Total cross sections for electron scattering on ${\mathrm{CH}}_3\mathrm{Cl},{\mathrm{CH}}_2{\mathrm{Cl}}_2,$ and ${\mathrm{CHCl}}_3$ have been measured by an absolute method in the (75–4000)-eV energy range. The overall experimental error is below 5%. A formulation of the additivity rule is proposed. The molecular cross sections are approximated by a Born-like two-parameter formula. We show that the low-energy parameter is correlated to the molecular polarizability. We show also that the high-energy parameter for a given molecule can be expressed as the sum of the high-energy parameters of the constituent atoms. The model has been successfully verified for two groups of halomethanes: the ${\mathrm{CH}}_4,{\mathrm{CH}}_3\mathrm{Cl},{\mathrm{CH}}_2{\mathrm{Cl}}_2,{\mathrm{CHCl}}_3,$ and ${\mathrm{CCl}}_4$ series and the ${\mathrm{CF}}_4,{\mathrm{CF}}_3\mathrm{Cl},{\mathrm{CF}}_2{\mathrm{Cl}}_2,{\mathrm{CFCl}}_3,$ and ${\mathrm{CCl}}_4$ series. The model has been successfully extended to molecules containing Si and S atoms, such as ${\mathrm{H}}_2\mathrm{S},{\mathrm{SF}}_6,{\mathrm{SiH}}_4,$ and ${\mathrm{SiF}}_4.$

Experimental cross sections are reported for electron excitation of the allowed $2s{}^2\overrightarrow{S}2p{}^{2}P$ transition in ${\mathrm{C}}^{3+}.$ Center-of-mass energies are from below threshold (7.00 eV), through threshold (8.00 eV), and up to approximately 1.5 times the threshold (12 eV). The present results are found to overlap earlier energy-loss data reported to energies of 8.45 eV. The data are also compared with other optical-emission results for this dipole-allowed transition and with several theoretical calculations. An “electronic aperture” is described that allows discrimination against elastically scattered electrons, with their larger Larmor radii, in experiments with magnetically confined ions or electrons.

An explicit solution is obtained for the four-wave frequency mixing ${\omega }_d={\omega }_a-{\omega }_b+{\omega }_c$ of two strong fields a and c and two weak fields b and d in a four-level system with large Doppler broadening in collinear geometry, where the frequencies of weak fields are nearly equal, ${\omega }_b\simeq {\omega }_d,$ and the medium is optically thin. Without weak fields there are two independent two-level systems. A pair of weak fields probes two other allowed transitions. A peak of the mixing coefficient as a function of intensity is found around an equal Rabi splitting of both two-level systems. The effect is based on a resonance between two closed cycles of four-wave mixing via different dressed states. Three, four, or six peaks are predicted in the dependence of the mixing coefficient on the frequency of the weak field; two of them are a consequence of averaging over velocities. The model allows an interpretation of the dependence of the output wave power on the intensity and detuning in recent experiments on frequency mixing in sodium vapor.

We present a spatiotemporal analysis of high harmonic generation, showing evidence for the presence of several quantum path contributions to the atomic dipole moment. We show that the harmonic radiation can largely be described as a sum of two fields having a phase proportional to the intensity of the generating field. We compare our results to recent experimental results demonstrating this separation [M. Bellini et al., Phys. Rev. Lett. 81, 297 (1998)]. We show how the temporal and spatial coherence properties are influenced by this effect, and discuss how it could be used to obtain better control of the generated harmonic radiation.

Generally, high optical intensity is required for effective multiphoton excitation of quantum systems to highly excited states. In certain situations, however, lower-intensity, chirped pulses can provide more efficient transfer of population to the upper states by the process of adiabatic passage. We have studied the relative importance of these two mechanisms in the anharmonic $5s-5p-5d$ quantum ladder system of rubidium using frequency chirped laser pulses from an amplified Ti:sapphire laser (λ=780 and Δλ=10 nm). We measure simultaneously the three-photon ionization signal due to the Ti:sapphire and the population that remains in the $5d$ state with a postionizing 532-nm Nd:YAG pulse (where YAG denotes yttrium aluminum garnet). At low infrared fluences (80 μ${\mathrm{J}/\mathrm{c}\mathrm{m}}^2$), the transfer to the $5d$ state is significantly enhanced when the pulse frequency is swept from the red to the blue, such that it follows the frequency spacing of the rubidium ladder. Counterintuitively, population is also transferred efficiently for the blue-to-red chirp at high fluences (>5 ${\mathrm{m}\mathrm{J}/\mathrm{c}\mathrm{m}}^2$). We attribute both of these effects to adiabatic passage from the $5s$ state to the $5d$ state. Even at the highest fluences, more efficient transfer occurs for either direction of chirp than occurs at zero chirp, where the intensity is maximal. A comparison to theoretical predictions reveals striking agreement in both absolute magnitude and functional form. These results have important implications for the understanding of population transfer in complex ladder systems, such as molecular anharmonic vibrational ladders.

