The vacuum-polarization correction (given by the expectation value of the Uehling potential) to the energies of the S states of muonic molecules is calculated using very accurate wave functions. The present results indicate that the values accepted so far in the literature for this correction had been underestimated by a factor of 2–8 (depending on the state of interest) due to insufficient quality of the wave functions used. Expectation values of the interparticle distances, the distances squared, and the inverses of the distances are presented.

The molecular expansion method within the framework of the semiclassical approximation is applied to quenching of low-lying excited states of Rydberg atoms colliding with ground-state He at thermal energies. Interactions between the colliding atoms are accounted for in terms of pseudopotentials, and their relative motion is described by a classical linear trajectory. A fairly large basis set of Slater-type orbitals is used to obtain molecular eigenstates, and subsequently a 14-state close-coupling calculation is performed to evaluate the total quenching cross sections and the contributions of individual transitions. The energy dependence of the calculated cross sections is investigated, and possible mechanisms responsible for individual transitions are explained. Finally, the collision rates are calculated and compared with experimental results.

Electron-impact ionization cross sections for ${\mathrm{U}}^{16+}$ and ${\mathrm{U}}^{82+}$ are calculated in the distorted-wave approximation. Excitation-autoionization contributions are found to enhance the direct ionization cross section for ${\mathrm{U}}^{16+}$ by a factor of 3 in the threshold energy region. Excellent agreement is found between the calculated total ionization cross section for ${\mathrm{U}}^{16+}$ and recent crossed-beams experimental measurements. Only the direct ionization cross section for ${\mathrm{U}}^{82+}$ is calculated since excitation-autoionization contributions are assumed to be small due to strong radiation field damping.

The $Z_1^2$-dependent (Bethe) stopping power of metallic Fe, Co, Ni, Cu, and Zn targets for protons in the energy range 2 keV<$E_p$<100 MeV is calculated using the kinetic theory of stopping. The necessary electron velocity distributions are obtained from measured solid-state Compton profiles. The mean excitation energies are obtained by matching the calculated stopping curves to experimental data in the projectile energy range 1 MeV<$E_p$<30 MeV, a range where the projectile velocity is too large for deviations from Bethe stopping to be important, and too small for relativistic corrections to be unimportant. The results obtained in this manner yield stopping curves which agree quantitatively with experimental measurements in the whole projectile energy region where Bethe theory should be valid, and which behave in the expected manner outside this region. We demonstrate that the decomposition of the mean excitation energy into orbital contributions is important for determination of the stopping cross section. We conclude that assigning different orbital mean excitation energies to different shells leads to rather large modifications of the commonly accepted total mean excitation energy.

The spectra of Mg Kα x rays, emitted in collisions of 33-MeV Mg ions with noble-gas atoms, have been measured using a curved-crystal spectrometer. Quenching of the metastable 1s2p ${}_{}{}^3{}_{}{}^{}P$ state of He-like Mg and the 1s2l2p ${}_{}{}^4{}_{}{}^{}P$ states of Li-like Mg caused the intensities of x rays from these states to decrease, relative to the intensities of x rays from the promptly decaying ${}_{}{}^1{}_{}{}^{}\mathrm{P}$ and ${}_{}{}^2{}_{}{}^{}\mathrm{P}$ states as the gas pressure increased. Quenching cross sections were determined by fitting the observed pressure dependence of the x-ray intensity ratios to a function derived from the rate equation for the production and decay of these states, with additional terms to account for contributions from prompt spectator-electron transitions. The quenching cross sections for the ${}_{}{}^3{}_{}{}^{}\mathrm{P}$ state compare favorably with outer-shell electron-capture cross sections estimated from semiempirical scaling rules and recoil time-of-flight measurements.

A method to evaluate thin-film thickness inhomogeneities is presented. It is based on the measurements of the first two moments of the energy-loss distributions of swift ions traversing thin foils at two different beam incidence angles. We apply the method to a set of thin (∼200-Å) aluminum foils, resolving thickness fluctuations of the order of 10%.

