Low-energy electron spectra are reported for 60- and 100-keV multicharged ions interacting at an incident angle of 20° with Au and Cu surfaces. Analysis of the spectra indicate that at least two features contribute. The first feature represents the major contribution to the total electron yield and consists of 5–10-eV electrons emitted over a wide range of angles. The angular distribution of this component is not symmetric with respect to the surface normal, but shows an increase in the forward direction of the incident ions. It will be shown that this component arises predominantly from below the surface. Possible potential-emission mechanisms which may contribute will be discussed. The second feature, which constitutes a minor part of the overall electron emission, occurs at higher electron energies (∼20 eV), and is peaked at the extreme forward angles. Binary encounters between incident ions and metal electrons at the surface-vacuum interface will be shown to describe the main features of this component. At even higher electron energies (>40 eV) the spectra show a tail whose slope does not depend on either the initial charge state or kinetic energy of the incident ions. The invariance with kinetic energy is in sharp contrast with the corresponding experimental results from ion-atom collisions.

The quantum theory for channeling radiation from mega-electron-volt electrons is extended to include higher-order corrections from phonon scattering. The formalism developed to describe radiation line profiles may be applied to any radiating system exposed to external perturbations. For channeling radiation, it is shown that at electron energies above a few mega-electron-volts, the dominant correction to line energies is of third order in the strength of the interaction giving rise to phonon scattering. Detailed analytical expressions as well as simple estimates are derived for both the second- and third-order thermal corrections. Their magnitude, relative to line energies, is typically of the order of ${10}^{\mathrm{-}3}$ or less. Corrections due to the periodic structure of axial and planar potentials are shown to be of the same order or smaller. Hence the present investigation results in a firm theoretical foundation for channeling-radiation spectroscopy which may then be used as a valuable probe for solid-state parameters, potentially in the study of vibrational anomalies in high-${\mathit{T}}_{\mathit{c}}$ superconductors. For electrons of a few mega-electron-volts, a shift due to photon refraction is important which previously has been included in an ad hocc way. The formalism contains this effect and the semiclassical formula for photon refraction in a free-electron gas is rederived within a fully-quantum-mechanical framework.

The K-shell photoabsorption spectrum of C ii has been obtained with a technique using two laser-produced plasmas. Both the discrete spectrum as well as the photoionization continuum have been observed. Precise wavelength measurements and atomic structure calculations have been performed. The discrete lines have been identified and correspond to transitions of the 1s inner electron starting from the ground or low excited levels. The 1${\mathit{s}}^2$2${\mathit{s}}^2$2p-1s2${\mathit{s}}^2$2pnp series has been observed up to n=6. The limits were estimated as 2 549 000 ${\mathrm{cm}}^{\mathrm{-}1}$ (316.0 eV) and 2 570 000 ${\mathrm{cm}}^{\mathrm{-}1}$ (318.6 eV) and checked against the corresponding C iii levels. The relative photoionization cross section from the threshold, at about 318 eV, up to 500 eV, has also been measured.

In a recent laser spectroscopic study of the ${\mathrm{H}}_2$, ${\mathrm{D}}_2$, and HD molecules [J. M. Gilligan and E. E. Eyler, Phys. Rev. A 46, 3676 (1992)], the energies of transitions between the X ${}_{}{}^1{}_{}{}^{}\mathrm{\Sigma }_{\mathrm{g}}^{+}{}_{}{}^{}$ ground state and the metastable EF ${}_{}{}^1{}_{}{}^{}\mathrm{\Sigma }_{\mathit{g}}^{+}{}_{}{}^{}$ state were measured precisely with respect to seven transitions in the iodine molecule. We report the absolute wavelengths of these iodine transitions with an accuracy of 250 kHz (5 parts in ${10}^{10}$) by interferometric comparison with the iodine-stabilized He-Ne standard at 633 nm. The full accuracy of the transitions to the EF state can now be realized. Improved ionization potentials for ${\mathrm{H}}_2$, ${\mathrm{D}}_2$, and HD are obtained using these results in combination with other previous measurements connecting the EF state to high Rydberg states. These ionization potentials are generally, but not completely, in agreement with ab initio theory. We also compare our measurements of the iodine wavelengths with less accurate values predicted from previously determined empirical molecular constants of the iodine B←X transition. We discuss the possibility of determining these constants more accurately and thereby improving the overall accuracy with which the widely used visible and near-infrared spectrum of ${\mathrm{I}}_2$ is known.

