We calculate electronic excitation cross sections for the b${\mathrm{}}^3$${\mathrm{\Sigma }}_{\mathit{u}}^{+}$ a${\mathrm{}}^3$${\mathrm{\Sigma }}_{\mathit{g}}^{+}$ c${\mathrm{}}^3$${\mathrm{\Pi }}_{\mathit{u}}$, and d${\mathrm{}}^3$${\mathrm{\Pi }}_{\mathit{u}}$ states of ${\mathrm{H}}_2$ by electron impact. Our results were obtained with the Schwinger multichannel method with pseudopotentials and real potentials at the two-channel level of approximation. Pseudo-H atoms are used to generate ${\mathrm{H}}_2$ molecules with almost the same low-energy spectrum as the real molecules. We show that the dynamics of the electronic excitation process of the pseudomolecules by electron impact is very similar to the real case. Our results support the idea that pseudopotentials can be used to obtain reliable molecular electronic excitation cross sections by low-energy electron impact, confirming the expectations of previous studies with ${\mathrm{CH}}_2$O [Bettega et al., Phys. Rev. A 25, 1111 (1993)] and HBr [Rescigno, J. Chem. Phys. 104, 125 (1996)]. © 1996 The American Physical Society.

We compute the absorption coefficient in the far infrared of collisional He-Ar pairs for comparison with measurements at four temperatures (140, 165, 200, and 240 K) in mixtures of helium and argon. An accurate ab initio dipole surface and an advanced model of the He-Ar interaction potential are input. The comparison with the measured absorption spectra shows agreement, generally within the accuracy of the measurement of roughly 10%. This fact suggests that this ab initio dipole surface of He-Ar pairs is probably as accurate as it was claimed to be (±3–5 %) in the range where the optical interactions take place at the above temperatures. Three types of line profiles are considered: classical, quantal, and a ‘‘symmetrized’’ quantal profile, which has been believed to approximate the classical profile. Classical and symmetrized profiles diverge, however, with increasing frequency. This fact suggests that analyses of measured spectra based on symmetrized profiles should be reconsidered. © 1996 The American Physical Society.

Data about antiprotons slowing down in gaseous targets at very low energies (E<1 keV) show that the stopping power in ${\mathrm{D}}_2$ is lower than in ${\mathrm{H}}_2$; the right way to explain this behavior seems to be through a nuclear stopping power derived from the classical Rutherford formula. © 1996 The American Physical Society.

We consider the vortex state solution for a rotating cloud of trapped, Bose-Einstein-condensed alkali-metal atoms and study finite-temperature effects. We find that thermally excited vortex waves can distort the vortex state significantly, even at the very low temperatures relevant to the experiments. © 1996 The American Physical Society.

Atomic band-gap structures are possible by exploiting resonant quantum tunneling of cold atomic beams through a positively detuned laser beam with a multipeaked transverse profile. Spontaneous emission inhibits but does not necessarily eliminate the resonance effects. © 1996 The American Physical Society.

The spontaneous emission rate of molecules depends on the refractive index of the surrounding environment. We present a photoluminescence lifetime study of sulforhodamine B molecules dissolved in water droplets (inverted micelles) which are stabilized by a surfactant and suspended in solutions of different dielectric constants. A good agreement is found between the experimentally observed and theoretically expected lifetime variations. The radiative and nonradiative lifetimes of sulforhodamine B molecules in micelles are determined. © 1996 The American Physical Society.

We observe optical third- and fifth-harmonic generation to wavelengths of ultraviolet and vacuum ultraviolet at an unconventional vacuum-glass interface without plasma formation. We extend this harmonic-generation technique to various other interfaces and suggest that odd-multipole harmonic generation at an interface by a focused light beam is a general phenomenon. © 1996 The American Physical Society.

Combining an electronic theory with molecular-dynamics simulations we present results for the ultrafast structural changes in small clusters. We determine the time scale for the change from the linear to a triangular structure after the photodetachment process ${\mathrm{Ag}}_3^{\mathrm{-}}$→${\mathrm{Ag}}_3$. We show that the time-dependent change of the ionization potential reflects in detail the internal degrees of freedom, in particular coherent and incoherent motion, and that it is sensitive to the initial temperature. We compare with experiment and point out the general significance of our results. © 1996 The American Physical Society.

Energy-resolved photoelectron spectroscopy in combination with femtosecond pump-probe and molecular beam techniques are used to map molecular dynamics along the internuclear coordinate. The capabilities of this method are demonstrated on the one-dimensional wave-packet motion on electronically excited states of the sodium dimer: by recording transient photoelectron spectra in a pump-probe experiment we are able to follow the molecular wave-packet motion on neutral electronic states along all energetically allowed internuclear distances simultaneously. © 1996 The American Physical Society.

