states which are ground-state independent. We use these to explain the locations of the zeros and their dependence on the initial conditions.

A general theory of four-wave mixing in correlated pump and probe fields is developed. The form of the signals, when the probe is delayed, is also given. The theory is applicable to both degenerate and nondegenerate four-wave mixing. The cross correlation between pump and probe leads to an extra contribution to the signal. This extra contribution is easily isolated in an experiment where the probe is delayed with respect to the pump and carries useful information about the physical parameters of the system. Various applications of the theory are discussed. The theory is shown to lead to the existence of fluctuation-induced resonances in a natural way.

The Hamiltonian describing, without the rotating-wave approximation (RWA), the linear interaction between M bosonic modes with an Einstein spectrum and a single two-level atom is exactly and canonically transformed introducing M suitable collective independent field modes, in such a way that only one among them is coupled to the atom. Some physical consequences of this fact are analyzed and, in particular, the existence of radiation-trapping phenomena together with the possibility of atomic absorption suppression is established. The applicability of the RWA to this system is discussed and the importance of the effective-field statistics for the time evolution of the system is pointed out.

A new way to expand an arbitrary boson-field operator into ordered products of creation and annihilation operators is introduced: alternate ordering. A continuous order parameter s characterizes the order and takes the value s=+1 for normal-alternate order, s=0 for symmetric-alternate order, and s=-1 for antinormal-alternate order. All of the alternate moments of a number of well-known states of light are calculated. The expansion of density matrices and operators in terms of alternate products is briefly discussed as well as the relation with the concepts of phase and higher-order phase dependence. Finally, the relevance of the new ordering scheme in the study of photon statistics is outlined and illustrated in the derivation of moment equations for phase-sensitive light amplifiers and absorbers.

A theoretical analysis is presented describing the interaction of the field in a high-Q microwave cavity with an atom that is suddenly excited to the lower of two Rydberg levels that are resonantly coupled by the field. It is found that the field initially in the cavity is canceled by interference with the source field emitted by the atom. The absorption may be characterized as a ‘‘darkness wave packet’’ emitted by the atom and reflected by the cavity walls.

Picosecond time-resolved dephasing measurements of the Raman-active J=2 rotons in para-${\mathrm{H}}_2$ single crystals have been performed with the stimulated-Raman-gain technique. The three distinct J=2 rotons (‖$M_J$‖=0, 1, or 2) could be selected by a proper choice of the polarizations of the four laser beams with respect to the crystal axes. Excellent exponential fits were obtained for the decay curves, indicating little or no inhomogeneous broadening. The resulting $T_2$ times exhibit no temperature dependence between 5 and 10 K and are in good agreement with recent high-resolution spectral measurements. However, the present time-domain results are more reliable and accurate. In the explored range of ortho-${\mathrm{H}}_2$ impurity concentration of up to 5%, a linear increase is found for the relaxational linewidths (∼1/$T_2$) with nearly equal slopes for the different ‖$M_J$‖ rotons. These results extrapolated to pure para-${\mathrm{H}}_2$ yield $T_2$ times in the neighborhood of 100 ps which agree quite well with those of a recent microscopic calculation. However, in the experiments unpredicted and significant differences (up to 30%) are evident between $T_2$ times for different ‖$M_J$‖ values. The rotational relaxation was also measured in the liquid phase of para-${\mathrm{H}}_2$ and a value $T_2$=7.5 ps was obtained. It too showed a small decrease with increasing ortho-${\mathrm{H}}_2$ concentration.

A particular four-photon Hamiltonian is studied. The model is exactly solved obtaining normal and higher-order squeezing.

The generalized coordinate representation of the optical field is developed to describe dynamics of the squeezed field, and is particularly applied to deal with the linear master equation. The endurance time of squeezing is calculated for a linear optical system with the initial squeezed field. An idea about the symmetry concerning the two quadrature components in the master equation is discussed also. It is shown that the linear master equation is symmetric and that the equation of motion of the probability distribution of each quadrature component is closed.

A quantum theory describing N two-level atoms in a coherently driven single-mode optical ring cavity is presented. The usual approximation that one may adiabatically eliminate either the cavity or atomic variables is not made. Hence the results are valid for a range of relative cavity and atomic relaxation rates. The theory is based on the linearized equations derived in the positive P representation. A simple analytical solution is presented, including the effect of detunings, for the steady-state transmitted spectrum. The transmitted intensity and squeezing spectra are discussed.

Small-scattering-wave-vector structure-factor measurements have been made for model hard-sphere suspensions undergoing a steady linear shear flow. The samples are comprised of sterically stabilized silica particles in cyclohexane and have been well characterized previously by rheological, light scattering, and neutron scattering measurements. These combined measurements provide a strict test of recent theories of microscopic order in suspensions undergoing shear and suggest a picture which unifies several intuitive notions about suspensions undergoing shear flow: distortion of the pair correlation function, clustering, layering, and nonequilibrium phase transitions.

The electron drift mobility has been measured in a wide range of temperatures and densities in neon gas and saturated vapor. The ‘‘zero-field’’ density-normalized mobility ${\mu }_0$N exhibits a strong density dependence, which cannot be accounted for by the existing multiple-scattering theories. The data, however, can be well fitted by assuming a density dependence in the e-Ne scattering cross section.

A phenomenological model is developed to explain single spots or regular patterns of spots at the electrode of gas discharges. The electrodic discharge parts are described by bistable layers, governed by a time-dependent Ginsburg-Landau equation or reaction-diffusion equation; the space between the electrodic layers is taken into account by a resistive region. The steady state of a bi- stable layer is characterized by an S-shaped voltage-current density characteristic. If one bistable layer dominates the interelectrodic space, one single spot may occur. A resistive region may help to stabilize a system of spots and causes a Coulombic interaction of the spots. From a Lyapunov function, equations of motion of the spots can be derived. An initial system of spots relaxes to a stable or metastable state. Experimentally found patterns of anode spots in a glow discharge can be reproduced.