Single-, double-, and triple-photoionization processes of carbonyl sulfide (OCS) have been studied in the photon-energy region of 20–100 eV by use of time-of-flight mass spectrometry and a photoion-photoion coincidence method together with synchrotron radiation. The single (${\mathrm{\sigma }}^{+}$), double (${\mathrm{\sigma }}^{2+}$), and triple (${\mathrm{\sigma }}^{3+}$) photoionization cross sections of OCS are determined; the double-photoionization cross section is found to be considerably larger at hν=60 eV, giving a ratio ${\mathrm{\sigma }}^{2+}$/${\mathrm{\sigma }}^{+}$ of 0.34, than found from previous observations of other molecules (${\mathrm{SO}}_2$ and ${\mathrm{CH}}_4$). The ratio is in close agreement with that for the isoelectronic zinc atom. Ionic fragmentation ratios of the parent ${\mathrm{OCS}}^{+}$ and ${\mathrm{OCS}}^{2+}$ ions are determined separately at the excitation energies where the single and double photoionization takes place concomitantly. The results show that both the (meta)stable ${\mathrm{OCS}}^{+}$ and ${\mathrm{OCS}}^{2+}$ ions amount to about 10–15 % at higher photon energies in the respective single- and double-photoionization processes. Ionic branching ratios and the partial cross sections for the individual ions produced from the parent ${\mathrm{OCS}}^{+}$ and ${\mathrm{OCS}}^{2+}$ ions are also presented. High-lying electronic states of the ${\mathrm{OCS}}^{+}$ and ${\mathrm{OCS}}^{2+}$ precursors are also observed.

The observation of a long-lived ${\mathrm{NeAr}}^{2+}$ molecular ion is reported. This ion was obtained by charge stripping of a 900-keV ${\mathrm{NeAr}}^{+}$ beam in an Ar gas target, and its mean lifetime was determined to be 275±25 nsec. This is a direct measurement of the mean lifetime of a doubly charged rare-gas dimer. The ion decay is decribed well by a single exponential decay, suggesting that only one metastable state is populated in the collision. The cross section for the ${\mathrm{NeAr}}^{2+}$ production was determined to be (3±2)×${10}^{\mathrm{-}18}$ ${\mathrm{cm}}^2$. The existence of this weakly bound molecular ion and the fact that its mean lifetime is known can serve to motivate better theoretical calculations of molecular structure and the dissociation process.

The relative photoionization cross section for starting from a particular fine-structure level of the initial state of an atomic system and leading to a particular fine-structure level of the residual ion is presented for the case of atomic systems having one open subshell using a simple theoretical model in which emphasis is placed on the analysis of geometrical effects. This model parametrizes photoionization cross sections in terms of LS-dependent, single-configuration dynamical amplitudes, unlike previous treatments which assume LS-independent transition amplitudes from the start. In special cases our general formulas are shown to reduce to those of previous workers. We highlight photoionization of inner s subshells in open-shell atoms and show that when the total angular momenta for the initial atomic and final ionic fine-structure levels differ by more than 1/2 then such transitions are quasiforbidden. This prediction is tested in detail for the case of Cl 3s-subshell photoionization by comparison of our single-configuration results with the multiconfiguration, eigenchannel R-matrix results of Robicheaux and Greene [Phys. Rev. A (to be published)]. All of our results apply also to photoionization of open-shell positive ions or photodetachment of open-shell negative ions.

Based on a previous Dirac R-matrix calculation [U. Thumm and D. W. Norcross, Phys. Rev. A 45, 6349 (1992)], we have derived elastic and inelastic angle-differential and elastic momentum-transfer cross sections for slow electrons (${\mathit{E}}_{\mathrm{kin}}$≤2.8 eV) colliding with neutral Cs atoms. Our results for the angle-differential cross sections are in good agreement with scaled experimental data and, depending on the incident electron energy, in qualitative or fair quantitative agreement with previously published theoretical work. The inelastic angle-differential cross sections for 6${\mathit{p}}_{1/2}$ and 6${\mathit{p}}_{3/2}$ excitation differ by more than a statistical branching ratio due to relativistic effects. For the momentum-transfer cross sections, we hope to resolve existing discrepancies in the literature and to provide more reliable input for transport calculations.

