Kinetic electron emission is studied for the impact of 3-keV electrons and of 4-MeV ${\mathrm{He}}^{2+}$ ions on polycrystalline Al, Cu, and Au targets, as a function of oxygen exposure. For electron-induced emission, the yield and the change of the work function is measured, whereas for ions the distribution of the number of emitted electrons is measured. The distributions found are very well fitted by Pólya distributions, giving the ion-induced emission yield and the Pólya parameter $b.\mathrm{}$ For Au targets, no influence of oxygen exposure upon electron emission was found. For Al targets, exposure up to 50 L gives a reduction of the emission yield; further exposure results in oxidation of aluminum and increases the yield. The work function is found to decrease with increasing oxygen exposure. The parameters $b$ of the Pólya distributions are almost independent of oxygen exposure. For Cu targets, oxygen exposure gives a decreasing emission yield, and a slightly increasing work function. The results are discussed in terms of existing models of oxygen adsorption.

The electron attachment to bound unoccupied levels of spherical neutral ${\mathrm{Na}}_n$ clusters $(n=8,$ 20, and 40) is analyzed using electron-cluster collisions. In the direct process here studied the captured electron loses its energy in a single step via the emission of a photon. The aim of the present paper is to estimate the strength of this inverse photoemission (IP) mechanism, and its contribution to the total inclusive inelastic cross section recently measured. We perform a quantum-theoretical calculation for incoming electrons in the range 0–2 eV. The magnitudes of the total IP cross sections are dominated by transitions to the less bound unoccupied levels while their patterns depend mainly on quasibound states in the incoming channel. It is found that in the range of incoming energies here analyzed the IP process has a negligible contribution to the inelastic inclusive experimental data.

We discuss the mechanism of harmonic generation for a one-dimensional model atom with several bound states in a strong, driving laser field. We extract the bound- and continuum-components of the harmonic radiation and investigate their temporal and spectral characteristics. We compare the bound-bound contributions from a fully numerical solution of the Schrödinger equation with the results from a two-level atom, and study the influence of effective ionization rates in the latter. In the generation of the low harmonics, bound-bound transitions play a decisive role. However, a two-level atom model is not adequate for reproducing their characteristics. High-harmonic generation is almost entirely determined by bound-continuum transitions, the transitions involving the ground state being dominant.

We have observed an interference in four-wave mixing involving two Rydberg states that play the key role of near two-photon resonant intermediate levels. This interference, due to the coherence of the nonlinear process, can be explained either classically or quantum mechanically and it depends on the lasers’ polarizations and detunings. We have performed calculations that are in very good agreement with the experimental data. Our results provide new information about the system, such as ionization rates of the Rydberg levels and the ratio of reduced matrix elements involved.

We describe a method of ${}^3\mathrm{He}$ polarimetry relying on the polarization-dependent frequency shift of the Rb Zeeman resonance. Our method is ideally suited for on-line measurements of the ${}^3\mathrm{He}$ polarization produced by spin-exchange optical pumping. To calibrate the frequency shift we performed an accurate measurement of the imaginary part of the $\mathrm{Rb}{-}^3\mathrm{He}$ spin-exchange cross section in the temperature range typical for spin-exchange optical pumping of ${}^3\mathrm{He}.$ We also present a detailed study of possible systematic errors in the frequency shift polarimetry.

Steady-state saturation of Doppler-broadened spectra of gases by strong near-resonant field in the presence of weak velocity-changing collisions (VCC), which causes the Gaussian-Markovian frequency fluctuations, is considered nonperturbatively. A stochastic rate equation is derived, and its applicability to different systems is discussed. The field-matter interaction is approximated by a δ-function sink in the velocity space. The necessary and sufficient conditions for the validity of this contact model (CM) are obtained. The CM is shown to hold for not too strong fields and/or sufficiently frequent VCC. The CM has allowed us to obtain nonperturbative analytical results for the velocity distribution of the populations of the resonant transition, the integral populations, and the absorption (induced emission) rate. We show that the CM, together with the quasistatic limit, describes the saturation for all values of the parameters, except for the region of the transition between the above models. For the exactly resonant case the CM, together with the previously obtained nonperturbative solution, provides the full solution of the problem. The most striking feature of the saturation in the case of weak VCC is the intermediate saturation, i.e., the existence of the interval of the field intensities, where the populations and absorption (emission) rate are almost independent of the field. In this region the sink is very strong, whereas the coupling of the field with nonresonant atoms is still weak, so that the saturation is controlled by the diffusion of the active atoms in the velocity space towards the sink. The results of the present theory are shown to differ significantly from the results of the strong-collision model of the VCC. The present theory is verified by independent numerical calculations.

