In a recent publication [Phys. Rev. Lett. 92, 233202 (2004)] two different electron impact double ionization $(e,3e)$ mechanisms were identified and the way in which two-electron momentum distributions for atoms and molecules could be obtained by triple coincidence $(e,3e)$ measurements was discussed. The apparatus used detected the two ejected electrons both in and out of the scattering plane at an angle of $45°$ to the momentum transfer direction in triple coincidence with the scattered electron. Ejected electrons detected out of the scattering plane were shown to be a result of two-step double ionization processes. With the same apparatus we have made double coincidence $(e,2e)$ measurements of electron impact single ionization cross sections for ionization of magnesium $3s$ (valence) and $2p$ and $2s$ (inner) shell electrons at incident energies from 400 to $3000\mathrm{eV}$ in order to obtain more information about two-step ionization. The experimental results were compared with distorted-wave and plane-wave Born approximations carried out to second order. For the experimental conditions, two-step ionization processes involving one ionizing collision and a second elastic collision with the atomic core are the dominant contribution to the measured cross sections. Calculations are in moderate agreement with the data. The angular distributions of the ionized electrons in these two-step ionizations reflect the initial momentum distributions of the target electrons, a result that is analogous with the earlier $(e,3e)$ measurements.

The light scattered by a cold trapped ion, which is in the stationary state of laser cooling, presents features due to the mechanical effects of atom-photon interactions. These features appear as additional peaks (sidebands) in the spectrum of resonance fluorescence. Among these sidebands the literature has discussed the Stokes and anti-Stokes components: namely, the sidebands of the elastic peak. We show that the motion also gives rise to sidebands of the inelastic peaks. These are not always visible, but, as we show, can be measured in parameter regimes which are experimentally accessible.

Laser pulses can create rotational wave packets in molecules that periodically revive as field-free aligned distributions. These rotational wave packets can be approximately annihilated by applying another laser pulse during a half revival, an effect corresponding to quantum antiresonance in chaotic kicked rotor studies. We theoretically explore causes of deviation from perfect annihilation. We experimentally demonstrate rotational wave packet annihilation in nitrogen gas, measuring the evolution of alignment by Coulomb explosion imaging. As a test, we apply the pulse pair to an existing rotational wave packet and observe the restoration of the original revival structure after the zero-effect pulse pair.

Optical dipole traps and fractional Talbot optical lattices based on the interference between multiple copropagating laser beams are proposed. The variation of relative amplitudes and phases of the interfering light beams of these traps makes it possible to manipulate the spatial position of trapped atoms. Examples of spatial translation and splitting of atoms between a set of the interference traps are considered. The prospect of constructing all-light atom chips based on the proposed technique is presented.

The alignment dynamics of polyatomic molecules subject to two temporally overlapping nonresonant laser pulses is studied theoretically and experimentally. We examine the potential advantage of combining a long (compared to the molecular rotational periods) with a short pulse, where both laser fields are linearly polarized with common polarization direction. Experimentally, iodobenzene molecules are irradiated by a $2\mathrm{ps}$ pulse at $800\mathrm{nm}$ synchronized to the peak of a $9\mathrm{ns}$ pulse at $1064\mathrm{nm}$. The alignment dynamics following the short laser pulse is measured using time-resolved Coulomb explosion with a delayed $25\mathrm{fs}$-long intense laser pulse. The alignment attained with the two-pulse combination is significantly enhanced compared to that attainable with either a short or a long pulse alone under nonionizing conditions. Numerically, we solve the time dependent Schrödinger equation nonperturbatively for asymmetric top, symmetric top, and linear molecules subject to similar combinations of long and short excitation pulses. The alignment dynamics, and in particular the alignment enhancement, are explained and their generality is tested.

We analyze coherence effects during the splitting of a quasi one-dimensional condensate into two spatially separated ones and their subsequent merging into a single condensate. Our analysis takes into account finite-temperature effects, where phase fluctuations play an important role. We show that, at zero temperature, the two split condensates can be merged into a single one with a negligible phase difference. By increasing the temperature to a finite value below the critical point for condensation $\left(T_c\right)$, i.e., $0⩽T∕T_c<1$, a considerable enhancement of phase and density fluctuations appears during the process of splitting and merging. Our results show that if the process of splitting and merging is sufficiently adiabatic, the whole process is quite insensitive to phase fluctuations and even at high temperatures, a single condensate can be produced.

