The effect on product rotational distributions of the energy released during exothermic processes is investigated with the model of collisions with a rigid, anisotropic shell. With increasing exothermicity, rotational-rainbow maxima become apparent in model integral cross sections for rotational excitation, due to the increasing importance of the final-state repulsion with respect to that in the initial state. Such rainbow maxima are expected to occur in product distributions of excitation transfer, chemical reactions, and dissociation processes. Two different rotational-rainbow maxima can appear even for homonuclear product molecules when the interaction potential contains substantial anisotropy described by a fourth-order Legendre polynomial $P_4$. This type of anisotropy can explain the bimodal rotational distribution of the product ${\mathrm{N}}_2$(C) of the exothermic excitation-transfer collision ${\mathrm{Ar}}^{\mathrm{*}}$${(}^3$${\mathrm{P}}_0$,2)+${\mathrm{N}}_2$(X), with an energy release ΔE=701 and 521 meV for the ${}_{}{}^3{}_{}{}^{}\mathrm{P}_0^{}{}_{}{}^{}$ and ${}_{}{}^3{}_{}{}^{}\mathrm{P}_2^{}{}_{}{}^{}$ states, respectively, as measured by Nguyen and Sadeghi at a temperature of 90 K (=11 meV). The scaling of the rainbow positions with the exothermicity is reproduced. The best agreement with the experimental data is obtained with a ratio of anisotropy parameters $c_4$/$c_2$=1.8. Such anisotropy may be caused by the promotion of a 2${\sigma }_u$ electron to a 2${\pi }_g$ orbital when going from the X to the C state of ${\mathrm{N}}_2$. .AE

Cross sections for s-state to s-state electron capture from a target atom to a completely stripped projectile ion are calculated using a relativistic form of the second-order Oppenheimer-Brinkman-Kramers (OBK) approximation to the leading order in the fine-structure constant. These theoretical cross sections are compared with the experimental data of Crawford and co-workers [Crawford, Ph.D. thesis, University of California (1979); Gould et al., Nucl. Instrum. Methods B 10, 32 (1985)], analyzed by Anholt [Phys. Rev. A 31, 3579 (1985)], for bare C, Ne, and Ar projectile ions incident with 140, 250, 400, 1050, and 2100 MeV/amu energies on various atomic targets. Quite good results are obtained, the calculated capture-cross-section values lying between the minimum and maximum values derived from the experimental data in most cases and often fairly close to the most probable values of the capture cross sections given by Anholt.

We report the first quantitative theoretical and experimental studies of frequency shifts of nuclear-magnetic-resonance (NMR) lines of noble-gas atoms and the corresponding frequency shifts of electron-paramagnetic-resonance (EPR) lines of alkali-metal atoms in mixtures of nuclear spin-polarized noble gases and vapors of spin-polarized alkali-metal atoms. The shifts are primarily due to the Fermi-contact hyperfine interaction. For the heavier noble gases there can be relatively large (≊10%) contributions to the shifts from van der Waals molecules. The NMR frequencies of polarized ${}_{}{}^{83}{}_{}{}^{}\mathrm{Kr}$ and ${}_{}{}^{129}{}_{}{}^{}\mathrm{Xe}$ were measured in a vapor of spin-polarized alkali-metal atoms oriented parallel and antiparallel to the external magnetic field. The shift in the EPR frequency of the alkali-metal vapor due to the polarized noble-gas nuclei was measured simultaneously, thus providing a measurement of the absolute noble-gas nuclear polarization. The frequency shifts can be used as a convenient way to measure the absolute nuclear spin polarization of noble gases.

A simple technique for measuring absolute photoionization cross sections σ of excited atoms has been applied to the 7$D_{3/2}$ level in Cs. Past experiments on excited-state cross sections have relied on measurements of the total number of ions or photoelectrons produced by photoionization to deduce σ. Our technique is novel in that it utilizes the simple measurement of the reduction in the excited-atom fluorescence due to photoionization to find σ. We report measurements of cross sections at six different photon energies.

The x-ray spectrum due to the deexcitation of a muonic molecule pαμ with total angular momentum J=1 composed of ${}_{}{}^4{}_{}{}^{}\mathrm{He}$, ${}_{}{}^1{}_{}{}^{}\mathrm{H}$ (and its isotopes), and a muon is investigated. There is a small second peak in the spectrum of pαμ. The lifetime of the muonic molecules is three times longer than that determined by a previous calculation.

We report fully numerical calculations of correlation energies in second-order perturbation theory based on Hartree-Fock and g-Hartree mean fields for Be and Ne atoms and show that basis-set techniques give essentially the same results. An extension to higher-order perturbation theory is relatively straightforward in the basis-set approach. We report third-order calculations that yield in the case of Be significantly improved values for the g-Hartree correlation energy which now agrees within 4% with the total correlation energy known from complete configuration-interaction estimates, while third-order Hartree-Fock correlation energies calculated in the same basis set still disagree by 10% due to difficulties in describing the near degeneracy of the 2s and 2p orbitals. In the case of the Ne atom, we find that for the g-Hartree mean field still higher orders of perturbation theory are required to obtain agreement with experiment.