We investigate the external and internal dynamics of a two-level atom in a standing wave cavity. In the strong coupling regime, where the atom field coupling $g$ dominates the atomic and cavity decay rates (Γ,κ), a cooling mechanism entirely different from free-space Doppler cooling appears. Under suitable operating conditions, the cavity dynamics induces a Sisyphus type cooling, which is the dominant contribution to the total friction force acting on a moving atom. Simple equations describing the key properties of this effect are derived from a completely classical picture and confirmed by a semiclassical approach. The model is investigated in the bad and good cavity limits, and analytic expressions for the friction coefficient and the momentum diffusion for slow atoms are derived. Using a continued fractions expansion the cooling force for arbitrary velocities is evaluated numerically. The result is used to calculate the equilibrium temperatures of the atom of the order of $\mathrm{kT}\approx ħ\kappa ,$ which can be much lower than the free-space Doppler limit and agree well to those obtained by quantum wave-function simulations.

We present scaled-energy spectra on helium Rydberg atoms in a static electric field. $|M|=1\mathrm{}$ states were studied in excitation from the $2{}^1{S}_0$ metastable state. Spectra were recorded for $\epsilon =-2.940\left(4\right),$ $\epsilon =-2.350\left(4\right),$ both below the saddle point, and $\epsilon =-1.760\left(4\right),$ above the saddle point. Closed-orbit theory was applied to interpret the spectra. A recent extension to closed-orbit theory, incorporating core effects, was used. This significantly improved agreement between experiment and theory.

We introduce a model of laser pulse ionization of a high Rydberg state whose core is a chain of coupled V-type transitions with exact resonance in each arm of the V. Through this chain the population is allowed to migrate resonantly from the initial state of low angular momentum to other bound states of higher angular momenta, both of low and high energy. Along with the redistribution of the initial population over different bound states, we discuss the effect of the redistribution on atomic stabilization, exploiting an analytical solution of the model that holds in a limiting case. The resonant migration mentioned is shown to be able to enhance atomic stabilization against ionization by an optical-frequency pulse.

We have investigated multiphoton ionization of Ba $6snl$ Rydberg states by intense 200-fs laser pulses. Specifically, we have excited Ba atoms to the $6sns$ and $6snd$ 15${\mathrm{Ba}}^{+}$ Rydberg states and ${\mathrm{Ba}}^{2+}$ photoions is relatively insensitive to the femtosecond laser wavelength. In addition, we find that, as the Kepler orbital period of the Ba Rydberg electron becomes comparable to this very small laser pulse width, the inner electron ionization process no longer appears to be instantaneous to the outer electron, and the outer electron is not projected onto ${\mathrm{Ba}}^{+}$ Rydberg states.

We present experimental and theoretical studies of polarization-gradient cooling of metastable neon atoms confined to the dark center of a ${\mathrm{TEM}}_{01}^{*}$ (doughnut) mode. A slow beam of neon atoms is guided and focused inside a blue-detuned and focused doughnut-mode laser beam to a spot size below 10 μm. The transverse motion inside this doughnut mode is cooled by means of two-dimensional optical molasses. We observed non-Gaussian two-component velocity distributions of which the cold component has a width of down to three recoil velocities. These results are found to be in qualitative agreement with a quantum Monte Carlo simulation of cooling in one dimension in the presence of an external light-shift potential. For the simulation we apply the recently developed technique of quantum-state diffusion with adaptive noise.

The angular cross section of a rotationally frozen individual molecule is investigated by considering the polarizability of free rotating neutral molecules induced by laser pulses short compared to a single molecular rotation. We show that in a femtosecond interaction the polarizability of a diatomic or symmetric-top molecule exhibits a dramatic dependence on the relative orientation of the molecular axis and the polarization vector of the incident radiation. The angular cross section is dominated by the angular dependence of a molecular orientation overlap factor whose effects are prominent in nonlinear processes involving ultrashort pulse intense laser fields.

We present experimental observations of phase-conjugate degenerate four-wave mixing in a two-level atomic system (the ${3s}^2{S}_{1/2},\hspace{0.5em}F=2,\hspace{0.5em}{m}_F=\overrightarrow{2}{3p}^2{P}_{3/2},\hspace{0.5em}F=3,\hspace{0.5em}{m}_F=3\mathrm{}$ transition in a diffuse, collision-free, thermal beam of atomic sodium) driven by cw, narrow-band $\left(\sim 250\hspace{0.5em}\mathrm{kHz}\right),$ stabilized pump and probe fields. The primary measurements that we report are of the peak signal strength of the four-wave-mixing interaction and the bandwidth of the four-wave-mixing spectrum (as the laser frequency is tuned through the resonant frequency) as a function of the intensity of the pump field. We compare these measurements with the predictions of a theoretical analysis in which extension to include the effect of the small transverse velocity $(\sim 3\hspace{0.5em}\mathrm{m}/\mathrm{s}\mathrm{e}\mathrm{c})$ of the atomic beam is critical. These measurements provide a direct verification of the theory of this important nonlinear interaction.

