The Ba(6s6p ${}_{}{}^1{}_{}{}^{}$${\mathit{P}}_1$${\to }^3$${\mathit{P}}_2$) inelastic transition induced by argon has been investigated in a crossed beam as a function of the collision energy between 0.14 and 0.40 eV. The differential cross sections have been obtained by the Doppler technique probing the Ba(6s6p ${}_{}{}^3{}_{}{}^{}$${\mathit{P}}_2$) with a single-mode cw laser. Oscillations are observed in the differential cross sections that are assigned to Stückelberg interferences resulting from a crossing between the ${}_{}{}^1{}_{}{}^{}\mathrm{\Pi }_1^{}{}_{}{}^{}$ and ${}_{}{}^3{}_{}{}^{}\mathrm{\Sigma }_1^{}{}_{}{}^{}$ Ba-Ar potential-energy curves. A very large polarization effect is observed by comparing the differential cross sections corresponding to a pσ or a pπ preparation of the Ba${(}^1$${\mathit{P}}_1$) orbital with respect to the relative Ba-Ar velocity.

The rate coefficients for electron transfer from Ar and ${\mathrm{H}}_2$ to ${\mathrm{Ar}}^{\mathit{q}+}$ ions (3≤q≤6) have been measured using an ion-storage technique in a Penning ion trap. The ions were produced in the trap by K-shell photoionization of Ar atoms, using broadband synchrotron x-ray radiation. K-electron removal resulted in vacancy cascading, yielding a distribution of argon-ion charge states peaked near ${\mathrm{Ar}}^{4+}$. The stored ion gas had an initial temperature near 480 K. The basic data determining the rate coefficients k(${\mathrm{Ar}}^{\mathit{q}+}$) are the storage time constants of each charge state in the trap, in the presence of a measured pressure of target gas. The results of the measurements (in units of ${10}^{\mathrm{-}9}$ ${\mathrm{cm}}^3$ ${\mathrm{s}}^{\mathrm{-}1}$) are k(${\mathrm{Ar}}^{3+}$,${\mathrm{H}}_2$)=4.3(0.7), k(${\mathrm{Ar}}^{3+}$,Ar)=1.6(0.2), k(${\mathrm{Ar}}^{4+}$,${\mathrm{H}}_2$)=5.2(0.6), k(${\mathrm{Ar}}^{4+}$,Ar)=2.5(0.3), k(${\mathrm{Ar}}^{5+}$,${\mathrm{H}}_2$)=5.9(0.7), k(${\mathrm{Ar}}^{5+}$,Ar)=2.9(0.3), k(${\mathrm{Ar}}^{6+}$,${\mathrm{H}}_2$)=8.5(1.2), and k(${\mathrm{Ar}}^{6+}$,Ar)=2.5(0.3).

Measurements and calculations of differential cross sections for direct scattering, single charge transfer, and double charge transfer in collisions of 1.5-, 2.0-, 6.0-, and 10.0-keV ${}_{}{}^3{}_{}{}^{}\mathrm{He}_{}^{2+}{}_{}{}^{}$ with a ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ target are reported. The measurements cover laboratory scattering angles below 1.5° with an angular resolution of about 0.03°. A quantum-mechanical molecular-state representation is employed in the calculations; in the case of single charge transfer a two-state close-coupling calculation is carried out taking into account electron-translation effects. The theoretical calculations agree well with the experimental results for direct scattering and double charge transfer. For single charge transfer, the agreement is less satisfactory because of the limited number of molecular states incorporated in our calculations. The present calculation identifies the origins of oscillatory structures observed in the differential cross sections.

We have obtained the spectrum of the Ba 5djnfj′ J=3 and 5djnpj′ J=3 states above and below the Ba 5d3/2 ionization limit from the 5d2 D12 state. Below the 5d3/2 limit all observed levels are spectrally narrower than 0.3 cm-1. Above the 5d3/2 limit the 5d5/2npj states become very broad while the 5d5/2nfj states remain relatively narrow. We have carried out a quantum-defect-theory analysis using an extension of the formalism of Cooke and Cromer.

The impulse approximation for the Rydberg atom-atom collisions is analyzed on the basis of the Faddeev equations. A modified expression is derived for the cross sections, including the interference between the Rydberg-electron–perturber scattering and the ion-core–perturber scattering. This expression is used for the calculations of the collisional quenching and elastic scattering in Rb(nS)-Rb and Rb(nP)-Rb collisions. We use the electron-Rb scattering phase shifts obtained from ab initio close-coupling calculations and their extrapolation, applying the modified effective-range theory. Our results show that the impulse approximation agrees with the experimental data for the quenching cross sections at n>20 but underestimates the quenching cross sections at lower n. The position of the maximum of the cross section is in good agreement with experimental data. We also obtain reasonable results for the elastic cross section at n>20, but for the lower n the results are too large. The interference effect plays a minor role in the process, which means that simultaneous electron-perturber and ion-perturber interaction is unlikely.