An anharmonic-potential–effective-charge approach for computing relative Raman intensities of a gas is developed. The equations of motion are set up and solved for the driven anharmonic molecular vibrations. An explicit expression for the differential polarizability tensor is derived and its properties discussed. This expression is then used within the context of Placzek’s theory [Handbuch der Radiologie (Akademische Verlagsgesellschaft, Leipzig, 1934), Vol. VI] to compute the Raman cross section and depolarization ratio of a gas. The computation is carried out for the small molecules ${\mathrm{CO}}_2$, ${\mathrm{CS}}_2$, ${\mathrm{SO}}_2$, and ${\mathrm{CCl}}_4$; results are compared with experimental measurements and discussed.

Two radio-frequency transitions in the n=27 manifold of molecular hydrogen have been observed, where the vibrational quantum number ν=0 and the rotational quantum number R=2. For each transition, one of the two well-resolved hyperfine components appears to autoionize, creating a significantly broadened line profile for that one component. This behavior is explained by including a spin Hamiltonian which mixes a nearby autoionizing state into an otherwise stable state. This analysis is presented; the experimental measurements agree with the calculation.

In previous work [Phys. Rev. A 40, 6931 (1989)] the interaction-induced dipole moments of ${\mathrm{H}}_2$ pairs have been obtained by treating the complex of the two molecules like one molecule in the self-consistent-field and size-consistent, coupled-electron pair approximations; from this dipole surface, the binary collision-induced absorption spectra have also been computed for the rototranslational and the fundamental band. In the present work, the radial transition matrix elements of the induced dipole components are obtained for the first overtone band of ${\mathrm{H}}_2$ at 1.2 μm. Two cases are here considered: ${\mathit{v}}_1$=0→0, ${\mathit{v}}_2$=0→2 (‘‘single’’ vibrational transition) and ${\mathit{v}}_1$=0->1, ${\mathit{v}}_2$=0→1 (‘‘double’’ transition), where the ${\mathit{v}}_{\mathit{i}}$ are the vibrational quantum numbers of two interacting ${\mathrm{H}}_2$ molecules (i=1 or 2). The dependence of these dipole elements on the most important initial and final rotational states (j=0,. . .,3) is also evaluated. From these results, the spectral profiles of the collision-induced absorption of molecular hydrogen pairs in the infrared 1.2-μm (the first ${\mathrm{H}}_2$ overtone) band are obtained. The calculations are based on a proven isotropic potential model which we have extended to account for the effects of vibrational excitations. The comparison of the computated spectra with the measurements available at temperatures from 24 to 300 K shows agreement within the estimated uncertainties of the best measurements (≊10%). This fact suggests that theory is capable of predicting these spectra reliably at temperatures for which no measurements exist, with an accuracy that compares favorably with that of good laboratory measurements.

In this work, measurements of the Kα line from pure samples (Z=20, 22, and 23), using synchrotron radiation of different energies, were carried out. Experimental evidence of the presence of double K holes and their effects on x-ray-fluorescence spectra is reported. The influence of multielectron transitions in calculations related to spectrochemical analyses is discussed.