We present coupled-channel optical (CCO) calculations of the elastic and the 3 ${}_{}{}^2{}_{}{}^{}$S–3 ${}_{}{}^2{}_{}{}^{}$P transitions in electron-sodium scattering at 1 to 40 eV. The results are compared with the measurements of spin asymmetries ${\mathit{A}}_{\mathrm{ex}}$, the ratio of final- to initial-spin polarization perpendicular to the scattering plane P’/P, and the angular momentum transferred to the atom perpendicular to the scattering plane for singlet ${\mathit{L}}_{\mathrm{\perp }}^{\mathit{S}}$, triplet ${\mathit{L}}_{\mathrm{\perp }}^{\mathit{T}}$, and spin-averaged ${\mathit{L}}_{\mathrm{\perp }}$. We find excellent agreement with measurements of all of these parameters at all energies. For energies above the ionization threshold we find that inclusion of the continuum target states in the CCO formalism has a very large effect on the spin asymmetries, particularly the 3 ${}_{}{}^2{}_{}{}^{}$P channel, and brings about excellent agreement with experiment. The treatment of the dynamics of the interaction is such that the quality of the structure approximation of the sodium atom may be readily studied.

A track scope, composed of an imaging quartz fiber and a position-sensitive photon counter, was developed. Luminescence spectra, their specific intensities dL/dx, and efficiencies dL/dE were measured as functions of the helium density and of depth along the N- and He-ion tracks in dense gas and liquid helium. Observed luminescences are limited to those due to Rydberg states of excimers, ${}_{}{}^3{}_{}{}^{}$d, ${}_{}{}^3{}_{}{}^{}$D, ${}_{}{}^3{}_{}{}^{}$H, and ${}_{}{}^3{}_{}{}^{}$J, according to Herzberg’s notation. It is concluded that they are produced only by the bimolecular reaction of the lowest triplet excimer ${}_{}{}^3{}_{}{}^{}$a and that the excess energy of this reaction must be removed by the third body. Therefore, with increasing density of excitation, dL/dx and dL/dE are enhanced, but in an extreme case near the track termination, dL/dx is suppressed because the three-body reaction canno t find the third body except for the excimer ${}_{}{}^3{}_{}{}^{}$a. The appearance of an additional peak of dL/dE near the N-ion-track termination suggests the importance of additional processes such as direct excitation and charge exchange. The drastic suppression of excimer luminescences with increasing helium density is explained by instabilities of helium-excimer bubbles.

Angular correlations between K${\mathrm{\alpha }}_1$ x rays and subsequent ${\mathit{L}}_3$ x-ray transitions were measured using a ${}_{}{}^{233}{}_{}{}^{}\mathrm{Pa}$ radionuclide source and high-resolution x-ray detectors. The results provide separately the values of ${\mathit{A}}_{22}$(K${\mathrm{\alpha }}_{1\mathrm{-}}$L${\mathrm{\alpha }}_1$) and ${\mathit{A}}_{22}$(K${\mathrm{\alpha }}_{1\mathrm{-}}$L${\mathrm{\alpha }}_2$) as opposed to the compound quantity ${\mathit{A}}_{22}$(K${\mathrm{\alpha }}_{1\mathrm{-}}$Lα). For the Ll and L${\mathrm{\alpha }}_2$ transitions, the ${\mathit{A}}_{22}$ values agreed closely with those based upon theoretical (Hartree-Fock) E1 and M2 transition rates. For the less intense L${\mathrm{\beta }}_6$ and L${\mathrm{\beta }}_{2,15}$ transitions, agreement was also observed, although within larger uncertainties. In contrast, the value of ${\mathit{A}}_{22}$(K${\mathrm{\alpha }}_{1\mathrm{-}}$L${\mathrm{\alpha }}_1$) was 0.085±0.007, which is somewhat larger than the predicted value of 0.073. Possible causes for this discrepancy are explored.

An experimental investigation of the atomic-Na 3s ${}_{}{}^2{}_{}{}^{}$${\mathit{S}}_{1/2–}$5s ${}_{}{}^2{}_{}{}^{}$${\mathit{S}}_{1/2}$ two-color two-photon polarization spectrum has been made. Measurements of the linear polarization degree of the two-photon transition were made for one laser tuned within a ±100 ${\mathrm{cm}}^{\mathrm{-}1}$ range of the atomic-Na 3p ${}_{}{}^2{}_{}{}^{}$${\mathit{P}}_{\mathit{j}}$ levels, with the second adjusted to satisfy the two-photon resonance condition. The relatively short-pulse (∼0.5 ns) lasers used eliminate effects on the spectrum due to atomic-Na hyperfine interactions and radiation trapping, while greatly reducing depolarization due to collisions with Ar gas. In the absence of collisions, the measured polarization spectrum is in excellent agreement with calculations. Measurements of the Ar pressure dependence of the spectrum revealed strong depolarization in the vicinity of resonant excitation through the 3p ${}_{}{}^2{}_{}{}^{}$${\mathit{P}}_{3/2}$ level. A collisional depolarization cross section of 200(30) Å for this process was deduced from the data.

