Three distinct phenomena are found to affect the autoionization of a quasicontinuum. For certain initial superpositions of the states within the quasicontinuum, the autoionization may be inhibited by quantum interference and the population may be trapped. For those initial superpositions that do decay, a self-trapping effect can be observed which causes the superposition to evolve into a trapped state. The self-trapping is due to a redistribution of the population within the quasicontinuum via Raman-like transitions driven by the channel interaction with the continuum acting as the intermediate state. The physical system of a two-electron atom is studied. There the autoionizing continuum is created by an isolated core excitation that embeds a range of highly excited Rydberg states into the continuum. Another mechanism appears, in this system, that affects the autoionization. When the channel interaction is strong compared to the strength of the core excitation a stabilizing effect occurs which slows the autoionization. Analytical models examine these effects in a system with degenerate quasicontinua. Perfect population trapping, self-trapping, and stabilization are all clearly displayed by this system. Numerical models of nondegenerate quasicontinua show that the stabilization process is virtually unchanged by the lifting of the degeneracy. These models also show that, in general, the trapping effects largely disappear. However, a synchronization of the free evolution of the quasicontinuum states with Rabi oscillation of the core excitation returns them to significant roles in the decay process. A wide variety of nondecaying states can be created. Competition between these mechanisms is explored and the impact of these phenomena on experiments is considered.

Results of semirelativistic $R$-matrix calculations for electron-impact excitation of the ${(np}^6{)}^1{S}_0\to {\mathrm{np}}^5[n+1\mathrm{}]s$ $\mathrm{}(J=1\mathrm{})\mathrm{}$ transitions in the noble gases Ne $\mathrm{}(n=2\mathrm{}),$ $\mathrm{Ar}\mathrm{}(n=3\mathrm{}),$ Kr $\mathrm{}(n=4\mathrm{}),$ and Xe $\mathrm{}(n=5\mathrm{})\mathrm{}$ are compared with experimental values for light polarizations and angle-integrated magnetic sublevel cross sections. The calculations focus on the near-threshold regime, where negative-ion resonances have a significant effect on the results. In light of the complexity of the structures, the agreement between theory and experiment is very encouraging. Nevertheless, further improvements in the theoretical treatment of both the target structure and the collision dynamics seem necessary.

Secondary-electron yield induced by MeV protons specularly reflected from a SnTe(001) surface is measured. From the observed secondary-electron yield, the position-dependent secondary-electron production rate $P\left(x\right)\mathrm{}$ is derived as a function of distance $x$ from the surface. The obtained $P\left(x\right)\mathrm{}$ can be explained in terms of the direct electron excitation as well as the decay of both bulk and surface plasmons excited by the proton. The probability of surface plasmon decay to an electron-hole pair is estimated to be $\sim 30\%.$

The dynamical interaction effects on the Coulomb explosion and the stopping power are studied for MeV ${\mathrm{H}}_2^{+}$ ions in grazing collisions with a solid surface. The surface dynamical potential induced by the molecular ions is obtained by using a local frequency-dependent dielectric function for a semi-infinite electron gas with damping. A set of equations describing the motion of the center of mass and the Coulomb explosion is obtained by including the repulsive force of the surface atoms, the bare Coulomb force, and the dynamical forces. The total energy loss for the molecular ion is calculated, taking into account a contribution from both the collective excitations of the conduction electrons and the single-electron excitations of each of the substrate atoms.

We study the interaction of a two-level atom with two lasers of different frequencies and amplitudes: a strong laser of Rabi frequency $2{\Omega }_1$ on resonance with the atomic transition, and a weaker laser detuned by subharmonics $(2\mathrm{}{\Omega }_1/n)$ of the Rabi frequency of the first. We find that under these conditions the second laser couples the dressed states created by the first in an $n$-photon process, resulting in “doubly dressed” states and in a “multiphoton ac Stark” effect. We calculate the eigenstates of the doubly dressed atom and their energies, and illustrate the role of this multiphoton ac Stark effect in its fluorescence, absorption, and Autler-Townes spectra.

We discuss a method of solving the time-dependent Schrödinger equation for atoms with two active electrons in a strong laser field, which we used in a previous paper [A. Scrinzi and B. Piraux, Phys. Rev. A 56, R13 (1997)] to calculate ionization, double excitation, and harmonic generation in helium by short laser pulses. The method employs complex scaling and an expansion in an explicitly correlated basis. Convergence of the calculations is documented and error estimates are provided. The results for He at peak intensities up to ${10}^{15}\hspace{0.5em}{\mathrm{W}/\mathrm{c}\mathrm{m}}^2$ and wavelength 248 nm are accurate to at least 10%. Similarly accurate calculations are presented for electron detachment and double excitation of the negative hydrogen ion.