One-electron and two-electron capture in slow ${\mathrm{B}}^{4+}$-He and ${\mathrm{Be}}^{4+}$-He collisions are studied within the semiclassical close-coupling description with two-electron atomic basis sets. For ${\mathrm{B}}^{4+}$-He collisions, we find large two-electron capture cross sections to 2l2l’ doubly excited projectile states. Double-capture cross sections to 2${\mathit{s}}^2$ ${}_{}{}^1{}_{}{}^{}$${\mathit{S}}^{\mathit{e}}$, 2s2p ${}_{}{}^1{}_{}{}^{}$${\mathit{P}}^{\mathit{o}}$, and 2${\mathit{p}}^2$ ${}_{}{}^1{}_{}{}^{}$${\mathit{D}}^{\mathit{e}}$ states and the relative M distributions are found to be in good agreement with recent experiments. Earlier data on ${\mathrm{B}}^{3+}$ production are found to contain large portions of double transfer followed by autoionization. For the ${\mathrm{Be}}^{4+}$-He system, the summed cross sections from this work agree with an earlier theoretical study within the adiabatic model. Discrepancies remain, however, for this system in the distribution of double transfer over final states.

The production of binary-encounter electrons in heavy-ion–atom collisions at high impact velocities is studied. The theoretical model used is the continuum-distorted-wave–eikonal-initial-state approximation. Deviations from classical predictions for the height and position of the associated peak in double-differential cross sections are determined. A simple adiabatic resonant-tunneling model is developed to interpret the physical meaning of the shift in the position of the binary-encounter peak.

The Sternglass theory [Sternglass, Phys. Rev. 108, 1 (1957)] for fast-ion-induced secondary-electron emission from metals has been modified to predict the secondary-electron yield from metals impacted by energetic (several keV to about 200 keV) electrons. The primary modification of the theory accounts for the contribution of the backscattered electrons to the production of secondary electrons based on a knowledge of the backscattered-electron energy distribution. The modified theory is in reasonable agreement with recent experimental data from gold targets in the 6–30-keV electron energy range.

Room-temperature mixtures of hydrogen and deuterium in the fluid and solid states have been studied to 60.5 GPa (605 kbar). Raman-scattering measurements performed on these mixtures showed substantial deviations in the pressure dependence of the vibrational stretching frequencies of the molecules as compared with spectra from monomolecular hydrogenic samples. Most notably, the maximum in the vibron-frequency shift occurs at higher pressures. These observations are explained in terms of the coupling interaction between identical molecules, the same interaction that governs the ${\mathit{Q}}_1$(0) and ${\mathit{Q}}_1$(1) frequency shifts in ortho-para hydrogen mixtures.

Modeling an atomic excited state as a simple charged dipole oscillator immersed in a random (zero-point) radiation, we discuss the effects of two metallic plates on the properties of a microscopic system. The spectral distribution of the zero-point electromagnetic field, characteristic of stochastic electrodynamics, and the rate of emission of the oscillator are modified by the boundaries of the cavity. As a result, the lifetime of the oscillator excited states are different from the free-space values. A comparison with recent experimental results [W. Jhe et al., Phys. Rev. Lett. 58, 666 (1987)] exhibiting suppression of spontaneous decay of excited Cs atoms shows a good agreement with our simplified model calculation.

Using tunable far-infrared spectrometers with high-frequency stability and accuracy, we have measured the self-pressure broadening and shift of ${\mathrm{CH}}_3$CN. We obtained clear evidence of absorber-perturber resonance effects on the collisional line shape. This tests the theoretical model and its possible improvements and also allows us to make broadening and shift predictions for a large class of molecules. Moreover, the resonance effect produces a theoretical temperature dependence of self-broadening that is very different from what is commonly assumed.

The pump-and-probe method of laser spectroscopy in conjunction with a jet-expansion molecular beam was used to record the excitation spectrum arising from E1(6 ${}_{}{}^3{}_{}{}^{}$${\mathit{S}}_1$)←A$0^{+}$(5 ${}_{}{}^3{}_{}{}^{}$${\mathit{P}}_1$) vibronic transitions in the CdAr excimer. Two lasers were employed for the two-step excitation. A Nd:YAG (where YAG denotes yttrium aluminum garnet) laser-pumped dye laser (pump laser) was tuned to the A$0^{+}$(v=5)←X$0^{+}$(v=0) transition, producing a population of the A$0^{+}$ state that was subsequently excited with a ${\mathrm{N}}_2$ laser-pumped dye laser (probe laser) to the E1(6 ${}_{}{}^3{}_{}{}^{}$${\mathit{S}}_1$) state. The probe laser was scanned in the range 4735–5060 Å, producing a vibronic excitation spectrum that consisted of about 20 components. The spectrum was subjected to vibrational analysis, and the assignments of the components were made with the aid of a computer-simulated spectrum based on Franck-Condon factors calculated assuming the Morse potential as representative of the upper and lower states. The analysis yielded ${\mathit{D}}_{\mathit{e}}$, ${\mathrm{\omega }}_{\mathit{e}}$, ${\mathrm{\omega }}_{\mathit{e}}$${\mathit{x}}_{\mathit{e}}$, and ${\mathit{r}}_{\mathit{e}}$ for the E1 state. The potential-energy curve for the E1 state is compared with a theoretical curve calculated by Czuchaj and Sienkiewicz [J. Phys. B 17, 225 (1984)] and by Czuchaj, Stoll, and Preuss [J. Phys. B 20, 1487 (1986)].

