Relative photoionization cross sections have been measured for xenon 4d in the 75–140-eV photon energy range and compared with published results. Agreement with many-body theory is good, except for a ∼5-eV shift in the position of the maximum. Our results show that double ionization at photon energies beyond the cross section maximum is not as important as previously thought.

The statistically averaged L-shell fluorescence yields for the doubly excited 3l3l’ and 3l4l’ configurations of Na- and Ne-like ions have been calculated for ions with 18≤Z≤92 using Dirac-Fock wave functions. The average n=3→2 radiative rates and Auger rates for the 3l3l’ states of the Na-like ions scale as $Z^{5.1}$ and $Z^{0.71}$, respectively. The procedure for calculating the average fluorescence yield as ratio between the average x-ray rate and average total decay rate underestimates the average fluorescence yield by as much as a factor of 3 for low-Z ions and overestimates by 50% for heavy ions. Inclusion of E2 transitions leads to a 25% increase in the average fluorescence yield for the Na-like 3l3l’ states at Z=80.

We report measurements of the energies of electrons ejected from Rydberg atoms by strong microwave fields. For high-n Rydberg states we observe double-peaked electron-energy spectra, with peaks at both one and three times the ponderomotive energy. The observation of both peaks confirms the validity of the classical picture used to describe above-threshold ionization at low frequencies.

We have measured the energy spectrum of photoelectrons emitted in multiphoton ionization of atoms by a short-pulse, tunable laser. The spectrum shows prominent resonances, confirming the observations of Freeman et al. [Phys. Rev. Lett. 59, 1092 (1987)]. The study of the position and intensity of such resonances, as a function of laser wavelength and intensity, probes the structure of the atom in the presence of the strong electromagnetic field and allows their spectroscopic assignment. A systematic study of the ac Stark shift of one of the resonances, the 7s[3/2]° level of xenon, is presented.

Recent measurements of the electron-energy distribution for high-intensity short-pulse multiphoton ionization in xenon have yielded a spectrum unexpectedly rich in well-resolved peaks, apparently arising from optical Stark shifts of atomic levels into resonance with harmonics of the ionizing laser. Here we show that such a resonance structure is, quite generally, accompanied by a surprisingly complex spatial structure of the total ionization yield. We predict this spatial structure to arise in a wide variety of multiphoton experiments.

The Keldysh approximation to treat the multiphoton ionization of atoms is reconsidered. It is shown that, if one consistently uses the hypothesis under which the approximation should be valid (essentially, that of a weak, short-range binding potential), a Keldysh-like term results as an approximation to the first term of a uniformly convergent series in powers of the binding potential. No cancellation occurs when higher-order terms are taken into account. This result allows one to consider the Keldysh approximation as a well-defined theoretical model, without implying, however, that it is adequate to describe multiphoton ionization of real atoms.

Dimensional analysis is proposed as a technique for studying the functional dependence of energy on coupling constants for a class of singular potentials. Some exact consequences are derived. In conjunction with WKB a useful approximation to the $x^6$ anharmonic oscillator energies is proposed.

We study the time evolution of SU(1,1) coherent states in an exactly solvable model of a single-mode quantized electromagnetic field in a four-photon medium that has previously been shown to give squeezed states from ordinary coherent states. If the SU(1,1) coherent state initially exhibits squeezing in the $X_1$ quadrature, the squeezing is revoked. We also show that for certain choices of phase in the initial state, enhanced squeezing may appear in the $X_2$ quadrature. Numerical results are also presented for the case when the model is not exactly solvable.

Recently Siegman has discussed how the nonorthogonality of the transverse eigenmodes of an open laser resonator can lead to an excess spontaneous-emission rate [Phys. Rev. A 39, 1253 (1989); 39, 1264 (1989)]. In the present paper we show how this treatment can be extended to also incorporate the case of nonorthogonality of the longitudinal eigenmodes. This is particularly relevant for a description of the spectral properties of lasers with large outcoupling, e.g., semiconductor lasers.

We present a simple method allowing computation of the intensity-intensity correlation function for the resonance fluorescence emitted by a two-level atom driven by a weak, noisy laser. The method is then applied to the two most commonly used models of the noisy laser light: phase diffusion and chaotic colored noise.

The nature of the nonlinear response of a quantum system in the presence of broadband squeezed radiation is examined. The relaxation of the system in the presence of squeezed radiation leads to enhancement of the four-wave-mixing signal.

We discuss the slowing mechanisms of the intermediate scattering function for wave vectors near the position of the main peak of the structure factor. Simulation data in ordinary and supercooled liquid rubidium show that the decay rate at long times is much slower than that predicted by the usual viscoelastic theories, particularly in the quenched system. This feature is accounted for by a simple mode-coupling calculation.

The reactivation efficiency of αμ slowing down in a deuterium-tritium plasma has been calculated for a broad range of plasma conditions. The plasma stopping power has been obtained from the random-phase approximation, which includes both the quantum mechanics of short-range collisions and collective effects due to long-range plasma interactions. It is shown that muon reactivation increases with increasing plasma temperature and density. Near-complete reactivation is, however, reached only at temperatures higher than 1000 eV.

The hydration-dependent dc conductivity of the two main components (embryo and endosperm) of a viable maize seed has been measured by a dielectric-gravimetric technique. The protonic conductivity, plotted as a function of the sample water content, displays a percolative behavior characterized by a threshold and by a critical exponent in agreement with theory for a two-dimensional process. This percolative water-assisted process reflects the presence of an extended network of water molecules adsorbed on the surface of proteins and/or membranes inside intact cells. Moreover, the percolation threshold is found close to the onset of the biological function of these tissues, where the early enzymatic activity leads to the onset of metabolism, a prerequisite to the commencement of germination.

We study the ability of a Hopfield network with a Hebbian learning rule to extract meaningful information from a noisy environment. We find that the network is able to learn an infinite number of ancestor patterns, having been exposed only to a finite number of noisy versions of each. We have also found that there is a regime where the network recognizes the ancestor patterns very well, while performing very poorly on the noisy patterns to which it had been exposed during the learning stage.

A two-dimensional field model was proposed to interpret the director waves in nematic liquid crystals on the basis of the Ericksen-Leslie theory. It is different from previous models and can explain most of the experimental phenomena. The theoretical results are compared in connection with the experimental data. An experimental method was suggested to test the theory.

There are many potentially useful generalizations of Nosé’s mechanics that are both deterministic and time reversible despite the lack of an underlying Hamiltonian. These generalizations are particularly useful in simulating systems far from equilibrium.

We examine the necessary and sufficient conditions that a dynamics is suited for mimicking a specified statistical ensemble. Some delicate points related to the Nosé scheme and its generalizations are clarified.

Differential and total elastic and total reaction cross sections are calculated for electron-hydrogen scattering at 30, 100, and 400 eV using an ab initio optical potential that treats bound and continuum nonelastic channels in the distorted-wave Born approximation. Multichannel and partial wave expansions are carried out to numerical convergence. Convergence criteria and quadratures for the continuum-energy integration are chosen for 1% overall accuracy. Results are compared with experiment and less detailed calculations.