The method of Auger electron spectroscopy was utilized to measure cross sections for the production of K-Auger electrons in ${\mathrm{N}}^{7+}$+Ne and ${\mathrm{N}}^{6+}$+Ne collisions at an impact energy of 35 keV. In addition to the K-Auger electrons from the nitrogen projectile following multiple electron capture, K-Auger electrons of the Ne target are observed for the system ${\mathrm{N}}^{7+}$+Ne. This observation is attributed to dielectronic excitation, produced by electron-electron interaction, where a K- and an L-shell electron of Ne are transferred into the K shell of ${\mathrm{N}}^{7+}$. © 1996 The American Physical Society.

Absolute doubly differential cross sections (DDCS’s) for the resonant inelastic scattering of quasifree target electrons on H-like projectiles have been measured. Electron spectra for 20.25-MeV ${\mathrm{O}}^{7+}$ projectiles on an ${\mathrm{H}}_2$ target were measured. The spectra contain a resonant contribution from the 3l3${\mathit{l}}^{\prime }$ doubly excited states of ${\mathrm{O}}^{6+}$, which decay predominantly to the 2l states of the ${\mathrm{O}}^{7+}$ via autoionization, and a nonresonant contribution from the direct excitation of the projectiles to the ${\mathrm{O}}^{7+}$(2l) state by the quasifree target electrons. Close-coupling R-matrix calculations for the inelastic scattering of free electrons on ${\mathrm{O}}^{7+}$ ions were performed. The relation between the electron-ion inelastic scattering calculation and the electron DDCS’s for the ion-atom collision was established by using the inelastic scattering model (ISM). We found excellent agreement between the theoretical and measured resonant peak positions and relative peak heights. The calculated absolute double differential cross sections for the resonance processes are also in good agreement with the measured data. The implication is that collisions of highly charged ions on hydrogen can be used to obtain high-resolution, angle- resolved differential inelastic electron-scattering cross section. © 1996 The American Physical Society.

Asymptotic atomic-state branching ratios for the dissociative recombination of both ${}_{}{}^3{}_{}{}^{}\mathrm{HeD}_{}^{+}{}_{}{}^{}$ and ${}_{}{}^4{}_{}{}^{}\mathrm{HeH}_{}^{+}{}_{}{}^{}$ have been studied using the CRYRING and TSR heavy-ion storage rings. The kinetic-energy release in the recombination process was measured for incident electron energies between 0 and 15 eV. It was found that the He(1${\mathit{s}}^2$)+D,H(n=2) channel completely dominates at zero electron energy. The branching ratios observed slightly above the threshold for the n=3 state of the H (D) atom indicate a rapid switchover of the final-state population to this level. At collision energies above 10 eV many channels leading to excited He atoms are found to contribute, and also a strong angular anisotropy of the dissociation products is observed. © 1996 The American Physical Society.

Quantum-mechanical analysis is presented on the photodissociation dynamics near the resonances. It is shown that the product angular distributions exhibit very different variations depending on the characteristics of resonances. The fragment anisotropy parameters in OH photodissociation are predicted to be independent of energy along isolated Lorentzian resonances. Interferences between neighboring Lorentzian resonances give significant variations of the anisotropy parameters. Quantum interference between the direct and indirect dissociation pathways also contributes to very strong variations of the anisotropy parameters across asymmetric resonances. © 1997 The American Physical Society.[S1050-2947(97)501412-6]

We study an elementary model of two tunneling Bose-Einstein condensates with an atom detector in one of the traps. Stochastic simulations of atom detection in one trap show an oscillation of atom number between traps even though the condensates are initially in equal number Fock states. The oscillation has a random amplitude depending on the phase difference between the condensates, which coincides with that given by a spontaneously broken symmetry argument. © 1996 The American Physical Society.

We analyze theoretically an optical scheme to detect the phase of a Bose-Einstein condensate associated with spontaneously broken gauge symmetry. Two condensates in different Zeeman states are confined to the same trap, and two phase-coherent laser beams are used to drive Raman transitions between the condensates. Amplification of one of the Raman beams immediately following the turn-on of light is an unambiguous sign of the presence of phase coherence in the atoms. © 1996 The American Physical Society.

By using a mean-field approach, based on the Popov approximation, we calculate the temperature dependence of the condensate fraction of an interacting Bose gas of N atoms confined in an anisotropic harmonic trap. For systems interacting with repulsive forces we find a significant decrease of the condensate fraction and of the critical temperature with respect to the predictions of the noninteracting model with the same value of N. An analytic result for the shift of the critical temperature holding to first order in the scattering length is also derived. © 1996 The American Physical Society.

We present semianalytic expressions for scattering probabilities of cold trapped atoms in the presence of two weak, independently tunable laser fields, both tuned near molecular resonances. We consider two experimental situations. In the first, an Autler-Townes doublet appears and is shown in certain circumstances to exhibit unequal peak heights. In the second, the pair of lasers is employed to produce translationally cold ground-state molecules, in which case our formula suggests the optimal conditions for such production. © 1996 The American Physical Society.