Recent improvements in the determination of the running of the fine-structure constant also allow an update of the hadronic vacuum-polarization contribution to the Lamb shift. We find a shift of $-$3.40(7) kHz to the $1S$ level of hydrogen. We also comment on the contribution of this effect to the determination by elastic electron scattering of the rms radii of nuclei.

General model-independent relationships among radial expectation values of the one-particle densities in position and momentum spaces for any quantum-mechanical system are obtained. They are derived from the Stam uncertainty principle and the recently reported lower bounds to the Fisher information entropy of both densities. The results are usually expressed in terms of some uncertainty products of the system. The accuracy of the bounds is numerically analyzed for neutral atoms within a Hartree-Fock framework.

We report measurements and theoretical calculations for the lifetimes of the ${5d}^{9}6p$ levels in Hg III with $J=0,2,3,4.\mathrm{}$ This is an extension of earlier measurements of the lifetimes of the ${5d}^{9}6p$ levels in Hg III with $J=1,$ and now provides data for all 12 of the levels. The results also provide an isoelectronic comparison with earlier studies of these levels in Au II of the Pt sequence, and a homologous comparison with earlier studies of the ${4d}^{9}5p$ levels in Ag II, Cd III, and In IV in the Pd sequence.

The $L_1-L_{2,3}M$ Coster-Kronig spectrum of argon has been measured by using photon excitation. The intensity ratio of $0.30\pm 0.02$ between the $L_1-L_{2,3}{M}_1$ main lines and the $L_1-L_{2,3}{M}_{2,3}$ lines is extracted from experiment. The experimental results for the ratio and partial decay rates are compared with present and previous theoretical estimates.

We have measured the ratio of the cross sections for double-to-single ionization in helium for several monoenergetic photon energies between 4.5 and 12 keV using time-of-flight ion charge state spectroscopy. In this energy range, both the photoelectric effect and inelastic (Compton) scattering contribute significantly to the total cross section. The ionization states caused by Compton scattering were distinguished from those caused by the photoelectric effect by the different recoil energies of the helium ion associated with the two processes. The ratios of the double-to-single ionization cross sections of helium for the photoelectric effect ${(R}_p)$ and for Compton scattering ${(R}_c)$ are given, and compared with previous measurements and theoretical calculations. The measured value for $R_c$ at 12 keV is $(1.21\mathrm{}\pm 0.27)\%,$ which agrees well with the theoretical calculations of Andersson and Burgdörfer [Phys. Rev. A 50, R2810 (1994)].

In this paper we report experimental measurements of the electron-impact-excitation cross sections out of the $2{}^{3}S$ metastable level of helium into higher excited levels, with emphasis on measurements at high electron energies. The agreement between our metastable results and recent theoretical calculations is comparable to the agreement between experimental and theoretical values for excitation from the ground state.

We report on the Autler-Townes spectra for a two-level atom driven by multiple fields. We investigate resonance phenomena and explain our results in terms of multiply dressed states.

The anomalous behavior of transient nutations is experimentally investigated in a set of two-level $(S=\frac{1}{2})$ spin systems differing only in spin concentration. Our results show that the non-Bloch power dependence of the decay rate of transient nutations is a concentration-dependent effect, which is more and more pronounced in more and more concentrated samples. The experimental results are interpreted in the framework of the recent theory by Shakhmuratov et al. [Phys. Rev. Lett. 79, 2963 (1997)] and support the hypothesis that the anomalous decay of transient nutations in solids originates from radiation-induced changes of the dipolar field, rather than from residual fluctuations of the nominally coherent field.

We present a perturbation solution of a model Bose-Einstein Hamiltonian derived by Bohn, Esry, and Greene. In our solution we use $1/N$ as the perturbation parameter, where N is the number of particles in the condensate. Ground-state energies are reported for parameters approximating the Joint Institute for Laboratory Astrophysics ${}^{87}\mathrm{Rb}$ experiments. We predict the critical number of atoms with negative scattering lengths that can be trapped using the effective trap frequency of the first-order equation. The $\overrightarrow{N}\infty$ perturbation limit, which retains a single term beyond the conventional Thomas-Fermi limit, gives ground-state energies that agree to three digits with converged results, thus providing a much improved limit for large N.

A method for producing macroscopic quantum superposition states (generally known as Schrödinger cat) states for optical fields is presented. The proposed method involves two modes of the field interacting dispersively in a Kerr medium where one of the modes is an arm of a Mach-Zehnder interferometer and the other mode is external to it. If the external mode initially contains a macroscopic quantum state, such as a coherent state, and the vacuum and a single photon state are the inputs to the interferometer, the external field state becomes entangled with the number states associated with the two paths of the interferometer. Selective measurement at the output ports of the interferometer project the external mode into the desired cat states. It is pointed out that the method can also be used to generate cat states out of multimode states initially containing correlations.

We object to the proposal that the mapping of the three-dimensional hydrogen atom into a four-dimensional harmonic oscillator can be readily used to determine the Wigner phase-space distribution function for the hydrogen atom.

