We search for nuclear fusion in the impact of macroscopic high-velocity iron particles on deuterated titanium and heavy-water targets. The consequences of our upper limits on proton and γ-ray emission for an explanation of the cluster-impact fusion observed in recent experiments are discussed. Our results constrain any explanation of cluster-impact fusion in terms of solid-state or shock-wave effects.

We have applied the laser-rf-laser double-resonance method to a molecular ion. Fifty-six hyperfine components of fine-structure transitions (ΔJ=±1) were measured in rotational levels from N=1 to 27 of the ν’’=1 vibrational level of the ${\mathit{X}}^2$ ${\mathrm{\Sigma }}_{\mathit{g}}^{+}$ ground electronic state of ${}_{}{}^{14}{}_{}{}^{}\mathrm{N}_2^{+}{}_{}{}^{}$. In order to fit the data, we required seven molecular constants, corresponding to the following interactions: electron spin-rotation fine structure and its centrifugal distortion, Fermi-contact hyperfine structure (hfs), dipolar hfs and its centrifugal distortion, and electric quadrupole and nuclear spin-rotation hfs. The results are in reasonable agreement with theoretical calculations based upon a Hartree-Fock-Roothaan wave function.

Line-shape analysis of transitions in a quantum system is a method widely applied to study relaxation processes on a microscopic scale. In many practical cases, this relaxation arises from stochastic modulation of the Hamiltonian due to interactions with a surrounding heat bath. Using a separable class of stochastic operators, the line-shape problem for a Hamiltonian with a single transition has been solved in closed form. This method is not appropriate for more complicated Hamiltonians with closely spaced multiplets, as the modulation may lead to significant coupling between the individual transitions in the multiplet. In this work, therefore, an investigation of the response function of a modulated multiplet is undertaken. The dynamical equation is derived from the stochastic Liouville equation for the density matrix. It is solved rigorously for two practical examples drawn from magnetic-resonance spectroscopy. These exact results are used to investigate the accuracy of approximations found in the literature.

A pseudopotential calculation of the total energy of small sodium clusters is presented. It is based on a local-pseudopotential scheme and local-density correlation and exchange. It includes a spherical distribution of electronic charge and minimizes the energy with respect to the position of the ions. It is found that the most stable clusters are (1) those that correspond to closed electronic shells for clusters with fewer than approximately 100 atoms, and (2) those that correspond to geometrical polyhedral clusters for numbers larger than that. A crossover between electronic-level dominance and geometrical-structure stability, found experimentally for high temperatures at larger numbers, is thus theoretically established.

A combined experimental and theoretical-computational study of three-body effects in collision-induced, depolarized light scattering in compressed hydrogen gas is reported. It includes measurements of the translational Raman line profile at a temperature of 50 K and gas densities from 20 to 530 amagats that are analyzed in terms of density-squared and -cubed components. The third virial coefficients of the zeroth, first, and second spectral moments are determined. The study also includes semiclassical calculations of the third virial coefficients of the zeroth and first spectral moments that are based on refined, empirical pair-polarizability models. Quantum corrections of the Wigner-Kirkwood type are significant. A comparison of measurement and theory suggests that at 50 K a positive irreducible component affects the measurement; its magnitude amounts to roughly 40% of the pairwise-additive component. Theoretical estimates of the long-range (classical) irreducible anisotropy of the polarizability of three-body complexes suggest a positive spectral contribution that, however, amounts to just a fraction (namely, about 1/4) of the difference observed between measurement and theory. At 297 K such measurements also exist and suggest a negative irreducible component that is probably nonclassical, of an overlap (short-range) nature.

A relativistic finite-basis-set method is used to calculate the ground-state energy of hydrogenic atoms in strong magnetic fields with B up to ${10}^{12}$ G, for several values of the nuclear charge Z. A modified Slater-type basis set with different values of the total angular momentum is used. Both accurate relativistic and nonrelativistic binding energies of hydrogen can be obtained with this basis set, with more than seven significant digits accuracy for magnetic fields in the range 0≤B≤${10}^{10}$ G. The relativistic corrections are found to differ substantially from previous calculations based on adiabatic approximations. In the case Z=1,B=4.7×${10}^9$ G, for example, our calculation yields a relativistic correction that is a factor of 7 larger, and of the opposite sign, than the previous result. For very strong B and small Z, a basis set that mixes Slater- and Landau-type functions is required to avoid a large expansion in angular momenta.