Photoionization of Rydberg atoms is considered in the framework of the quasiclassical (WKB) approach. Distribution of the photoelectron density in space and time is found and analyzed. This distribution is shown to consist of two main parts: the temporary population, which exists only as long as the light field is on; and the population corresponding to irreversible ionization. The irreversible-ionization wave packets are shown to experience the effect referred to as Coulomb narrowing. An origin of this effect and the conditions of its existence are discussed. In a strong field, rather simple analytical expressions are found for the partial probabilities of above-threshold ionization and for the total probability of ionization. In contrast to many earlier works, in the present theory neither the rotating-wave nor “pole” approximations are used, and the results are derived for an arbitrary pulse shape. Strong-field interference stabilization of Rydberg atoms is confirmed to occur at pulse durations shorter than the Kepler period. The arising regimes of interference stabilization are analyzed in comparison with the predictions of earlier works.

We study an extended model of intense-laser ionization of the hydrogenlike atom from a low excited state via a band of high Rydberg states. The essence of the extension is that the population from the directly excited low-angular-momentum Rydberg band is allowed to migrate resonantly toward Rydberg bands of higher angular momenta through a sequence of V-type degenerate Raman transitions. Each V-type transition exploits, as a resonant intermediary, the state of the same principal quantum number as that of the initial state but of a different angular momentum number. We find an analytical limit to this model, and obtain for it a closed-form solution for arbitrary length of the chain of the V-type transitions. This solution implies that, through the chain of V-type resonances, the population initially localized in a single state can be spread over a number of bound states of much higher angular momenta. The main result of the model is that, at high intensities, no population is left in high Rydberg states, but the population not released to the continuum is shared between the initial state and the higher-angular-momentum states of the same energy as that of the initial state. This sharing depends on the pulse duration, and, for the duration approaching one Kepler period of the resonantly excited high Rydberg state, the initial state was found to be completely depleted. This particular result emphasizes the effect of the orbital degeneracy of the initial level and V-type resonances on the high-intensity hydrogen-atom stabilization.

We investigate coherent control of atomic translational motion in an applied off-resonance dipole optical potential. Since spontaneous emission can be neglected in this regime, the problem is treated as one of coherent momentum transfer. We consider both adiabatic and nonadiabatic regimes of cooling in the framework of the Wigner function formalism. For the adiabatic case, an approximate solution is obtained corresponding to the cooling envelope for an arbitrary time dependence of the external field. The nonadiabatic cooling process is formulated in terms of optimal control theory in order to define the most favorable regime of cooling under imposed constraints on the intensity of the control field. We find that the applied control field yields a significant reduction of the effective temperature of the atoms.

We present a time-dependent theory of laser catalysis, a process in which a strong light source is used to affect tunneling through a potential barrier by inducing a transient electronic excitation to a bound state. We have performed detailed calculations of pulsed laser catalysis on a one-dimensional Eckart potential as a function of the collision energy and the laser’s central frequency. As in the cw case, the barrier transmission coefficients range from 100% tunneling on the blue side to complete suppression of tunneling on the red side of the radiatively broadened line. The point of perfect transmission is explained in the dressed state picture in terms of the equivalence between adiabatic laser catalysis and transmission through a double-barrier potential. The point of complete suppression of tunneling is shown to result from nonadiabatic destructive interference between the nonradiative tunneling and the optically assisted route.

We present a theory for two- and three-photon excitation, optical shifting, and four-wave mixing when a first laser is tuned onto, or near, a two-photon resonance and a second much more intense laser is tuned near or on resonance between the two-photon resonance and a second excited state. When the second excited state has a dipole-allowed transition back to the ground state and the concentration is sufficiently high, a destructive interference is produced between three-photon coupling of the ground state and the second excited state and one-photon coupling between the same states by the internally generated four-wave mixing field. This interference leads to several striking effects. For instance, as the onset of the interference occurs, the optical shifts in the two-photon resonance excitation line shape become smaller in copropagating geometry so that the line shapes for multiphoton ionization enhanced by the two-photon resonance eventually become unaffected by the second laser. In the same range of concentrations the four-wave mixing field evolves to a concentration-independent intensity. With counterpropagating laser beams the line shape exhibits normal optical shifts like those observed for both copropagating and counterpropagating laser beams at very low concentrations. The theoretical work presented here extends our earlier works by including the effect of laser bandwidth and by removing the restriction of having the second laser be tuned far from three-photon resonance. In this way we have now included, as a special case, the effect of both laser bandwidth and interference on laser-induced transparency. Unlike other effects related to odd-photon destructive interference, the effect of a broad bandwidth is to bring about the predicted effects at much lower concentrations. Studies in rubidium show good agreement between theory and experiment for both ionization line shapes and four-wave mixing intensity as a function of concentration.