The 3d-4f and 3d-5f emission patterns occurring between 4.5 and 6.5 Å in the spectra of laser-produced plasmas of highly ionized tantalum are studied, and the pseudocontinuum which underlies the former is interpreted. For the transition subarrays suspected to be responsible for the pseudocontinuum, the average wavelengths and widths are computed ab initio in the formalism of spin-orbit-split arrays; these subarrays are proven to cover a continuous band of wavelengths. For their intensities, the dielectronic recombination (DR) process populates efficiently the upper configurations, which, in most cases, lie above the ionization levels. The autoionization and radiative-transition probabilities, and the DR coefficients, are computed ab initio, in the form of averages for whole configurations or subconfigurations of Cu-like tantalum. The compact formulas for these averages are valid in intermediate coupling, avoiding any diagonalization. All the needed radial integrals are computed by means of a relativistic parametric potential central-field code.

Experimental studies of the quasibound Stark spectrum of cesium are reported with emphasis on the Stark redistribution of nonhydrogenic states onto the linear Stark manifold. We show that most characteristics of the spectra are understood with analytical models describing the interaction of one or several discrete states with one or several quasicontinua of discrete states. In particular, we give experimental evidence for the discrete versions of Fano profiles and stabilization of states due to interference effects. These are also important features of the continuum region which several experiments have recently demonstrated.

A study of the energy shift of x-ray Kα satellites relative to the Kα diagram lines for atoms having 10≤Z≤32 is presented. Nonrelativistic Hartree-Fock (HF) total energies are employed, as well as the hydrogenic analytic model of Burch et al. [Phys. Rev. A 9, 1007 (1974)] using both HF-derived and Slater-rule screening constants. The results are compared with an extensive body of experimentally derived ion-, electron-, and photon-excited shifts. The HF results are found to increasingly underestimate the measured shifts for Z≥17, while the analytic model using HF screening constants overestimates them uniformly by about 2.5 eV for almost all Z. Excellent agreement is found for the very recent self-consistent-field extension of Burch’s model by Bhattacharya et al. [Phys. Rev. A 37, 3162 (1988)]. A way of improving agreement between HF calculations and experiment is suggested.

Using ${}_{}{}^{201}{}_{}{}^{}\mathrm{Hg}$ (I=(3/2) atoms, we have demonstrated a new technique for studying the static quadrupole interactions of nuclei in ${}_{}{}^1{}_{}{}^{}\mathrm{S}_0^{}{}_{}{}^{}$ atoms. The nuclear-spin precession frequency is made sensitive to quadrupole energy shifts by inducing, through optical pumping, both dipole and quadrupole spin polarization in a vapor of ${}_{}{}^{201}{}_{}{}^{}\mathrm{Hg}$ atoms. The sensitivity predicted from measurements of the polarization agrees with the calibrated value found by inducing a known quadrupole light shift. By rotating the mercury-vapor cell, quadrupole interactions with the cell walls were observed with the expected cos2φ azimuthal variation. In an application of this technique, a search for possible dependence of the ${}_{}{}^{201}{}_{}{}^{}\mathrm{Hg}$ spin precession frequency on the orientation of the precession axis in space has yielded the null result Δf<5×${10}^{\mathrm{-}7}$ Hz, which reduces the previous limits on spatial anisotropy by over three orders of magnitude and places stringent new bounds on violations of Lorentz invariance.

We apply multichannel-quantum-defect theory (MQDT) to multiphoton ionization of rare gases in lowest-order perturbation theory. We determine MQDT parameters of J=1,3 odd-parity states, J=0,2 even-parity states in xenon and krypton by fitting experimental energy levels and constructing Lu-Fano plots. We present calculations of two- and three-photon ionization of xenon and krypton in the autoionization region between the two thresholds. We compare autoionization spectra and also photoelectron angular distributions to experimental data [S. T. Pratt, P. M. Dehmer, and J. L. Dehmer, Phys. Rev. A 35, 3793 (1986); J. L. Dehmer, S. T. Pratt, and P. M. Dehmer, ibid. $fat 36—, 4494 (1987)]. Agreement between theory and experiment is reasonable, especially for krypton.