This paper examines two collisional mechanisms by which cold alkali-metal atoms can escape from a neutral-atom trap, due to collison of excited- and ground-state atoms. One is fine-structure-changing collisions, by which the atoms are heated by the amount of the fine-structure splitting. The other produces hot ground-state atoms following emission of a red-shifted photon during the course of a collision. A rate expression is obtained that applies to both normal and ultracold temperatures (<1 mK). This expression assumes a canonical distribution of initial states, low-intensity excitation, and a semiclassical treatment of the survival probability relative to excited-state decay during the long time of the ultracold collision, and uses fine-structure-changing probabilities found by quantum scattering calculations. The known properties of the attractive molecular states of the alkaline-metal dimers are used to identify and calculate the probabilities for the specific mechanisms. We conclude that the earlier semiclassical analysis by Dashevskaya [Opt. Spectrosk. 46, 423 (1979)] of the fine-structure-changing mechanisms for the various alkali-metal species is qualitatively correct. We present quantum-mechanical calculations of the rate coefficient for fine-structure-changing collisions between ground- and excited-state Cs atoms from 1000 K to 100 μK. Collision-rate coefficients for the trap-loss processes are calculated for pairs of Li, Na, K, Rb, and Cs atoms at low temperature. There is a wide variation of predicted loss rate among alkali-metal species from the fastest for K and the slowest for Li. Retardation corrections to molecular lifetimes must be taken into account to predict the correct rate coefficients for Na, K, and Rb. Good agreement is obtained with the observed trap loss rate in a Cs trap.

We describe an approach for numerically integrating the time-dependent Schrödinger equation for an atom in a radiation field. The time propagation is based on the split-operator technique, with the full Hamiltonian split into two parts, the atomic Hamiltonian and the atom-field interaction. Both parts are represented on a complex Sturmian basis. The method is relatively efficient; ionization yields and level populations for atomic hydrogen can easily be computed on a workstation for modest pulse durations (e.g., 50 cycles or so) and modest intensities (e.g., of order ${10}^{15}$ W/${\mathrm{cm}}^2$ for a frequency of 0.2 a.u.). We present results of an application of the method to atomic hydrogen, and to illustrate the performance we compare our results with those obtained previously by Kulander [Phys. Rev. A 35, 445 (1987)]. We also illustrate the stabilization of atomic hydrogen against ionization by an intense high-frequency field, and the sensitivity of the ionization yield to the relative phase in the case where the field is bichromatic with one field a harmonic of the other.

We report on the results of nonlinear ionization and dissociation of gas-phase oxygen molecules in an intense (${10}^{11\mathrm{-}13}$ W/${\mathrm{cm}}^2$) laser field. The investigations were performed with the second- and third-harmonic wavelengths of two different laser systems; a 50-ps Nd:YLF (where YLF denotes yttrium lithium fluoride) and a 10-ns Nd:YAG (where YAG represents yttrium aluminum garnet). Measurements include energy resolved photoelectron and mass spectroscopy. We report on an interesting observation made with excitation of the neutral with green (∼2.3 eV) photons, in which the vibrational distribution of the molecular ion shows a strong intensity dependence. We have analyzed this intensity-dependent behavior to be a result of a complex interplay between different resonant and nonresonant electronic channels, which to first approximation does not require dynamical Stark shifting of states. A model is presented that describes this behavior and results in the determination of the molecular constants for the intermediate states.

The multielectron dissociative ionization of diatomic molecules is investigated using an intense laser field at a wavelength of 615 nm, in the ${10}^{15}$ W/${\mathrm{cm}}^2$ intensity range, with a 100-fs pulse duration. Comparison with previous experiments performed with 2-ps and 600-fs pulse durations shows that the atomic-ion kinetic-energy releases depend neither on the laser-pulse duration nor on the peak laser intensity. The simple model of Coulomb repulsion between atomic ions is therefore insufficient to account for the dynamics of the multiple-ionization mechanism. Comparison between ${\mathrm{N}}_2$, CO, and ${\mathrm{O}}_2$ experiments indicates that the channel ${\mathit{AB}}^{2+}$→${\mathit{A}}^{+}$+${\mathit{B}}^{+}$ remains molecule specific. Conversely, the channels involving molecular ions carrying higher charges (${\mathit{AB}}^{(\mathit{p}+\mathit{q})+}$→${\mathit{A}}^{\mathit{p}+}$+${\mathit{B}}^{\mathit{q}+}$) exhibit similar dissociation energies for ${\mathrm{N}}_2$, CO, and ${\mathrm{O}}_2$.