Resonant excitation of an inhomogeneously broadened ensemble of two-level atoms (TLA) by a stochastic field with phase noise is theoretically investigated. Free-induction decay (FID), hole burning (HB), and transient nutation (TN) are studied. We consider two kinds of driving fields, one with a free walking phase and another with the phase locked in a limited domain. It is shown that the resonant excitation behavior depends strongly on the noise property. Noise induced by a walking phase gives a simple contribution to the dephasing time, $T_2,$ of two-level atoms whereas phase locking qualitatively changes the laser-atom interaction. In the latter case, it is shown that even when the central part of the driving field spectrum is narrower than homogeneous absorption line of the TLA, the wide, low intensity wings of the spectrum (sidebands produced by the locked phase noise), have a strong effect on the FID, TN, and HB induced by the central, narrow part of the spectrum. The influence of sidebands on photon echoes is also discussed.

Rotationally resolved spectra of the $b{}^1{\Sigma }_g^{+}\mathrm{}(v=0\mathrm{})\mathrm{}\leftarrow X{}^3{\Sigma }_g^{-}\mathrm{}(v=0\mathrm{})\mathrm{}$ band of molecular oxygen are recorded by cavity-ring-down (CRD) spectroscopy in magnetic fields up to 20 T. Measurements are performed in a 3-cm-long cavity, placed in the homogeneous field region inside a Bitter magnet. CRD absorption spectra are measured with linearly and circularly polarized light, leading to different $\Delta M$ selection rules in the molecular transition, thereby aiding in the assignment of the spectra. Dispersion spectra are obtained by recording the rate of polarization rotation, caused by magnetic circular birefringence, using the polarization-dependent CRD detection scheme. Matrix elements for the Hamiltonian and for the transition moment are presented on a Hund’s case a basis in order to calculate the frequencies and intensities of the rotational transitions of the oxygen A band in a magnetic field. All spectral features can be reproduced, even in the highest magnetic fields. The molar magnetic susceptibility of oxygen is calculated as function of the magnetic-field strength and the temperature, and a discussion on the alignment of the oxygen molecules in the magnetic field is given.

We present a generalization of the S-matrix theory of above-threshold ionization (ATI), which includes both the Coulomb and rescattering effects. The Coulomb effects are responsible for a considerable increase of the ionization rates, while the rescattering effects give rise to the appearance of the second plateau of the ATI spectrum. Coulomb effects change the behavior of the rates at the first plateau, but they do not influence the qualitative behavior of the spectrum of the high-energy electrons in the second plateau. Our results are in excellent agreement with recent experiments. We show that the sidelobe positions correspond to the last rounded top before the cutoff of the ATI spectrum for the corresponding angle. Our model is simple, and can be easily generalized to different inert gases.

We discuss temporal dark solitons in confined nonuniform Bose condensates. As a kind of localized high excitation, these solitons can be viewed as macroscopic quasiparticles, having relative motion to the background condensate. We get an analytic expression for one dark soliton under a slowly varying approximation and discuss its special propagation properties in a nonuniform condensate, then we numerically prove that this approximation is reasonable and this kind of soliton exhibits their propagation properties in the nonuniform condensate. Finally, we simulate the generation of dark-soliton-like pulses in the condensate, and indicate that the excitation experiment, done by Ketterle and co-workers [Phys. Rev. Lett. 79, 553 (1997)], can also be interpreted in terms of temporal dark soliton.

We study the Bose-Einstein condensation phenomenon in a two-dimensional (2D) system of bosons subjected to a harmonic-oscillator-type confining potential. The interaction among the 2D bosons is described by a δ function in configuration space. Solving the Gross-Pitaevskii equation within the two-fluid model we calculate the condensate fraction, ground-state energy, and specific heat of the system. Our results indicate that interacting bosons have similar behavior to those of an ideal system for weak interactions.

An analytical theory of the time-orbiting-potential quadrupole magnetic trap for cold atoms is developed. It is shown that the rotating magnetic field used to create the time-averaged harmonic potential is responsible for the formation of quasienergy states of an atom in the trap. It is found that the motion of an atom near the origin of the trap can be represented as consisting of slow motion in the time-averaged potential and fast oscillations with small amplitude. Eigenstates and eigenfunctions for the motion are found. The eigenfunctions are used to calculate the coordinate and momentum distributions for a single atom. It is concluded that at low temperature the quantum-statistical momentum distribution for a single atom exhibits a ring shaped structure due to the fast oscillations in the atomic linear momentum.

We develop a finite-temperature perturbation theory (beyond the mean field) for a Bose-condensed gas and calculate temperature-dependent damping rates and energy shifts for Bogolyubov excitations of any energy. The theory is generalized for the case of excitations in a spatially inhomogeneous (trapped) Bose-condensed gas, where we emphasize the principal importance of inhomogeneity of the condensate density profile and develop the method of calculating the self-energy functions. The use of the theory is demonstrated by calculating the damping rates and energy shifts of low-energy quasiclassical excitations, i.e., the quasiclassical excitations with energies much smaller than the mean-field interaction between particles. In this case the boundary region of the condensate plays a crucial role and the result for the damping rates and energy shifts is completely different from that in spatially homogeneous gases. We also analyze the frequency shifts and damping of sound waves in cylindrical Bose condensates and discuss the role of damping in the recent MIT experiment on the sound propagation.