We report the observation of an anticrossing in solid-state laser spectroscopy produced by cross relaxation. Spin-spin cross relaxation between the ${}_{}{}^{141}{}_{}{}^{}\mathrm{-}$ and ${}_{}{}^{19}{}_{}{}^{}\mathrm{spin}$ reservoirs in ${\mathrm{Pr}}^{3+}$:${\mathrm{LaF}}_3$ and its influence on the ${}_{}{}^{141}{}_{}{}^{}\mathrm{Pr}$ NMR spectrum is detected by means of optical pumping. The technique employed combines optical pumping and hole burning with either external magnetic field sweep or rf resonance saturation in order to produce slow transient changes in resonant laser transmission. At a certain value of the external Zeeman field, where the energy-level splittings of Pr and F spins match, a level repulsion and discontinuity of the ${\mathrm{Pr}}^{3+}$ NMR lines is observed. This effect is interpreted as the ‘‘anticrossing’’ of the combined Pr-F spin-spin reservoir energy states. The Zeeman-quadrupole-Hamiltonian spectrum of the hyperfine optical ground states of ${\mathrm{Pr}}^{3+}$:${\mathrm{LaF}}_3$ is mapped out over a wide range of Zeeman magnetic fields. A new scheme is proposed for dynamic polarization of nuclei by means of optical pumping, based on resonant cross relaxation between rare spins and spin reservoirs.

The Keldysh approximation as applied to above-threshold ionization (ATI) is compared with conventional (weak-field) perturbation theory and the Kroll-Watson theory of electron scattering in intense fields. Unlike the latter ‘‘nonperturbative’’ theory, the Keldysh approximation treats the effects of the field on the initial and final states differently, and this leads to a spurious contribution from the ${\mathrm{A}}^2$ part of the Hamiltonian in the A⋅p gauge. We attempt to clarify the role of the ponderomotive potential in ATI and the influence of the ${\mathrm{A}}^2$ term on the polarization dependence of the transition amplitude. The fundamental defect of the Keldysh approximation with regard to ${\mathrm{A}}^2$ is shown to be of little practical consequence for the experiments carried out thus far. However, it is shown that the Keldysh amplitude is not ‘‘exact’’ in the applied field, as sometimes claimed, and is best regarded as an ansatz rather than a leading term in a gauge-invariant perturbation expansion. A simple modification of the Keldysh theory is proposed.

The L${\beta }_2$,15 spectra of elements between ${}_{41}{}^{}{}_{}{}^{}\mathrm{Nb}$ and ${}_{51}{}^{}{}_{}{}^{}\mathrm{Sb}$ have been measured at high instrumental resolution from solid samples. The satellite structure on the high-energy side of L${\beta }_2$,15 was resolved and the natural widths of diagram lines were determined. Two different satellite groups due to LM and LN double-hole states were clearly seen. The relative intensity of satellites was examined and it was noticed that the Coster-Kronig channels which create extra holes in the M shell are closed at ${}_{51}{}^{}{}_{}{}^{}\mathrm{Sb}$. A faint structure was observed between the two main groups of satellites after ${}_{48}{}^{}{}_{}{}^{}\mathrm{Cd}$. The $L_3$$O_1$ transition falls in this region but it may also be partly the result of triple-hole states.

The variational R-matrix approach for photoionization has been modified in two ways to treat Rydberg excited states of large radius: (1) the large residual contributions to dipole transition and normalization integrals outside the R-matrix radius are treated analytically, and (2) the amplitude of Rydberg functions on the boundary is considered explicitly. Photoionization cross sections as a function of energy are reported for 2$p^4$ O${(}^3$P) ground-state atomic oxygen and for ${\mathrm{O}}^{+}$${(}^4$S)3p ${}_{}{}^3{}_{}{}^{}\mathrm{P}$ excited-state oxygen and compared with previous work, where available. The excited-state cross section is in excellent agreement with limited experimental data. A Cooper minimum is predicted in the excited-state partial cross section to the ${\mathrm{O}}^{+}$${(}^4$S)εd continuum. A systematic study of the effect of electronic correlation in the configuration-interaction (CI) expansion of the wave function inside the R matrix is reported. The location of the Cooper minimum is the feature that is most sensitive to the CI representation used.