With the help of molecular-dynamics computer simulations, we study the equilibrium configurations of systems of N=2–5000 strongly correlated charged particles under the influence of a radial harmonic external confining force and their mutual Coulomb forces. The temperature is well below the crystallization point; i.e., the ratio of Coulomb to kinetic energy is as large as Γ=${10}^9$. The particles arrange in concentric spherical shells with approximately constant intershell distances. On the surfaces plane hexagonal structures are well pronounced. The calculated radii, occupation numbers, and energies per particle are compared with results of classical geometrical and shell models with homogeneously charged shells corrected for hexagonal surface occupation. The closed-shell particle numbers also agree well with those of multilayer icosahedra. From the computer simulations we extract a Madelung (excess) energy of -0.8926, which is close to the theoretical value of the shell model corrected for plane hexagonal surfaces, -0.8923, but larger than the one of the infinite geometrical lattice, -0.8944, and of the bcc value of -0.8959. Surface-energy effects are positive and of the order of ${\mathit{N}}^{\mathrm{-}1/3}$.

We analyze a method based on discretized basis sets to calculate multiphoton ionization effects in two-electron systems. One-electron systems will serve as a benchmark; then the case of two-photon ionization of helium will be investigated. Emphasis will be placed on the role of correlations. Finally the choice of different type of basis sets (Slater-type orbitals, B-spline functions) will be discussed.

We present experimental and theoretical results on highly excited Rydberg atoms passing through a waveguide. The waveguide is excited in a coherent mode with a superimposed component of technically generated noise. In the theoretical part of the paper we derive and solve a master equation for a Rydberg atom driven by a monochromatic coherent microwave field in the presence of noise. We show that a Rydberg atom subjected to a mixture of coherent modes and noise fields exhibits four dynamical regimes: (i) diffusive broadening, (ii) localization, (iii) destruction of coherence and localization, and (iv) relaxation to equilibrium. The four regimes are passed one after the other as a function of irradiation time. They occur on different time scales and are thus temporally well separated from each other. The theory is checked by an experiment on the time dependence of the population distribution of highly excited rubidium Rydberg atoms initially prepared in a unique and well-defined Rydberg state and irradiated by a strong microwave field. The localization regime, characterized by a ‘‘freezing’’ of the width of the wave packet with respect to the Rydberg levels, has been observed. The addition of a small noise component was shown to lead to delocalization after times inversely proportional to the noise power, as predicted by our theory.

The interaction of a two-level system with a single quantized mode of the radiation field of finite bandwidth in a thermal state and coupled to background vacuum modes is considered theoretically. A hierarchy of equations for atom-field moments is derived and solved in terms of continued fractions. The population inversion, the spectrum of resonance fluorescence, and the photon statistics of the cavity mode are calculated.

Two correlated photons incident upon two distant interferometers can produce a coincidence rate that depends nonlocally on the sum of the phases of the interferometers. This effect has been observed for the case in which the optical path length between the two interferometers was 102 m. The results of the experiment are in good agreement with the quantum theory predictions and violate a recently derived inequality for semiclassical field theories.

A recent theory of the fundamental laser linewidth developed by the authors is extended to include the effects of the Henry α parameter. The usual (1+${\mathrm{\alpha }}^2$) enhancement of the linewidth is shown to be increased for arbitrary output coupling by spatial hole burning. A total enhancement factor is derived that multiplies the Schawlow-Townes linewidth and takes into account line broadening due to amplification of quantum noise as well as that due to phase-amplitude coupling.

A one-dimensional model for the propagation of amplified spontaneous emission and a set of line-broadening equations for the frequency dependence of gain coefficients in the laser field are solved self-consistently. Results are presented for high-gain amplification from spontaneous emission, without and with saturation, and for injection into power amplifiers. Power broadening and Rabi splitting is seen to limit the effective gain-length product in high-gain collisionally excited neonlike lasers and to prevent some of the expected gain narrowing. For future power amplifier experiments, using high-gain preamplifier output fluxes as input, a general decrease in power gain is predicted. Rabi splitting of the output spectrum is expected, if power broadening is comparable to or larger than the intrinsic line broadening.

We show that a laser may generate amplitude-squeezed or antibunched light in a regime well above threshold, where depletion of the ground state by incoherent pumping is significant. Simple analytical solutions are presented to illustrate which atomic-level schemes and pumping rates are optimal.

It is possible to determine whether the input state to a Kerr medium is nonclassical by measuring the total noise of the output state. For a classical input state the total noise at the output has a minimum value that depends on the photon number and the interaction time. If the total noise is found to be less than this value, then it can be concluded that the input state was nonclassical. A certain class of generalized coherent states can be detected in this way.

A singular perturbation theory for nonlinear Schrödinger solitons is extended into second order. An application is the calculation of the contribution of the radiation to the phase-independent interaction between soliton pairs, as suggested by Smith and Mollenauer [Opt. Lett. 14, 1284 (1989)].