The Lifshitz formalism for determining the attractive force between material bodies with generalized electromagnetic susceptibility is applied numerically to gold, copper, and aluminum. The deviation from the perfect conductivity Casimir force approximately agrees with a first-order plasma model calculation at large separation, but at separations corresponding to the plasma frequency, deviations of over 50% from the first-order model are found, while deviations from corrections up to second order are about 10%. These results are discussed in the context of recent measurements of the Casimir force.

Two-photon detachment of the negative hydrogen ion, ${\mathrm{H}}^{-},$ has been performed in the vicinity of the threshold for one-photon detachment. The specific threshold behavior of negative ions, which results in a suppression of the one-photon process just above the threshold, is utilized to enhance the probability for observation of nonresonant two-photon excess-photon detachment of ${\mathrm{H}}^{-}.$ It is found that the electron angular distribution for the two-photon process at threshold closely resembles that of a pure d wave. The general application of an imaging electron spectrometer for ion-beam studies is briefly discussed.

We report triple-coincidence measurements of Auger electrons, scattered projectile, and target recoil ions in slow collisions of highly charged ions with many-electron atoms. Subpartial Auger-electron spectra corresponding to specific final projectile and recoil-ion charge states have been obtained by means of time-of-flight and position imaging techniques for the 60-keV ${\mathrm{O}}^{7+}+\mathrm{Ar}$ collision system. The spectra exhibit marked differences and provide insights into the production and relaxation pathways of multiply excited states populated during the collisions.

A deeper understanding of the physics and chemistry of clusters has been constrained by the inability to observe experimentally the structure and structural variations for size selected clusters. We report here measurements of trapped ion electron diffraction, a technique that presents the possibility to directly observe the evolution of cluster structure with size and temperature. The results of electron diffraction from ${\mathrm{C}}_{60}^{+}$ ions stored in a radio-frequency Paul trap are reported.

We have examined the dynamics of noble-gas clusters, heated with high-intensity laser radiation, using pump-probe experiments to temporally resolve the expansion of the clusters. Absorption of the probe radiation is observed to reach a maximum for a particular time delay between pump and probe, dependent on the cluster size. For single-pulse experiments, we find that there is an optimal pulse width to maximize absorption for a given cluster size. Model calculations suggest that these effects are due to resonant heating of the spherical cluster plasma in full support of a hydrodynamic interpretation of cluster interactions.

Using circular and linear laser polarization for strong-field dissociative ionization we show that iodine molecules are rotationally fixed by their own moment of inertia during the dissociative ionization with a 80-fs pulse. What seems to be alignment of the fragments is created by an enhancement of the ionization rate, which is strongly dependent on the angle between the molecular axis and the laser electric-field vector. By the introduced method we can measure the degree of alignment for ${\mathrm{I}}_2,\hspace{0.5em}{\mathrm{Br}}_2,\hspace{0.5em}{\mathrm{Cl}}_2,$ and ${\mathrm{O}}_2$ and independently estimate the enhancement of the ionization rate for ${\mathrm{I}}_2.$

A multidimensional semiclassical description of excitation of a Rydberg electron by half-cycle pulses is developed and applied to the study of energy- and angle-resolved ionization spectra. Characteristic phenomena observable in these spectra such as interference oscillations and semiclassical glory and rainbow scattering are discussed and related to the underlying classical dynamics of the Rydberg electron. Modifications to the predictions of the impulse approximation are examined that arise due to finite pulse durations.

A simultaneous observation of the optical-clock ${}^2{S}_{1/2}{-}^2{D}_{5/2}$ transition at 674 nm, in two separately trapped single ${}^{88}{\mathrm{Sr}}^{+}$ ions, is reported. Two nominally identical miniature rf Paul traps were used together with a 674-nm sideband-injection-locked extended-cavity diode laser. This “slave” laser was optically phase locked about 650 MHz away from a “master” diode laser, which was itself locked to a high-finesse, ultra-low-expansion (ULE) cavity. The ULE cavity was temperature-stabilized and suspended in an evacuated enclosure, and provided an “optical flywheel” reference standard with a relative drift rate of better than 1 part in ${10}^{11}$ per hour. The difference between center frequencies of the single ${}^{88}{\mathrm{Sr}}^{+}$-ion 674-nm (445-THz) transition multiplet in two traps was 120(90) Hz (one standard uncertainty). Thus the two trap center frequencies agree to 3 parts in ${10}^{13}.$

In a recent experiment for ionization of the neutral Xe atom with a femtosecond Ti:sapphire laser pulse, large yields of high charge states up to sixfold ionizations have been observed. Here, we present a dynamical model of laser-induced nonsequential multiple ionization of a complex atom. The calculated results for the ion yields are found to agree remarkably well for the six charge states measured. Analyses show a significant presence of nonsequential (or direct) escapes of up to four electrons in the experimental data. For future experimental references, predictions for the sevenfold and eightfold ionizations are also made.