We show that parametric amplification is capable of generating squeezed light over a wide band if materials with large ${\chi }^{\mathrm{}\left(2\right)\mathrm{}}$ nonlinearities can be found, and that the squeezing bandwidth can be enhanced considerably by phase matching away from degeneracy. We compare our results with similar results recently found for four-wave mixing in an optical fiber.

It is pointed out that the emission of photons in the GeV range produced by the annihilation of channeled positrons can be stimulated by superfluorescent emission of characteristic radiation in the MeV range from a beam of channeled electrons or positrons. The threshold current for the entire process is ∼${10}^{10}$ times lower than that of stimulated Compton scattering of visible light into the MeV range of frequencies.

The modified-hypernetted-chain approximation is applied to the study of inhomogeneous transverse structure-factor and pair distribution functions for a model Ar-${\mathrm{CO}}_2$ interface. It appears that the manner in which transverse correlations decay at long range characterizes the different approach to prewetting and complete wetting. Specifically, in the approach to complete wetting, transverse correlations slowly decay with the interparticle distance in a relatively large portion of the fluid; on the other hand, when prewetting is approached, such a slow decay is found only in a confined slice. This different behavior is clearly reflected in the shape of the density profiles, which are also reported. Correlations between particles placed on a line perpendicular to the ${\mathrm{CO}}_2$ substrate are also investigated, and these show qualitative features which conform to the general picture emerging from the analysis of density profiles and transverse pair distribution functions.

The occurrence of Fréedericksz transitions giving rise to initially splay-twist distortions is theoretically investigated. We consider an experimental situation where a homogeneous nematic-liquid-crystal layer is placed in crossed electric and magnetic fields, which are orthogonal to the undistorted director. The electric field E and the magnetic field H couple, respectively, with the splay and twist components of the initial distortion. Both fields are destabilizing. Four types of distortions are considered: initially pure splay; pure twist; periodic splay-twist (PST), predominantly splay; and periodic twist-splay (PTS), predominantly twist. It is shown that the occurrence of the various types of deformations depends on the ratio r=$k_2$/$k_1$ between the twist and splay elastic constants and on the field strengths. For fixed H, between zero and the threshold field $H_{c2}$ for twist distortion, an increase of E gives rise to a PST transition for r<$r_c$(H), and to a splay transition for r>$r_c$(H). A simple analytic expression for $r_c$(H) in strong-anchoring conditions is given. The limiting values of $r_c$(H) are 0.303 25. . . for H=0, as already found by F. Lonberg and R. B. Meyer [Phys. Rev. Lett. 55, 718 (1985)], and (1/3 for H=$H_{c2}$. For r>3 similar phenomena occur, involving twist and PTS distortions. In this case the distortions are induced by an increase of H, whereas E plays the role of a control parameter. For r within the interval ((1/3, 3) only aperiodic splay and twist distortions are allowed.

The goal of one three-dimensional random-parking (occupation) -limit problem is to determine the mean fraction of space that would be occupied by fixed equal-size spheres, created at random locations sequentially until no more can be added. No analytical solution has yet been found for this problem. Our earlier simulations, done for ratios of cubic-region side length (L) to sphere radius (R) up to L/R=20, predicted a parking-limit volume fraction (as volume becomes very large) of 0.37–0.40, using regression equations that indicated the approach to this limit as a function of the ratio of the volume of spheres tried to the volume of the region and the ratio L/R. Our results for random parking in a volume with penetrable walls can be adjusted with a multiplicative correction factor to give the results for the same volume with impenetrable walls. An improved algorithm, almost an order of magnitude faster than our earlier one, was used to extend our simulations to L/R=40 and confirm the original predictions, for a series of six runs totaling 9×${10}^6$ attempts at sphere placement. The results supported a narrower estimate of the parking-limit volume fraction: 0.385±0.010.

It is shown that all four-dimensional symplectic quadratic maps generated by a generating function should be in a form that can be transformed into two coupled (two-dimensional) Hénon maps. The latter is called the standard form of the four-dimensional symplectic quadratic map.

In this Reply to the Comment by G. W. Ford, J. T. Lewis, and R. F. O’Connell [Phys. Rev. A 37, 3609 (1988)] we refute the objections to our result on the partition function of a quantum oscillator interacting with the radiation field [Eq. (12) of D. P. L. Castrigiano and N. Kokiantonis, Phys. Rev. A 35, 4122 (1987)]. Furthermore, we show that the inclusion of the $A^2$ term, which they propose, leads to a strong divergence in the limit of infinite cutoff frequency ${\omega }_c$. This divergence cannot be removed by the methods of renormalization applied so far. Therefore it does not appear that the inclusion of the $A^2$ term would reproduce the result of G. W. Ford, J. T. Lewis, and R. F. O’Connell [Phys. Rev. Lett. 55, 2273 (1985)] obtained earlier. Moreover, this fact constitutes an additional objection to that result [see p. 4124 of D. P. L. Castrigiano and N. Kokiantonis, Phys. Rev. A 35, 4122 (1987)] where only analogous methods of renormalization are applied.

Numerical calculation of the linear instability to oscillatory and stationary convection in binary-fluid mixtures between rigid, impermeable boundaries shows that the critical wave number and frequency of the oscillatory solution jump discontinuously as the threshold Rayleigh number is followed as a function of fluid parameters, contrary to a recent claim of B. J. A. Zielinska and H. R. Brand [Phys. Rev. A 35, 4349 (1987)]. A degenerate codimension-2 point is found only if the wave numbers of the onset solutions are constrained.