Collinear fast-beam laser spectroscopy has been used to measure the hyperfine splittings and the isotope shifts in the 4${\mathit{p}}^4$4d ${}_{}{}^4{}_{}{}^{}$${\mathit{D}}_{7/2}$–4${\mathit{p}}^4$5p ${}_{}{}^4{}_{}{}^{}$P${\mathrm{°}}_{5/2}$ (λ=729 nm) transition of singly ionized Kr. The measured isotope shifts are very large due to a large specific-mass effect that is caused by the strong momentum correlations of the 4d electron with the core electrons. Hartree-Fock and multiconfiguration Hartree-Fock (MCHF) calculations of the specific-mass shift were carried out and are compared to the measurements. The MCHF calculation yields excellent agreement with the experimental results.

We argue that it is possible to bind a neutral atom in stable orbits around a wire charged by a time-varying sinusoidal voltage. Both classical and quantum-mechanical theories for this system are discussed, and a unified approach to the Kapitza picture of effective potentials associated with high-frequency fields is presented. It appears that cavities and waveguides for neutral-atomic-matter waves may be fashioned from these considerations.

An analysis of the Hamiltonian of a molecule interacting with time-dependent external fields leads to the identification of various time scales governing the motion of the electrons and the nuclei. The most important role is played by the Born-Oppenheimer approximation, which considers the nuclear kinetic energy to be bounded and small compared to the gradient of the electronic-potential surfaces. The relative electronic-excitation operator is defined and is shown to be slowly dependent on time. This is a consequence of the Born-Oppenheimer approximation in the context of the time-dependent Schrödinger equation and the interaction with an external electric field. We show how to capitalize on this identified slow motion in the presence of rapid Rabi oscillations driven by the external field. The stroboscopic-integration technique is used in an illustrative calculation for the case of two electronic states coupled by the external field.

We describe a configuration for a linear Paul rf ion trap. This trap can store a long string of ions with a small second-order Doppler shift, comparable to that achieved with a single ion in a quadrupole Paul trap. Crystallized strings of trapped ions, as well as more complicated structures, have been observed in the trap. We report an observation of the 40.5-GHz ground-state hyperfine interval of ${}_{}{}^{199}{}_{}{}^{}\mathrm{Hg}_{}^{+}{}_{}{}^{}$ by microwave-optical double-resonance spectroscopy and discuss prospects for a microwave frequency standard based on a trapped string of ions.

Electron-impact excitation of the hydrogen metastable 2s state to the 3s, 4s, 3d, and 4d states is studied in the presence of a resonant laser field using the rotating-wave approximation. The frequency of the circularly polarized laser field is chosen to match the 2s-3p transition frequency in the hydrogen atom. The role of the laser field in individual magnetic substate (${\mathit{m}}_{\mathit{f}}$=0,±1,±2) excitation is analyzed. It is found that for these optically forbidden transitions (s-s, s-d), a significant increase in the excitation cross sections is obtained through joint collisions with electrons and photons.

We investigate the effect of a high-intensity laser on the spin-dependent energy of a hydrogen atom. We use the high-frequency approximation for arbitrary polarization of the laser field. Spin is an intrinsically relativistic effect so we use our relativistic generalization of this method. We find a spin-dependent term which can dominate the usual spin-orbit form in many cases. The two terms are discussed in detail for both linear and circular polarization. We find that in general the effect of the laser is to suppress the spin-dependent energy splittings significantly.

We describe a theoretical approach to laser cooling with broadband real Gaussian laser fields that is based on adiabatic elimination methods and the Fokker-Planck equation for the Wigner function. This approach is used to model the recent broadband cooling experiment of Zhu, Oates, and Hall [Phys. Rev. Lett. 67, 46 (1991)] and gives correct order-of-magnitude results. The theory is then applied to J=0→J=1 and J=1→J=2 transitions in a ${\mathrm{\sigma }}_{+\mathrm{-}}$${\mathrm{\sigma }}_{\mathrm{-}}$ optical molasses configuration with correlated broadband laser fields. An instability in the cooling scheme at zero velocity is found for red laser detunings, and sub-Doppler-width velocity distributions are predicted for J=1→J=2 transitions with both red and blue laser detunings.

We have derived reduced-density-matrix equations of motion for a pair of two identical atoms moving in the radiation field as the first step in establishing a theory of collisional redistribution of light from neutral-atom traps. We use the Zwanzig projection-operator technique to average over spontaneous field modes and establish the conditions under which Born and Markov approximations can be applied to the system of moving atoms. It follows from these considerations that when these conditions hold, the reduced-density-matrix equation for moving atoms has the same form as that for the stationary case: time dependence is introduced into the decay rates and interaction potentials by making the substitution R=R(t).