The alignment dynamics of HCN, in a linear configuration, interacting with linearly polarized infrared laser pulses are studied numerically by exact (nonperturbative) solutions of the time-dependent Schrödinger equation. The alignment, with respect to the laser field polarization vector, is measured from the angular distribution of the molecule using a defined half angle ${\theta }_{1/2}.$ It is shown that, at intensities $\mathcal{I}{=10\mathrm{}}^{13}{\mathrm{W}/\mathrm{c}\mathrm{m}}^2,$ alignment can be achieved on a subpicosecond time scale with a single laser frequency, in the presence of simultaneous dipole- and polarizability-field interactions. The results are compared to those of a laser-driven rigid-rotor analytical model that is thoroughly developed. The importance of the permanent and field-induced dipole moments on the alignment process is investigated, as well as the role of vibrational excitation of the two molecular bonds.

Mass spectrometry experiments have been performed to probe the morphology of dissociative ionization of a ring molecule benzene when it is immersed in a laser field of 35-ps duration whose intensity is in the range ${10}^{13}\hspace{0.5em}{\mathrm{W}\mathrm{}\mathrm{cm}}^{-2}.$ The field-induced fragmentation pattern is grossly different from that obtained by electron impact. The intense laser field clearly gives rise to much more fragmentation. Covariance mapping has been applied to study field-induced formation of highly charged molecular ions. Results indicate that the overall fragmentation dynamics is complex and is likely to involve both fast ladder switching as well as multiple ionization of fragment molecules at some intermediate steps in the ladder. Angular distributions of fragment ions have been measured. Anisotropic distributions indicate that some fragments are produced from precursors that are strongly aligned along the direction of the laser polarization vector while at least one fragment is formed from a precursor whose angular distribution is markedly isotropic. The nature of the precursors remains to be discovered.

Simple semiclassical formulas are derived for the probability of excitation and ionization of Rydberg atoms irradiated by a half-cycle pulse whose duration is shorter than the Kepler period. The calculated ionization probabilities are in good agreement with the experimental data of Jones, You, and Bucksbaum [Phys. Rev. Lett. 70, 1236 (1993)] and with previous calculations.

We investigate several configurations for complete population transfer on the basis of the dressed state picture in a three-level system. This picture allows us not only to confirm an atom staying in the only dark state all the time under the well-known stimulated Raman adiabatic passage but also to find the considerably different dynamics from the conventional adiabatic process in cases where the pulses are significantly overlapped or concurrent (optimal detuning method). We expand the concept of the adiabatic process in order to explain all these cases.

Optical-pumping-induced population-grating transfer between hyperfine levels of the cesium $D_2$ line is observed through four-wave mixing in a sample of cold atoms. Diffraction efficiencies of order of 1% have been measured for a large range of angular apertures. We have studied the angular dependence of the diffracted signal in the limit of small atomic velocities and discussed its application for a nondestructive diagnostic of the trap dynamic. Image processing with nearly degenerate frequency conversion was also demonstrated using this specific mechanism.

We examine the revival features in wave-packet dynamics of a particle confined in a finite square-well potential. The possibility of tunneling modifies the revival pattern as compared to an infinite square-well potential. We study the dependence of the revival times on the depth of the square well and predict the existence of superrevivals. The nature of these superrevivals is compared with similar features seen in the dynamics of wave packets in an anharmonic-oscillator potential.

We calculated the rate of electron detachment from the ground state of ${\mathrm{H}}^{2-}$ in a high-frequency linearly polarized laser field for various different intensities I and laser frequencies $\omega .$ The calculations are performed within the Floquet framework in the Kramers-Henneberger frame. We find that at fixed frequency the rate of detachment decreases with increasing intensity, i.e., stabilization against photodetachment is found. In addition, the rate of detachment decreases as the frequency increases, illustrating the convergence to the high-frequency limit $\overrightarrow{\omega }\infty$ in which photodetachment is absent. In the ground state of ${\mathrm{H}}^{2-}$ the three widely separated electrons quiver along the same straight line parallel to the polarization axis with frequency $\omega$ [Phys. Rev. Lett. 77, 3759 (1996)]. A comparison of the rates of detachment of the individual electrons shows that the middle electron is by far the most stable electron.

