We present experimental results on the study of the length dependence of molecular second-order nonlinear optical responses based on a new class of heteroaromatic compounds. We observed a power-law dependence of the second-order molecular susceptibility on the length of the molecule with an exponent of 2. The exponent value observed is in agreement with that obtained from the equivalent internal-field model using recently reported data for third-order susceptibility [Thienpont et al., Phys. Rev. Lett. 65, 214 (1990)].

An expression that allows one to evaluate the electron-electron coalescence density in terms of the one-electron density function at the Hartree-Fock level is derived. Then it is shown, by calculation with highly accurate wave functions of He-like ions, that this expression represents a simple and accurate upper bound to the electron-electron coalescence density for nearly exact wave functions.

Various small-angle experimental differential cross sections for the Xe 5${\mathit{p}}^{6\mathrm{-}}$5${\mathit{p}}^5$${(}^2$${\mathit{P}}_{3/2}$)6s dipole-allowed transition have been contrasted through the generalized oscillator strength (GOS) and with the GOS from our recently constructed universal extrapolation function. We conclude that the Suzuki et al. [Phys. Rev. A 43, 5867 (1991)] measurement is more reliable and that for small momentum transfer squared, ${\mathit{K}}^2$(≤0.05 a.u.), dynamical effects are unimportant.

We here report Stark-broadened widths with error bounds of Li-like ions carbon to neon considered in recent experiments. Good agreement is obtained except for neon, for which the calculated widths, while within the error bounds given, seem too narrow.

We have computed the dc-field-induced tunneling rates for ${\mathrm{H}}^{\mathrm{-}}$, He, Li, and ${\mathrm{Li}}^{\mathrm{-}}$ by applying two methods. The first used ab initio theory, which not only produces reliable results but also allows the systematic analysis of the effects of electronic structure, of the multichannel continuum, and of field-induced mixings. The second used previously published formulas derived from semiclassical models. Comparison shows large quantitative differences, especially for strong fields and for He, where the electrons are equivalent. However, for ${\mathrm{H}}^{\mathrm{-}}$, Li, and ${\mathrm{Li}}^{\mathrm{-}}$, the plots of the widths on a logarithmic scale as a function of the field strength yield similar shapes.

Cross sections and rate coefficients for excitation of the 2${\mathit{s}}^2$2${\mathit{p}}^5$ ${}_{}{}^2{}_{}{}^{}$${\mathit{P}}_{3/2}$–2${\mathit{s}}^2$2${\mathit{p}}^5$ ${}_{}{}^2{}_{}{}^{}$${\mathit{P}}_{1/2}$ transition in fluorinelike Fe xviii by proton (p), deuteron (d), triton (t), and α-particle (α) impact have been calculated using the close-coupled impact-parameter method. These data, in conjunction with R-matrix calculations of electron-impact excitation rates, are used to derive the theoretical emission line ratio R=I(2${\mathit{s}}^2$2${\mathit{p}}^5$ ${}_{}{}^2{}_{}{}^{}$${\mathit{P}}_{3/2}$–2${\mathit{s}}^2$2${\mathit{p}}^5$ ${}_{}{}^2{}_{}{}^{}$${\mathit{P}}_{1/2}$)/I(2${\mathit{s}}^2$2${\mathit{p}}^5$ ${}_{}{}^2{}_{}{}^{}$${\mathit{P}}_{3/2}$–2s2${\mathit{p}}^6$ ${}_{}{}^2{}_{}{}^{}$${\mathit{S}}_{1/2}$)=I(974.8 Å)/I(93.4 Å) as a function of electron and heavy-particle number density, and electron and ion temperature, for values applicable to tokamak plasmas. A comparison of our results with observations of R from the JIPP T-II-U tokamak at the Institute of Plasma Physics, Nagoya, Japan, for which the plasma parameters have been independently determined, reveals excellent agreement between theory and experiment, with discrepancies of typically ≤10%. This provides observational support for the accuracy of the atomic data adopted in the line ratio calculations.

We observed the phenomenon of chemical fusion of fullerenes in a laser-desorption mass-spectrometric investigation of adduct compounds ${\mathit{R}}_{\mathit{n}}$${\mathrm{C}}_{60}$ (R=${\mathrm{C}}_6$${\mathrm{H}}_5$${\mathrm{CH}}_2$ or ${\mathrm{C}}_9$${\mathrm{H}}_7$) in the negative-ion channel. The radicals were believed to play a key role in the formation of stable higher-mass fullerenes and doubly charged negative ions of ${\mathrm{C}}_{60}$ in the gas phase, in our case. These results imply that the heat of fusion has to be released by evaporating small even-numbered fragments or by collison with the radicals ruptured from the ${\mathrm{C}}_{60}$ cage.

The diffracted intensities of sodium atoms by a standing evanescent light wave near the three-Doppleron resonance are calculated using a multislice technique. This calculation predicts a sharp dip in the reflected intensity of the specular beam for a detuning slightly below resonance. Such phenomena have been observed experimentally and can be understood using the dressed-state picture.

Curious ‘‘doublet’’ resonances in the KL double photoexcitation spectrum of argon are explained as a strong admixture of 3${\mathit{d}}^2$ excited states to the principal [1s2p]4${\mathit{p}}^2$ excitation. A simulated ‘‘ab initio’’ KL photoabsorption spectrum including shake-up and shake-off contributions agrees well with the experimental data.