We study the fluctuation properties of large-scale quantum spectra of the hydrogen atom in a uniform magnetic field. The present data are of a statistical significance not reached by any other study of these topics. This allows us to tackle a number of problems, whose treatment has been prevented hitherto by the lack of sufficiently long spectral sequences. We mainly focus on the dependency of fluctuation properties on the parameter describing the classical transition from regularity to irregularity. Our results strongly indicate that there are no universal distributions for the nearest-neighbor spacings in the transition regime. Generally, fluctuation measures can show extraordinary behavior, which can be understood only by a detailed knowledge of the underlying classical dynamics rather than by a knowledge of the global classical phase-space structure.

The linear Stark effect in 1snp ${}_{}{}^1{}_{}{}^{}$,3P Rydberg states (n≊40) of the fundamental two-electron atom helium was studied with the resolution of cw laser spectroscopy. The evolution of the angular-momentum manifolds was followed up to the regime where Stark states originating from different n values interact. Narrow avoided level crossings were detected with high precision. Stark manifolds were also calculated by diagonalization of the complete energy matrix in the presence of an electric field. In these calculations, even at moderate values of the field up to five n values have to be included to accurately reproduce the experimental data.

Total and partial (4${\mathit{d}}^{\mathrm{-}1}$,4${\mathit{f}}^{\mathrm{-}1}$,5${\mathit{s}}^{\mathrm{-}1}$5${\mathit{p}}^{\mathrm{-}1}$,5${\mathit{d}}^{\mathrm{-}1}$6${\mathit{s}}^{\mathrm{-}1}$) sections of atomic Ba, La, and Ce have been determined by absorption and photoelectron spectroscopy in the range of the giant 4d→(4f,εf) resonances. Many electron theories that take polarization and relaxation effects into account describe the gross features of the spectra but in many cases fail to reproduce the exact energy positions, line shapes, and amplitudes of the resonances in the cross sections of the main and satellite lines.

We use molecular-dynamics simulations to study the competition between sequential and collective ionization in low-Z atoms by strong laser fields. The model shows sequential ionization at low fields, changing to collective ionization at large fields. The field strength at which this occurs is lower for higher frequencies. We also study the ionization as a function of pulse length and find that collective ionization is favored for shorter pulses.

The transient dynamical evolution of spatial pattern formation in a nonlinear passive optical system in a Fabry-Pérot cavity driven by an external coherent field is studied. The associated decay of the unstable homogeneous state of the system is described analytically, with the inclusion of noise effects, in terms of a stochastic amplitude equation for the unstable mode in the direction of instability. As a result, the time scale on which the pattern is formed as well as the anomalous transient fluctuations of the field intensity are explicitly calculated. Our results show that the behavior of these fluctuations depends strongly on the position within the cavity. Finally, the symmetry-restoring effect of fluctuations is also discussed.

In a series of experiments Formvar-supported circular silver-dot targets were simultaneously irradiated with 1.06-μm and 0.35-μm laser light in the mid-${10}^{14}$-W ${\mathrm{cm}}^{\mathrm{-}2}$ range. In some of the shots, only 0.35-μm light was used. The 3000-Å thickness of the silver was chosen to assure an electron density in excess of the critical ${10}^{21}$ ${\mathrm{cm}}^{\mathrm{-}3}$ at the time of superposition of the 1.06-μm beam, thereby facilitating the generation of suprathermal electrons by resonance absorption. Both high-resolution and broadband spectral data reveal that higher states of excitation and ionization of neonlike and fluorinelike silver were achieved in the presence of the 1.06-μm beam. Evidence that this enhanced excitation and ionization is due to suprathermal-electron pumping is provided by the broadband-continuum data which show little or no increase in the thermal-electron temperature, when the 1.06-μm light is superimposed. Analysis of the data indicates that, in a similarly structured linear target, the gain of the 3p-3s J=2 to 1 neonlike line at 99.6 Å would increase from 0.4 to ∼1 ${\mathrm{cm}}^{\mathrm{-}1}$ when using both laser wavelengths. This conclusion is independent of the mechanism producing the increased excitation and ionization.

We derive two types of perturbative approximation for the response of collisionally broadened two-level atoms to a time-dependent field. The perturbative approximations for the inversion and polarization are valid to any desired order in the derivatives of the field. Comparison with a numerical solution of the full density-matrix equations of motion demonstrates that the new approximations significantly improve upon the adiabatic-following and collisional steady-state approximations for time-dependent pulses in the appropriate damping regime. Application of the approximations to pulse propagation, sideband instability, and population pulsations is discussed.