Various aspects of a rather general treatment of proton-spin relaxation in water are discussed within the framework of quantum-dynamical semigroup theory for a four-level system coupled to a reservoir in equilibrium. In particular, the specifications of the infinitesimal generator of time evolution, either in Kossakowski or in Davies form, are worked out in detail. With the help of the Lie algebra of SU(4), the results are used to derive, under suitable simplifications, generalized Bloch equations for the static and alternating-field case. The relevant correlation functions are calculated using conventional approaches but supplemented by taking into account explicitly results from a stochastic model for formation and breaking of hydrogen bridges. A further approximate reduction of the coupled general equations to simpler ordinary Bloch equations leads to an identification of the relevant relaxation times. This approach provides a somewhat different interpretation of rotational correlation times whose numerical values are estimated over a wide temperature range.

The x-ray-absorption near-edge structure (XANES) of sulfur hexafluoride, ${\mathrm{SF}}_6$, in the gas phase has been measured with high energy resolution at the sulfur ${\mathit{L}}_{2,3}$ and fluorine K ionization thresholds using synchrotron radiation from the SX700/II monochromator at BESSY. Besides dominant transitions to core-excited inner-well states, several series of Rydberg states with vibrational fine structure were resolved below the sulfur ${\mathit{L}}_{2,3}$ thresholds. Using the Rydberg formula, quantum defects of ${\mathrm{\delta }}_{\mathit{s}}$=1.80 and ${\mathrm{\delta }}_{\mathit{d}}$=-0.03 were obtained for the s and d Rydberg orbitals of the central sulfur atom. A Franck-Condon analysis was used to determine the vibrational spacing and the S-F bond length of the (S 2${\mathit{p}}_{3/2}$${)}^{\mathrm{-}1}$4${\mathit{s}}^1$ core-excited state. The presence of vibronically coupled transitions below the sulfur ${\mathit{L}}_{2,3}$ edges was confirmed. The derived natural linewidths of the (S 2${\mathit{p}}_{1/2,3/2}$${)}^{\mathrm{-}1}$ Rydberg states were found to be strikingly narrower than those of the (S 2${\mathit{p}}_{1/2,3/2}$${)}^{\mathrm{-}1}$ inner-well resonances. Line-shape analysis also revealed significant inhomogeneous broadening of the (S 2${\mathit{p}}_{1/2,3/2}$${)}^{\mathrm{-}1}$${\mathit{a}}_{1\mathit{g}}^1$ states, which is attributed to unresolved vibrational structure. Large Lorentzian contributions to the line shapes of the (S 2${\mathit{p}}_{1/2,3/2}$${)}^{\mathrm{-}1}$${\mathit{t}}_{2\mathit{g}}^1$ resonances suggest that vibrational effects are relatively small for those states.

We report here on high-resolution spectral measurements of Stark-broadened 3d-5f lines from lithiumlike ions and 3d-4f lines from heliumlike ions. The spectra were emitted from high-density laser-produced plasmas. Plasmas were produced by irradiating thin- and thick-foil targets made of magnesium, aluminum, phosphorus, and chlorine with the OMEGA laser in a line-focus geometry. Line profiles were compared to a Stark-broadening calculation that uses the static-ion approximation and an impact approximation for the electrons. The dependence of Stark broadening on atomic number is discussed. For the aluminum plasmas ∼10% narrowing was observed in the width of the 3d-5f line as the length of the plasma was increased from 3 to 6 mm.

The potential-energy surfaces of clusters composed of identical charged particles confined by an isotropic harmonic potential are investigated with particular reference to rearrangement mechanisms. A number of intrashell and intershell rearrangements in systems with 4–60 ions are characterized. The intrashell processes, in which only the ions of one roughly concentric shell are permuted, generally exhibit much lower barriers than the intershell processes that involve the migration of an ion between shells. The former rearrangements may generally be described as concerted multiple diamond-square-diamond processes. We can also identify trends in the size of the energy barriers as the relative number of ions in the concentric shells varies. These observations will be required to understand the evolution of the detailed dynamics of such systems at temperatures where the clusters begin to melt. The results are in good agreement with the dynamics observed in previous simulations.