The ${\mathrm{H}}_2$${\mathrm{-}}^4$He dimer and small ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ clusters are studied using Monte Carlo sampling techniques. We consider alternative wave-function forms in order to obtain high accuracy efficiently. For the smaller systems, both guided and unguided Metropolis walks are used and the effciencies are studied. Of particular concern is accurate sampling at small particle separations and the behavior of the local energy in this regime. As a final step, we compute exact energies by a diffusion Monte Carlo method. We obtain converged energies significantly below the Green’s-function Monte Carlo values, which employed an earlier He-He potential with a slightly shallower well. For ${\mathrm{He}}_3$ and ${\mathrm{He}}_{20}$, the Green’s-function Monte Carlo energies are reproduced when employing the same potential. However, for the 112-atom cluster, our converged energy lies below the Green’s-function Monte Carlo value. Second-order estimates of the exact density profiles and particle separation distributions, p, are also determined. For the 14- and 20-atom clusters, second-order estimates of p show enhanced structure in comparison to variational Monte Carlo results. Statistically meaningful oscillations in the second-order estimates of the exact density profiles are not observed.

A fast and accurate computer code has been developed to solve the Schrödinger equation for the one-dimensional hydrogen atom in an external spatially uniform, time-dependent electric field. Many calculations that previously required hours on a supercomputer can be done in minutes on a workstation. It has been applied to a variety of problems, starting with a system prepared in a hydrogenic eigenstate. In a weak field, for sufficiently short times, the Fermi golden rule has been verified. In very strong fields, the ionization probability oscillates in time, but the mean value increases. Ionization suppression is present only in the averaged sense and with a sudden turnon. For the case of above-threshold ionization, we see ponderomotive shifts and multiphoton resonance peaks in the continuous spectrum. Gauge independence is proved analytically and verified by numerical computations. It is demonstrated that prior numerical work was marred by problems of numerical accuracy and consistency of approximation.

We study the magnetic-field dependence of the cross sections for elastic and inelastic collisions of pairs of ultracold cesium atoms in a magnetic trap, calculated with the coupled-channels method. We pay special attention to atoms in the f=3, ${\mathit{m}}_{\mathit{f}}$=-3 weak-field seeking state of the lower hyperfine manifold. The cross sections show a pronounced resonance structure. We discuss its origin, starting from the pure bound singlet and triplet rovibrational ${\mathrm{Cs}}_2$ states and introducing perturbations due to the hyperfine and Zeeman interactions. We also discuss the role of the centrifugal barrier in the final collision channel in reducing the loss of atoms from the trap due to transitions induced by the magnetic dipole-dipole interaction.

The dynamics of a highly excited Morse oscillator in an intense femtosecond laser field are evaluated using the Heisenberg uncertainty time-energy principle. The resulting expressions provide a unique definition of dissociation times as directly measured in Zewail’s femtosecond transition-state experiments. Our method enables us to determine the uncertainties natural to the dynamics of the chemical bond, especially in the falling potential-energy surface at the different interfragment separations. Our methodology is applied to the femtosecond transition-state spectroscopy of the I-CN photodissociation.

Theoretical calculations are presented for the probe absorption spectra of atoms having magnetically degenerate ground states which are subjected to weak pump fields that produce spatial polarization gradients. Apart from the Raman resonance contribution to the spectrum which has been observed experimentally, we predict that additional resonant structures should appear as a result of the atomic recoil that occurs during the absorption or emission of radiation by the atoms. The width of the atomic recoil resonances is directly related to the Doppler width associated with the driven transition. For sub-Doppler cooled atoms, the width can be much narrower than the optical pumping rate that determines the width of the Raman signal.

Applying a method based on some results due to Kato [Proc. Phys. Soc. Jpn. 4, 334 (1949)], we show that series of Rydberg eigenvalues and Rydberg eigenfunctions of hydrogen in a uniform magnetic field can be calculated with a rigorous error estimate. The efficiency of the method decreases as the eigenvalue density increases and as γ${\mathit{n}}^3$→1, where γ is the magnetic-field strength in units of 2.35×${10}^9$ G and n is the principal quantum number of the unperturbed hydrogenic manifold from which the diamagnetic Rydberg states evolve. Fixing γ at the laboratory value 2×${10}^{\mathrm{-}5}$ and confining our calculations to the region γ${\mathit{n}}^3$<1 (weak-field regime), we obtain extremely accurate results up to states corresponding to the n=32 manifold.