In this paper we consider the time evolution of SU(1,1) coherent states driven by a coherence-preserving Hamiltonian containing periodic or quasiperiodic pulsing terms. This is a generalization of a system consisting of a two-level atom subjected to quasiperiodic pulsing that was recently studied by Milonni, Ackerhalt, and Goggin [Phys. Rev. A 35, 1714 (1987)]. The time-evolution operator in our case is given by a product of two finite group transformations of SU(1,1). Assuming an initial SU(1,1) coherent state, we determine the equivalent classical motion generated by a Poincaré map that is a Möbius transformation on the Lobachevski plane, the interior of the unit circle in the complex plane. The quantum-mechanical evolution of the state vector is calculated exactly and in closed form even though the Hilbert space is infinite dimensional. We also study the autocorrelation function which, as in the work of Milonni, Ackerhalt, and Goggin, is found to decay in the case of quasiperiodic pulsing that may possibly be associated with a manifestation of chaos in a quantum-mechanical system.

We describe a method suitable for the design of broadband propagators in two-level or pseudo-two-level systems via amplitude-modulated irradiation. Irradiation sequences are described in terms of a Fourier expansion. The Fourier coefficients of the expansion are used to calculate the time-independent Floquet Hamiltonian which represents the time-dependent Hamiltonian. This transformation removes the difficulty of integrating the time-dependent equations of motion. The infinite Floquet matrix is approximately diagonalized by an application of perturbation theory. A set of infinite matrix operators is introduced which assists in the diagonalization. The perturbation-theory expressions are used to derive approximate expressions for the propagator. Successive terms in the perturbation expansion can be nulled by considering additional Fourier coefficients. Broadband-irradiation sequences with low mean power or low peak power are described corresponding to coherent excitation in two-level systems through the flip angles 45°, 90°, 135°, and 180°.

We use an argon-ion laser, modulated at two or more frequencies, to pump a multimode dye laser. Time series of the modulated pump and the output of the dye laser are collected simultaneously. The dye-laser response is analyzed for the presence of chaotic and quasiperiodic attractors. We estimate and compare the correlation dimension and the order-2 Renyi entropy of both the modulated-pump and dye-laser output signals using an embedding-space technique. Fourier transforms of the time series are also compared.

We present experimental measurements of vacuum ultraviolet light emission processes in a 15-Torr rare-gas medium exposed to a strong 1064-nm laser field. Apart from a small number of lines which correspond to discrete transitions, and a broad continuum emission, we essentially observe the odd harmonics of the laser field, up to very high order. At a 3×${10}^{13}$ W ${\mathrm{cm}}^{\mathrm{-}2}$ laser intensity, the highest-order harmonics observed are the 33rd in Ar, the 29th in Kr, and the 21st in Xe. The harmonic distribution presents three regions: a steep decrease for the first harmonics, a plateau, and a sudden cutoff at high order. We study the variation of the conversion efficiency for some of the harmonics as a function of the incident laser intensity and the atomic density. Finally, we analyze the harmonic-generation results as thoroughly as possible, in particular, by characterizing the phase-matching conditions in our experiment.

The hyperfine structure of all levels of the metastable 3$d^2$ configuration in Sc ii has been measured with high precision by collinear fast-ion laser-rf double-resonance spectroscopy. The double-resonance technique improved the precision of the earlier results by more than a factor of 150. The enhanced precision made possible for the first time a meaningful comparison between experimental and multiconfiguration Dirac-Fock calculated B values. The comparison showed that, unlike the A values where disagreements between experiment and theory could be greater than 400%, the B values were well predicted.

The connection between the modulational instability (MI) and the evolution of a higher-order soliton is described. The amplitude ripples that appear on the higher-order soliton can be regarded as a MI. A perturbation theory for higher-order solitons is developed to show differences between the cw MI and the pulsed MI. Initiation of the MI ripples on the soliton pulse is produced by MI amplification of the Fourier components of maximum gain. This phenomenon is corroborated by the study of the breakup of super-Gaussian pulses. Higher-order effects such as the third-order dispersion, shock, and the self-induced Raman effect in a nonlinear Schrödinger equation (NLS) are studied from the point of view of the MI process. It is shown that the Raman effect can enhance generation of the ripple because the parametric noise can build up faster than by MI alone as described by the NLS without perturbations.

The two-mode inhomogeneously broadened ring laser with spontaneous-emission noise is investigated in the presence of backscattering. Using a linear transformation it is shown that this problem in the absence of detuning is equivalent to that of the ring laser without backscattering in the two following cases: (i) in-phase backcoupling case ($R_1$=$R_2^{\mathrm{*}}$), (ii) equal pump parameters and ‘‘symmetric’’ backscattering (‖$R_1$‖=‖$R_2$‖), where $R_1$ and $R_2$ are complex backscattering coefficients. We recover in this way the stationary probability density obtained by Christian and Mandel [Phys. Rev. A 34, 3932 (1986)]. Exact expressions for the mean values, fluctuations, and correlations of the two-mode intensities are derived. Our results show that the relative intensity fluctuations of both modes die out well above threshold. Mode-competition effects are reduced by symmetric backscattering. Exact formulas for the effective eigenvalue characterizing the initial decay of intensity correlation functions are given. These results are compared with the experimental data measured by Singh [Phys. Rep. 108, 217 (1984)].