Using fast-ion-beam laser spectroscopy, we have measured the 1s2s ${}_{}{}^1{}_{}{}^{}$${\mathit{S}}_0$–1s2p ${}_{}{}^1{}_{}{}^{}$${\mathit{P}}_1$ interval in Be iii to be 16 276.774(9) ${\mathrm{cm}}^{\mathrm{-}1}$. Frequency calibration was provided by an iodine absorption cell and the iodine atlas of Gerstenkorn and Luc [Atlas du Spectre d’Absorption de la Mole$aa—cule d’Iode: Partie III (Editions du CNRS, Paris, 1978 )]. This work represents an improvement in precision over previous measurements by an order of magnitude and is in excellent agreement with a very recent calculation of Drake [Can. J. Phys. 66, 586 (1988)].

Angle-resolved electron spectroscopy with the help of synchrotron radiation has been used to study resonant Auger processes near the core shells of Ar 2p, Kr 3d, and Xe 4d. Results for the lowest-energy resonances have received special attention: argon (Ar 2$p_{3/2}$→4s) at a photon energy of 244.4 eV, krypton (Kr 3$d_{5/2}$→5p) at 91.2 eV, and xenon (Xe 4$d_{5/2}$→6p) at 65.1 eV. The angular distribution parameters β are evaluated for each of the resolved Auger peaks. Most striking is the occurrence of large negative β values for some of the higher kinetic energy peaks. The results are most apparent under high electron resolution. The theoretical basis for having β values near -1 is discussed. In particular, the experimental results for argon are found to be in general agreement with the prediction of -1 β values by using either angular momentum transfer theory or normal Auger theory. However, a better understanding of the range of β values will have to await explicit calculations. Experimental results on the Auger spectra are also given at lower kinetic energies, for shakeup states, and for higher-energy resonances, especially those involving vacancies in the core shells of the lower spin states. It is shown that, although transitions having the same final state of the singly charged ion frequently have similar relative intensities and β values, occasionally they differ quite markedly. The theoretical consequences of variant behavior for processes having the same final states are also discussed.

A study of the kinetic energy distributions of ionic fragments produced by subpicosecond irradiation of ${\mathrm{N}}_2$ with 248-nm radiation at an intensity of ∼${10}^{16}$ W/${\mathrm{cm}}^2$ is reported. These measurements, in comparison to other findings involving molecular excitation with charged particles and soft x rays, reveal several important features of the nonlinear coupling. Four ionic dissociative channels are identified from the data on the multiphoton process. They are ${\mathrm{N}}_2^{2+}$→${\mathrm{N}}^{+}$+${\mathrm{N}}^{+}$, ${\mathrm{N}}_2^{2+}$→N+${\mathrm{N}}^{2+}$, ${\mathrm{N}}_2^{3+}$→${\mathrm{N}}^{+}$+${\mathrm{N}}^{2+}$, and ${\mathrm{N}}_2^{4+}$→${\mathrm{N}}^{+}$+${\mathrm{N}}^{3+}$, three of which are charge asymmetric. The data for the energy distributions are found to be in approximate conformance with a simple picture involving ionizing transitions occurring within a time of a few cycles of the ultraviolet wave at an internuclear separation close to that of the ground-state (X ${}_{}{}^1{}_{}{}^{}\mathrm{\Sigma }_{\mathit{g}}^{+}{}_{}{}^{}$) molecule. The implication follows that a strong nonlinear mode of coupling is present which causes a high rate of energy transfer. A simple hypothesis is presented which unites the ability for rapid energy transfer with the observed tendency to produce charge-asymmetric dissociation.

Recent theoretical analyses of the single-mode laser with colored-noise pump fluctuations have allowed the detailed prediction of the short-time behavior of the intensity autocorrelations of such a laser. In particular, a rounding off of the intensity correlation function on the time scale of the correlation time of the pump fluctuations has been predicted. We present results from experiments performed on a single-mode dye-laser system that clearly confirm these predictions. Numerical simulations of the laser Langevin equations including the colored-noise pump fluctuations are used to fit the experimental results.

Photoelectron counting sequences for single-atom resonance fluorescence are studied. The distribution of waiting times between photoelectric counts is calculated, and its dependence on driving-field intensity and detection efficiency is discussed. The photoelectron-counting distribution is derived from the waiting-time distribution. The relationship between photoelectron counting sequences and photon emission sequences is discussed and used to obtain an expression for the reduced state of the atom during the waiting times between photoelectric counts. The roles of irreversibility and the observer in atomic state reduction are delineated.