We have studied the Stark effect on high singlet Rydberg states of para-${\mathrm{H}}_2$ which converge to the ${\mathit{v}}^{+}$=0, ${\mathit{N}}^{+}$=0,2 ionization limits using multiphoton excitation and ionization, through monitoring the predissociative decay channel. We observe the formation of hydrogenic Stark manifolds with increasing field, as well as the appearance of forbidden transitions and interactions between states possessing different core rotational quantum numbers. Comparison of our experimental results with a perturbation calculation suggests that the predissociation rates of the Stark components are determined primarily by the degree of admixture of ns and nd character in their wave functions. Directly field-induced predissociation of the Stark states may also play a role in their decay.

We have studied theoretically and experimentally the effect of a relatively strong magnetic field on sub-Doppler laser cooling in a one-dimensional optical molasses. We used the operator description of laser cooling with the Larmor precession frequency ${\mathrm{\omega }}_{\mathit{Z}}$ being much higher than the optical pumping rate. We found velocity-selective resonances (VSR) in the force at velocities ${\mathit{v}}_{\mathit{r}}$=n${\mathrm{\omega }}_{\mathit{Z}}$, with n=0,±1,±2 for both the scattering and redistribution force operators. These depend on the relative direction of the magnetic field and the polarization vectors of the light beams. Analytical results for the force on the atom are obtained in two cases that illustrate the effect of the VSR on the force. These formulas are compared with numerical calculations of the force. We also discovered a redistribution mechanism that relies on the gradient of the eigenstates of the light-shift operator, with eigenvalues that are independent of position so that a ‘‘Sisyphus cooling’’ picture does not apply. The theory is compared with many experimental results and excellent agreement is found. We believe that all essential features of laser cooling at low intensity are well described by this operator theory.

We have developed an alternative method for the analysis of pulse statistics in modulated systems based on a generalization of the quasideterministic approach. With this method the stationary properties of pulses are easily obtained via the solution of an integral equation for the probability density of the passage time by a certain threshold. In general, this equation must be solved by numerical methods. However, analytical expressions can also be obtained in limiting cases. We have compared this method with simulations in a gas laser with on-off modulation (Q switching), obtaining an excellent agreement in all situations.

The influence of the high-pump-field intensity saturation and dynamic Stark effects on the resonant nearly degenerate four-wave-mixing signal (${\mathrm{\omega }}_3$=2${\mathrm{\omega }}_1$-${\mathrm{\omega }}_2$) is studied in a frequency space defined by the pump- and probe-field frequencies, ${\mathrm{\omega }}_1$ and ${\mathrm{\omega }}_2$, respectively. Variable inhomogeneous widths have been considered ranging from extremely narrow (homogeneous distribution) to extremely wide. Remarkable differences between nearly homogeneously broadened systems and extreme inhomogeneous ones are exhibited. It is shown, for the situation of homogeneous broadening, that the spectra carried on with fixed probe and fixed pump frequency values have substantial differences in shape. These differences disappear in the situation of extreme inhomogeneous broadening. Fixed-pump-frequency resonant and fixed-probe-frequency nonresonant spectra are proposed as very convenient tools to yield useful evidence about the width of the inhomogeneous molecular distribution.

We derive in closed form the solutions to the rate equations for sequential field ionization of gases by a focused laser beam. We obtain the low-intensity and the high-intensity limits of the ion yield for each charge state produced by the laser pulse. Furthermore we derive the scaling laws for the appearance intensity and the saturation intensity of the ion yield. We find that the appearance intensity depends not only on the binding energy of the ionized electron but also on the quantum numbers of the shell from which the electron ionizes. We interpret this dependence on the quantum number to be the species dependence in the appearance intensities discussed by Meyerhofer and co-workers [Phys. Rev. Lett. 63, 2212 (1989)]. Formulas for five ionization models are presented.

We study the quantum fluctuations of the electromagnetic field inside a waveguide or a rectangular cavity. Explicit expessions are obtained for the field correlation functions. We also find the Casimir energy and pressure in the cavity.