We investigate the generation of higher harmonics from a metal surface in a powerful laser field. The metal is described by the Sommerfeld free-electron model. Solving the Schrödinger equation of the problem perturbatively in the Kramers-Henneberger frame, a simple expression for the harmonic production can be derived and evaluated numerically. Good agreement for the calculated harmonic-generation rates for a gold surface is found if compared with recent experimental data of Farkas et al. [Phys. Rev. A 46, R3605 (1992)].

We present Floquet calculations of the total rate for multiphoton ionization of atomic hydrogen by a coherent superposition of the second and third harmonics of a Nd:YLF (neodymium-doped yttrium-lithium-fluoride) laser field. The total rate depends little on the relative phase of the two fields in the range of intensity considered.

The transient four-wave-mixing signal from a three-level system is calculated in the short-pulse limit including a general distribution function for the energies of the levels between which beating occurs. This allows an examination of the effects of correlation between the energetic splitting of the levels. As expected, the results demonstrate that the degree of correlation can significantly alter the four-wave-mixing signal. However, the results also show that in the case where the level energies are completely correlated, in a rigorous sense, but that the correlation is not a simple offset, a strong modification of the beat amplitude occurs. This case describes the type of correlation expected in semiconductor quantum wells. Examination of several representative cases demonstrates that these effects can easily be incorrectly ascribed to dephasing.

We report the operation of an argon-ion laser in pure (single-frequency) ‘‘doughnut’’ modes of order m=1, 2, and 3. The phase discontinuity at the center of these modes leads to striking two-beam interference patterns that clearly demonstrate the existence of a helical cophasal surface (wave front). The doughnut mode with m=1 (usually called ${\mathrm{TEM}}_{01}^{\mathrm{*}}$) displays a forking interference fringe pattern characteristic of a pure single helix. The m=2 mode shows a pattern with four extra prongs, establishing that the cophasal surface is a two-start or double helix; the m=3 mode is a triple helix with a six-extra-pronged pattern. Each pure doughnut mode is shown to have two possible states corresponding to output wave fronts of opposite helicity.

High-precision measurements (25–175 ppm) have been performed of selected 2p-3s and 2p-4d transitions in neonlike Fe and Cu that are of possible use in resonant photopumping x-ray lasing experiments. A marginal resonance of 600 ppm is found for the 2p-4d transition in neonlike Fe with the 1s-2p transition in hydrogenic Ne. By contrast, an excellent resonance of 200 ppm is determined for photopumping of the 2p-4d transition in neonlike Cu with the 1s-2p transition in heliumlike Mg, suggesting that the Cu-Mg pair is a good candidate for resonant photopumping experiments. Moreover, the separation of the 2p-3s transition in neonlike Fe and the 1s-2p transition in heliumlike F is measured to assess the resonance for use in a scheme to spoil x-ray lasing. The overlap (2000 ppm), however, appears insufficient.

We show that the information about a quantum system contained in the coherent evolution is still available for times much longer than the spontaneous lifetime. We consider the deflection of an atomic wave packet by a resonant quantized light field in the presence of spontaneous emission. We present an analytical expression for the momentum distribution of the deflected atoms which allows us to monitor the dynamics of the relaxation quantum system ‘‘two-level atom plus field’’ on a macroscopic scale.

Electron capture in collisions of ${\mathrm{O}}^{+}$ ions with He atoms is studied on the basis of a molecular-orbital expansion approach. The cross section for electron capture by metastable ${\mathrm{O}}^{+}$${(}^2$D, ${}_{}{}^2{}_{}{}^{}$P) ions is found to be larger by an order of magnitude than that for capture by ground-state ${\mathrm{O}}^{+}$${(}^4$S) ions over the entire energy region studied. The energy dependencies of cross sections for electron capture by the ground and metastable ions are found to differ considerably below 2 keV. The present result, obtained by combining the cross section of 90% of the ground state and that of 10% of metastable states, reproduces reasonably well the experimental result of Kusakabe et al. [J. Phys. Soc. Jpn. 59, 1987 (1990)], who reported using the mixture of the ground and metastable ${\mathrm{O}}^{+}$ ions. The present finding clearly contradicts another experimental result of Wolfrum et al. [Phys. Rev. A 45, R4218 (1992)], who reported no effect from the metastable states.

Accounting for the mistake in our earlier analysis [Phys. Rev. A 46, R3613 (1992)] that was pointed out by Zhu and Xiao [recent Comment, Phys. Rev. A 49, 2203 (1994)] we show that lasing without inversion is still achieved in our more general model description of a ssV-shaped system.

A dc hot thermal and an ac-modulated cold slow potassium beam copropagate and pass through an atom de Broglie–wave interferometer, consisting of a sequence of three microfabricated diffraction gratings. Talbot-vonLau interference fringes are formed and sensed by measuring transmission with a hot wire as a function of grating position. The hot beam produces diffraction-limited shadow Moiré fringes, while the longer de Broglie wavelength slow beam produces interference fringes with high visibility at the fifth and sixth spatial harmonics of the shadow Moiré fringes.

We examine the role of fictitious gauge structure in nonadiabatic transitions for transport in open paths. Local features of the gauge potential modify the nature of the intersection of the adiabatic energy surfaces and thereby affect crucially the Landau-Zener formula for a single-passage transition rate.