The rectilinear-trajectory coupled-equation method is extended in this paper to collisions of αμ(2S) with molecular ${\mathrm{D}}_2$. An efficient quadrature is formulated to integrate over the reaction probabilities and obtain the cross section that is dominated by trajectories passing close to one or the other nucleus. Molecular effects are found to increase the cross section by only 1–2 % above twice the atomic value. Coherent effects in multiple scattering by more than one molecule in a high-density (or condensed) medium are also examined and shown to be negligible.

We propose a cascade model to interpret a recent, novel experiment of ${\mathrm{S}}^{15+}$ + ${\mathrm{H}}_2$ collisions with simultaneous excitation capture in which two $K$ x rays were detected. Cascade effects leading to double $K$ x-ray emission are carefully examined to obtain $K\alpha -K\alpha$, $K\alpha -K\beta$, and $K\alpha -K\gamma$ (and higher states) coincidence cross sections. The overall spectral shapes and absolute magnitudes are consistent with the predictions of the model.

An experimental technique is described which enables determination of the partial cross sections for electron excitation of atoms as a function of changes in spin and orbital angular momentum. This method provides a good signal-to-noise ratio in the energy region near threshold, and could be used to study long- or short-lived excited states of many atomic systems. Measurements for Na $3S-3P$, near-threshold excitation are reported here. The results are generally in good agreement with the close-coupling calculations of D. L. Moores and D. W. Norcross [J. Phys. B 5, 1482 (1972)] for the largest $\Delta m_S$ and $\Delta m_L$ components of the cross section, but not for the component with $\left|\Delta m_S\right|=1\mathrm{}$ and $\left|\Delta m_L\right|=1\mathrm{}$, indicating that the exchange interaction is larger than previously recognized.

We report the first ab initio calculation of the photodetachment cross section of the bound ${\mathrm{Cu}}^{-}$ ion. The calculated cross section exhibits two shape resonances, the larger of which should be accessible experimentally. The magnitude of the cross section is substantial [(6.8-7.1) × ${10}^{-17\mathrm{}}$ ${\mathrm{cm}}^2$] at wavelengths associated with the copper-vapor laser.

We have solved the time-dependent Schrödinger equation for a one-dimensional model atom exposed to a spatially homogeneous laser field that is turned on smoothly. We have studied the ionization of this "atom" at a laser frequency close to a three-photon resonance and have found a spike on top of each of the above-threshold peaks of the ionization signal, in accordance with the findings of a recent experimental investigation.

Using high-resolution laser spectroscopy, we have studied the distribution of oscillator strength in parallel magnetic ($B=970\mathrm{}$ G) and electric ($0\leqq F\leqq 18$ V/cm) fields in the vicinity of the zero-field threshold of Ba. The Fourier transforms of the spectra exhibit transitions from quasi-Landau resonances to electric-field-induced oscillations. The transitions were followed by calculating classical trajectories and Poincaré surfaces of section.

Doubly excited $ml=9s \mathrm{and} 8d$ states of Ca have been produced by multistep laser excitation via $4{\mathrm{sn}}^{\prime }s$ Rydberg states, and the states in which $n$ is from 20 down to $m+2\mathrm{}$ have clearly been observed by detecting photoionization current of ${\mathrm{Ca}}^{2+}$. In some states, remarkable interference patterns and considerable line narrowing caused by the interaction with other autoionizing Rydberg series have been observed. The quantum defects of the observed states have also been obtained and discussed. In addition, we have seen some resonance peaks which are likely to be interpreted as transitions to the states with $n=m+1\mathrm{}$ and $n=m$, although they have not definitely been assigned.

A cw C${\mathrm{O}}_2$ laser with a predominantly homogeneously broadened lasing transition is shown to lose its stability under certain values of cavity detuning when more than two transverse modes are excited. The instability dynamics is found to be strongly dependent on the excited modes. In particular, we have observed period-doubling bifurcations, $\frac{f}{3}$ followed by $\frac{f}{5}$ periodic windows, and a frequency locking of two fundamentals at some rational ratio.

Antibunching and sub-Poissonian statistics of the 11-μm photon emission from the ${5d}^{10}{6p}^2{P}_{\frac{1}{2}}$ to ${5d}^6{6s}^2{}_{}{}^2{}_{}{}^{}D_{\frac{3}{2}}^{}{}_{}{}^{}$ transition in one and two trapped ${\mathrm{Hg}}^{+}$ ions have been inferred from the quantum jumps of the laser-induced fluorescence of the first resonance transition at 194 nm. Each downward step in fluorescence was assumed to mark the emission of an 11-μm photon. Signals from two ions were consistent with the assumption that quantum jumps in the two ions occurred independently. Mandel's $Q$ parameter was determined to be approximately -0.25 for one and two ions.

We calculate the spectrum of quadrature fluctuations of orthogonal polarizations of light for a beam propagating along a birefringence axis of a dispersive transparent Kerr medium. Reshaping and tuning of the spectrum is achievable by variation of the pump power or the linear birefringence: Wide-band squeezing is predicted, and its relationship to modulational polarization instabilities discussed.

The structure of the liquid-liquid interface of three very different mixtures (carbon disulfide+methanol, methanol+cyclohexane+deuterated cyclohexane, and $\mathrm{nitrobenzene}+n-\mathrm{d}\mathrm{e}\mathrm{c}\mathrm{a}\mathrm{n}\mathrm{e}$) has been studied using ellipsometry in the reduced temperature range $0.0009<t<\mathrm{}0.042\mathrm{}$. Although the elliptical thickness varies by an order of magnitude between these mixtures, the data from all three mixtures can be scaled to the same universal constant by a combined meanfield plus capillary-wave model of the interface. The universal constant determined experimentally is significantly less than the theoretical value.

The mean first-passage time of a particle moving in a spatially correlated, Gaussian random potential and subject to Gaussian white noise is calculated. Depending on the parameters of the problem, we find four different regimes of diffusional transport.