We report on experimental and theoretical studies of a collisionally pumped, optical-field-ionized soft-x-ray laser (SXRL) at $41.8\mathrm{nm}$ driven in capillary tubes with smooth inner surface. A detailed experimental study has been conducted in order to understand the key effects related to guiding in this configuration. The amplifying plasma was created inside few-cm-long capillary tubes, and maximum extreme ultraviolet emission was obtained when operating in a multimode guiding regime with an optimized lasing signal from a $25\text{−}\mathrm{mm}$-long capillary a factor of 3 higher than that of a gas cell and with a beam divergence reduced by a factor of 3. A numerical code, named COFIXE, has been developed to calculate the SXRL signal emitted by the plasma source. It includes the calculation of the pump beam propagation, the determination of the plasma state just after the interaction with the pump beam, the calculation of the evolution of the local properties of the plasma during the subsequent few ps, and the calculation of the amplification and transport of the SXRL emission. Excellent agreement has been obtained between experiment and theory for several features such as the divergence of the beam, the correlation between guided pump beam transmission and SXRL energy, and the enhancement factor induced by guiding.

Considering a three-level $\Lambda$-type atom with both transitions coupled to a single photonic-band-gap reservoir, we investigate the upper-state population and spontaneous emission spectra by using the resolvent operator for the isotropic dispersion model, and give their analytical solutions. For the case of two transitions coupled to the same reservoir, the upper-state population always displays an oscillatory behavior and finally reaches a steady-state value. For atomic transition frequencies in the vicinity of the band-gap edge or inside the gap, the spontaneous emission spectra become strongly non-Lorentzian.

We report an experimental investigation of the effect of electromagnetically induced transparency (EIT) using bichromatic laser radiation and an extremely thin cell filled with pure Rb with smoothly controllable thickness $L$ of the atomic vapor layer in the range $\sim 780–1600\mathrm{nm}$, for which $L$ is comparable to the laser wavelength $\lambda$ resonant with the $D_2$ line $\left(780\mathrm{nm}\right)$. It is revealed that the transmission spectrum of the probe laser contains two sub-Doppler peaks that have different linewidths. The narrow peak corresponds to EIT, whereas the broader one results from velocity-selective optical pumping (VSOP) and repumping processes. It is demonstrated that in the case of nonzero detuning of the coupling laser, the EIT resonance and VSOP are shifted with respect to each other on the frequency scale, which makes it possible to observe a competition between the two effects (this is not possible to realize in an ordinary cell). Also, the Dicke-type coherent narrowing effect depending on the ratio $L∕\lambda$ influences the absorption spectrum of the probe laser. Formation of an EIT resonance is substantially favored for the atoms with slow normal velocity, caused by their longer interaction time with the bichromatic laser field. As a result of the predominant contribution of these atoms, the observed linewidth of the EIT resonance is only $\sim 9\mathrm{MHz}$, which is more than ten times narrower than the inverse window-to-window flight time. In an external magnetic field, three and five EIT resonances have been observed for the vapor thickness of $L=2\lambda$, on ${}^{87}\mathrm{Rb}$ and ${}^{85}\mathrm{Rb}$, respectively.

Nearly unipolar, subpicosecond half-cycle pulses (HCPs) have been used to ionize ${\mathrm{Ba}}^{+}$ $ng$ Rydberg ions. The double ionization probability vs HCP field strength data are compared with analogous single ionization measurements for neutral Ba $6snd$ Rydberg atoms prepared in the same apparatus. We find that neutral and ionic Rydberg states with the same binding energies follow approximately the same ionization curves. Our findings are in agreement with the predictions of an impulsive ionization model and classical trajectory Monte Carlo simulations.

Femtosecond explosive processes of argon clusters irradiated by linearly chirped ultraintense laser pulses have been investigated by 90° side spectral scattering. The spectral redshift and blueshift, which correlate with the cluster explosion processes have been measured for negatively and positively chirped driving laser pulses, respectively. The evolution of the heated-cluster polarizability indicates that the core of the cluster is shielded from the laser field in the beginning of the explosion and enhanced scattering occurs after the fast explosion initiates. Evidence of resonant heating is found from the coincidence of enhanced scattering with enhanced absorption measured using the transmitted spectra. Anomalously large-size clusters with very low gas density have been observed in this way and can be used as clean and important cluster targets.

Using a classical Monte Carlo method, we have computed the three-body recombination (two free electrons and a proton scattering into one free electron and a hydrogen atom, $e+e+p\to \mathrm{H}+e$) in strong magnetic fields. The proton is fixed in space but the electrons are allowed their full, three-dimensional motion. We investigate recombination for temperatures and fields similar to those used in recent experiments that generated antihydrogen. The present rate is compared to that when the electrons’ motion is given by the guiding center approximation, validating previous results at low temperature and demonstrating the breakdown of this approximation at higher temperature. Unlike the $B=0$ case, strong $B$ gives preferential recombination to atoms with positive magnetic moment. Also, the canonical angular momentum in the field direction is often negative even when the magnetic moment is negative. Both results affect the trapping of antihydrogen using spatially dependent magnetic fields.