The kicked atom is realized experimentally by exposing potassium $\mathrm{np}$ Rydberg atoms with $n\sim 388$ to a sequence of up to 50 half-cycle pulses whose duration is much shorter than the classical electron orbital period. The Rydberg atom survival probability is observed to have a broad maximum for pulse repetition frequencies near the classical orbital frequency. Comparisons with detailed classical trajectory Monte Carlo simulations show that this behavior provides an unambiguous signature of dynamical stabilization. The classical simulations further show that the kicked hydrogen atom is, depending on the pulse repetition frequency, chaotic or characterized by a mixed phase space with various families of fully stable islands within which the atom is stable against ionization. Signatures of stabilization and chaotic diffusion are also observed in the final bound-state distribution of the surviving atoms.

We study the ir excitation of ${\mathrm{HeH}}^{+}$ using the counterintuitive arrangement of two pulsed lasers with the continuum as the intermediate state. The time-dependent Schrödinger equation describing the system is integrated numerically using the split-operator technique. We examine the maximum population transfer as a function of the pulse separation, pulse width, relative pulse strength, two-photon detuning, and relative position of the intermediate continuum state. The largest population transfer found is 63%.

The ionization dynamics of a one-dimensional model atom in ultrastrong dichromatic $1\omega -2\omega$ laser fields is investigated in the stabilization regime. We focus on the phase control and the quantum interference effects. The diagrams of total and partial yield versus relative phase show the similar structure to that of tunneling regime, although underlying mechanism is quite different. An enhancement or suppression of the total ionization rate by more than two orders of magnitude is observed by adjusting the relative phase, which is attributed to the significant effect of quantum interference in the stabilization regime. Asymmetry on the distribution of forward and backward ionization is also observed, i.e., for a proper choice of the phase, the photoelectrons can be confined almost to one direction. All of these findings can be explained satisfactorily by the first-order high-frequency Floquet theory.

We discuss the general features of spontaneous spatial symmetry breaking in trapped two-component alkali-metal–atom Bose-Einstein condensates, and give qualitative guidelines for when it will occur. We further show that the Hartree-Fock equations admit symmetry-broken solutions for as few as two trapped atoms, discussing the particular system of one ${}^{85}\mathrm{Rb}$ atom and one ${}^{87}\mathrm{Rb}$ atom. It is also shown that the critical value of the interspecies scattering length for a mixture of Na and Rb in an isotropic trap depends strongly on the number of atoms in a manner not described by the standard Thomas-Fermi phase separation condition.

We examine the Gross-Pitaevskii (GP) model for Bose-Einstein condensates in parabolic traps with attractive interactions. The decay of metastable condensates is investigated by applying the instanton formalism to the GP field theory. Employing various dynamical trial states, we derive within a coherent state path integral approach a collective coordinate description in terms of the condensate radius, in agreement with (and extending) earlier results. We then solve numerically for the complete instanton field configuration and compare with the collective coordinate approach. Adjusting only the effective mass of the collective coordinate, the two schemes are then in good agreement.

We examine the possibility of Bose-Einstein condensation in one-dimensional interacting Bose gas subjected to confining potentials of the form $V_{\mathrm{ext}}{\mathrm{}\left(x\right)=V}_0(|x|{/a)}^{\gamma },$ in which $\gamma <2,$ by solving the Gross-Pitaevskii equation within the semiclassical two-fluid model. The condensate fraction, chemical potential, ground state energy, and specific heat of the system are calculated for various values of interaction strengths. Our results show that a significant fraction of the particles is in the lowest energy state for a finite number of particles at low temperature, indicating a phase transition for weakly interacting systems.

We provide Monte Carlo simulation and analytic interface arguments for the absence of cylindrical symmetry in the density difference of a binary mixture of Bose-Einstein condensates in traps in the phase segregated regime. For different spin states of ${}^{87}\mathrm{Rb},$ the total density remains very symmetric however. There is a metastable symmetric phase in a substantial part of this phase segregated regime. The boundary between the symmetric and the nonsymmetric phase is a function of the relative concentration. The effect of different confining potentials is investigated.

The wave equation of low-frequency density waves in Bose-Einstein condensates at vanishing temperature in arbitrarily anisotropic harmonic traps is separable in elliptic coordinates, provided the condensate can be treated in the Thomas-Fermi approximation. We present a complete solution of the mode functions, which are polynomials of finite order, and their eigenfrequencies, which are characterized by three-integer quantum numbers.