Intracavity self-induced transparency of a three-level absorber is studied in the scope of solid-state laser generation of an ultrabroadband electromagnetic pulse that drives the population of all absorber levels through complete Rabi flopping. We show that at sufficient pump rates a Ti:sapphire laser forces an intracavity GaAs single quantum-well absorber, which provides an inter-valence-band transition in the THz domain in addition to two direct optical interband transitions, into the self-induced transparency regime and acts as an all-solid-state ultrabroadband pulse emitter. In dependence on the resonator bandwidth, the intracavity pulse energy and the absorber dipole moments we obtain a multilevel self-induced transparency pulse spectrum which extends from the THz domain up to the ultraviolet. The steady-state sub-10-fs pulse consists of only a few optical cycles with the high-frequency components at its leading edge and a single to subcyclic THz component at its trailing edge.

Classical periodic orbits play an important role in interpreting the complex structure of photoabsorption spectra of highly excited Rydberg atoms in external fields. In this paper, we proceed beyond the simultaneous excitation of many periodic motions implied by the photoabsorption spectrum, and investigate the selective excitation of a single periodic motion. Using dynamical averaging we synthesize a wave packet which is optimally localized on an entire classical structure such as a periodic orbit or an invariant manifold. The effect of electron scattering from the core can be documented clearly using these special wave packets, as can “antiscarring,” in which intensity placed on a hyperbolic fixed point avoids entering elliptic regions for many Kepler periods.

The yields and time-of-flight spectra for ions produced in three-photon ionization of atomic deuterium have been investigated as a function of laser intensity from 0.4 to $200{\mathrm{M}\mathrm{W}/\mathrm{c}\mathrm{m}}^2.$ A pulsed laser beam of 243 nm is used, so that the photoionization is resonant with the metastable $2s$ state of deuterium. The production of resonantly excited $2s$ atoms is measured by detecting the quenched Lyman-α radiation at time intervals both during and after the laser pulse. The near degeneracy of the $2s-2p$ state manifold provides a particularly sensitive way to monitor the presence of any electric field in the interaction volume. As the laser intensity is increased, we observe the effects of a collective field arising from the electron-ion plasma produced by the laser pulse. We present the results of a theoretical model where the collective motion of the ions and the electrons is analyzed in terms of the fluid approximation. The essential features of the experimental observations including ion yields, ion time-of-flight spectra, and metastable production versus time, are well described by the theory.

A precise measurement of the initial kinetic energy of photoelectrons born in a high-intensity, linearly polarized laser has been made in the long-pulse tunneling limit. The shape of the angular distribution of electrons outside the laser focus is related to the initial momenta the electrons gain from the field. The measurements of nonzero drift momenta for high-charge states of neon indicate tunneling ionization is occurring and agree with the predictions of the Ammosov-Delone-Krainov (ADK) model [M. V. Ammosov, N. B. Delone, and V. P. Krainov, Sov. Phys. JETP 64, 1191 (1986)]. The validity of this model allows the value of the electron’s initial kinetic energy to be limited to approximately 0.2% of its average quiver energy.