Following absorption of a single photon, angles of simultaneous emission of two electrons from a $\mathrm{He}\left(n{}^{1}S\right)$ atom become more correlated with increasing $n$. We find that the strength of this correlation is due to the two-electron continuum of the electron-impact ionization of the ${\mathrm{He}}^{+}\left(ns\right)$ ion. The strength is determined by the width of the momentum profile of the ionic $ns$ state but not the strength of the electron correlation in the He initial state. This can explain the increasing (over He) angular correlation strength found in double photoionization of targets such as Be, Ne, and ${\mathrm{H}}_2$.

We examine the thermal entanglement of $XXZ$-type Heisenberg chains in the presence of a uniform magnetic field along the $z$ axes through the evaluation of the negativity associated with bipartitions of the whole system and subsystems. Limit temperatures for nonzero global negativities are shown to depend on the asymmetry $\Delta$, but not on the uniform field, and can be much higher than those limiting pairwise entanglement. It is also shown that global bipartite entanglement may exist for $T>0$ even for $\Delta ⩾1$, i.e., when the system is fully aligned (and hence separable) at $T=0$, and that the bipartition leading to the highest limit temperature depends on $\Delta$.

A complete set of cross sections is presented for photodouble ionization of He at $0.1\mathrm{eV}$ above the threshold. Special care is taken to clear the asymmetry parameter and the energy differential cross section of any ionization-excitation contribution. As a result, their limiting behaviors for the fully asymmetric partitionings of the excess energy are elucidated, thus shedding light on pending discussions in the field. A reliable scheme follows for computing the fully integrated cross section. Very good agreement is observed between the calculated and measured fully differential cross sections after a detailed reassessment of the experimental normalization procedure. The present findings are compared with the assumptions underlying the Wannier picture of near-threshold double escape.

We use hydrodynamic equations to study sound propagation in a superfluid Fermi gas at zero temperature inside a strongly elongated cigar-shaped trap, with main attention to the transition from the BCS to the unitary regime. First, we treat the role of the radial density profile in the limit of a cylindrical geometry and then evaluate numerically the effect of the axial confinement in a configuration in which a hole is present in the gas density at the center of the trap. We find that in a strongly elongated trap the speed of sound in both the BCS and the unitary regime differs by a factor $\sqrt{3∕5}$ from that in a homogeneous three-dimensional superfluid. The predictions of the theory could be tested by measurements of sound-wave propagation in a setup such as that exploited by Andrews et al. [Phys. Rev. Lett. 79, 553 (1997)] for an atomic Bose-Einstein condensate.

The $2p$ photoionization and subsequent $L_{2,3}{M}_{2,3}{M}_{2,3}$ Auger decay spectra from free $4s_{1∕2}\to 4p_{1∕2}$ laser-excited potassium atoms are studied both experimentally and theoretically. The shake-up-structure of the $2p$ photoelectron spectrum of K is resolved. A direct experimental way for resolving the satellite structure due to conjugate shake-up transitions accompanying the hole creation in the $L_{2,3}{M}_{2,3}{M}_{2,3}$ Auger spectrum of nonexcited atoms is presented. Theoretical ab initio computations based on the multiconfiguration Dirac-Fock approach were performed to interpret the experimental findings.

By virtue of the fact that its low-lying ${}^2\mathrm{\Pi }_{3∕2}$ state is paramagnetic, nitric oxide $\left(\mathrm{NO}\right)$ can be an effective agent for inducing nuclear spin relaxation during isolated binary collisions with other species. At the same time, the strong coupling between the electronic angular momentum and the internuclear axis of this particular molecule leads to a situation in which its effective magnetic moment depends critically on molecular motions that take place on the time scale of the collision. Here we present the results of an investigation in which the NMR-detected longitudinal nuclear relaxation rate $T_1^{-1}$ of room temperature ${}^3\mathrm{He}$ gas adulterated with $\mathrm{NO}$ was measured as a function of $\mathrm{NO}$ density $\left(\right[\mathrm{NO}\left]\right)$ in two different magnetic fields. We find $T_1^{-1}=0.0502\left(3\right)\left[\mathrm{NO}\right]{\mathrm{s}}^{-1}$ at $1.50\mathrm{T}$, and $T_1^{-1}=0.0506\left(3\right)\left[\mathrm{NO}\right]{\mathrm{s}}^{-1}$ at $2.35\mathrm{T}$, where $\left[\mathrm{NO}\right]$ is expressed in amagat. Under these conditions approximately two-thirds of the $\mathrm{NO}$ molecules occupy the diamagnetic ${}^2\mathrm{\Pi }_{1∕2}$ ground state and one-third occupy the paramagnetic ${}^2\mathrm{\Pi }_{3∕2}$ state. Our data can be understood in terms of a semiclassical treatment of collision dynamics that involves the paramagnetic moment autocorrelation function.