We present a comprehensive theory of nuclear spin polarization of ${}^3\mathrm{He}$ and ${}^{129}\mathrm{Xe}$ gases by spin-exchange collisions with optically pumped alkali-metal vapors. The most important physical processes considered are (1) spin-conserving spin-exchange collisions between like or unlike alkali-metal atoms; (2) spin-destroying collisions of the alkali-metal atoms with each other and with buffer-gas atoms; (3) electron-nuclear spin-exchange collisions between alkali-metal atoms and ${}^3\mathrm{He}$ or ${}^{129}\mathrm{Xe}$ atoms; (4) spin interactions in van der Waals molecules consisting of a Xe atom bound to an alkali-metal atom; (5) optical pumping by laser photons; (6) spatial diffusion. The static magnetic field is assumed to be small enough that the nuclear spin of the alkali-metal atom is well coupled to the electron spin and the total spin is very nearly a good quantum number. Conditions appropriate for the production of large quantities of spin-polarized ${}^3\mathrm{He}$ or ${}^{129}\mathrm{Xe}$ gas are assumed, namely, atmospheres of gas pressure and nearly complete quenching of the optically excited alkali-metal atoms by collisions with ${\mathrm{N}}_2$ or ${\mathrm{H}}_2$ gas. Some of the more important results of this work are as follows: (1) Most of the pumping and relaxation processes are sudden with respect to the nuclear polarization. Consequently, the steady-state population distribution of alkali-metal atoms is well described by a spin temperature, whether the rate of spin-exchange collisions between alkali-metal atoms is large or small compared to the optical pumping rate or the collisional spin-relaxation rates. (2) The population distributions that characterize the response to sudden changes in the intensity of the pumping light are not described by a spin temperature, except in the limit of very rapid spin exchange. (3) Expressions given for the radio-frequency (rf) resonance linewidths and areas can be used to make reliable estimates of the local spin polarization of the alkali-metal atoms. (4) Diffusion effects for these high-pressure conditions are mainly limited to thin layers at the cell surface and at internal resonant surfaces generated by radio-frequency magnetic fields when the static magnetic field has substantial spatial inhomogeneities. The highly localized effects of diffusion at these surfaces are described with closed-form analytic functions instead of the spatial eigenmode expansions that are appropriate for lower-pressure cells.

We numerically calculate the zero-temperature density profile and excitation spectrum of a two-species Bose-Einstein condensate for the parameters of recent experiments. We find that the ground-state density profile of this system becomes unstable in certain parameter regimes, which leads to a phase transition to a new stable state. This state displays spontaneously broken cylindrical symmetry. This behavior is reflected in the excitation spectrum: As we approach the phase transition point, the lowest excitation frequency goes to zero, indicating the onset of instability in the density profile. Following the phase transition, this frequency rises again.

The properties of an ideal Fermi gas in an external potential in any dimensional space are studied, based on the semiclassical (Thomas-Fermi) approximation. The general analytical expressions of the total particle number $N,$ density of states $D\left(\varepsilon \right),$ Fermi energy $E_F,$ total energy $E,$ and heat capacity $C$ have been derived, where $N,$ $E,$ and $C$ are expressed by the Fermi integration. Moreover, the analytical expressions of the total energy $E,$ chemical potential μ, and heat capacity $C$ in the high- and low-temperature approximations are given. From these results, how the characteristics of the Fermi gas depend on an external potential and the dimension of space is discussed.

We use the quantum kinetic theory to calculate the steady state and fluctuations of a trapped Bose-Einstein condensate at a finite temperature. The system is divided in a condensate and a noncondensate part. A quantum-mechanical description based on the number-conserving Bogoliubov method is used for describing the condensate part. The noncondensed particles are treated as a classical gas in thermal equilibrium with temperature $T$ and chemical potential $\mu$. We find a master equation for the reduced density operator of the Bose-Einstein condensate, calculate the steady state of the system, and investigate the effect of one-, two-, and three-particle losses on the condensate. Using linearized Ito equations, we find expressions for the intensity fluctuations and the amplitude fluctuations in the condensate. A Lorentzian line shape is found for the intensity correlation function that is characterized by a time constant ${\gamma }_I^{-1}$ derived in the paper. For the amplitude correlation function, we find ballistic behavior for time differences smaller than ${\gamma }_I^{-1}$, and diffusive behavior for larger time differences.

Bose-Einstein condensation is described in terms of the condensate wave function and the pair-excitation function, the latter being responsible for the existence of phonons. This minimal description in terms of these two functions is generalized to the case with an external potential. For a dilute gas with short-range pairwise repulsive interaction and at very low temperatures when the Bose-Einstein condensation is nearly complete, a partial differential equation is obtained for the condensate wave function and an integro-differential equation for the pair excitation. Experimentally, the external potential is used to trap the atoms, i.e., to keep them together. Since the trap is of macroscopic dimensions, the resulting external potential is often slowly varying. In these cases and when the condensate is in the lowest state, the partial differential equation for the condensate wave function and the integro-differential equation for the pair excitation are solved approximately for the case of a time-independent trap.