For an electron in the presence of a Coulomb field and a weak uniform harmonic electric field, and for states which are Coulomb bound states of definite angular momentum in the remote past, we present analytic expressions for the tensorial fields, which are the basic quantities that are used to construct the second-order correction to the electron wave function. The calculation is based on solving the inhomogeneous differential equations connected with this correction in position space. The results are useful for the evaluation of several higher-order radiative processes.

An atom passing through a standing light field imparts a position-dependent phase shift to the field. By making a phase-sensitive measurement of the field, it is possible to resolve the atom’s position to much less than the wavelength of the light. The field measurement results in the creation of virtual slits, and diffraction and interference phenomena may be observed. The phase measurements give welcher Weg information and enable ‘‘quantum-eraser’’ experiments to be realized.

This paper reports on the evidence for large permanent electric-dipole moments of Rydberg atoms in crossed electric and magnetic fields. It was found that the dipole moments have a large value if the scaled electric-field strength ε=${\mathit{EB}}^{\mathrm{-}4/3}$ has the value ε≊0.75 (with electric- and magnetic-field strengths E and B in atomic units). The experimental technique employed to determine the atomic dipole moment was to add a slight inhomogeneity to the electric field and to measure the deflection of the atoms, this deflection being proportional to the field inhomogeneity and the atomic dipole moment. In order to discuss the experimental results, different theoretical approaches to the E×B problem are reviewed. The occurrence of Rydberg atoms with large dipole moments can be explained as being caused by a distinct type of classical orbit.

We present a method for measuring ion-oscillation frequencies in quadrupole traps (Paul, Penning, and combined traps). The method depends on a statistical analysis of the delays between detected fluorescence photons. We have used the method to verify the oscillation frequencies for magnesium ions in a Penning trap as a function of the dc voltage applied to the trap electrodes. We demonstrate that, as expected, the method measures the space-charge shifted (individual-ion) oscillation frequency and not the frequency at which the ion cloud as a whole moves. However, our data suggest that for most of our operating conditions, the space-charge shift is negligible. We also discuss the effect of the contact potential arising from areas of the trap electrodes that are coated by excess Mg from the atomic beam used. We show that this extra potential shifts the center of the Penning trap and makes it unstable for low values of the applied voltage.

The question is studied as to which state a radiating atom has to be reset to if a photon is detected and absorbed. This is important for applications of a recently proposed approach to photon counts from a radiating atom which is based upon repeated gedanken measurements and associated reductions as a technical tool. The general reset density matrix is determined. It may depend on the atomic state shortly before the photon detection, and may thus be time dependent. It may also have off-diagonal terms which may lead to interesting coherence effects. It is shown that the repeated measurements together with the reset matrix lead to the optical Bloch equations for a general N-level atom. The general photon-counting distribution for an N-level radiating atom is derived and the axiomatic continuous-measurement theory of Srinivas and Davies [Opt. Acta 28, 981 (1981); 29, 235 (1982)] for quantum-counting processes is shown to follow in the present framework from ordinary quantum mechanics.

We report a systematic study of optical pumping of ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ nuclei by metastability exchange using an arc-lamp-pumped neodymium-doped lanthanum magnesium hexaluminate (Nd:LMA) laser. The laser produces 6 W at 1083 nm in a 1.5-GHz bandwidth and yields a substantial improvement in attainable nuclear polarization and pumping rates. Spin-polarized ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ has applications in several fields of physics; the focus in this article is on targets for nuclear physics, which require high pumping rates to maintain the highest polarization. We have studied the influence of the ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ pressure, pumping transition, discharge intensity and frequency, cell size and shape, and laser power on both the polarization and pumping rate. Under ideal conditions, we obtain 85% nuclear polarization. With pumping rates of ${10}^{18}$ (${10}^{19}$) atoms/s, 82% (50%) polarization is obtained. We have found a pressure-dependent saturation of the increase in pumping rate with laser power. Based on our results, we discuss projections for the performance of both internal and external targets.

We have performed precise NMR measurements in order to calibrate a standard optical technique for measuring the polarization of ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ nuclei. This optical technique requires knowledge of the pressure-dependent relationship between the nuclear polarization and the degree of circular polarization of the 668-nm light emitted from a ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ discharge. We determined the nuclear polarization by comparing the adiabatic fast-passage NMR signal from samples of polarized ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ at pressures between 0.1 and 5 torr to an identical sample of water. For water, the polarization is known from statistical mechanics. The ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ is polarized using the metastability exchange technique for optical pumping of ${}_{}{}^3{}_{}{}^{}\mathrm{He}$. The accuracy of the calibration is ±2%, which is required for applications in nuclear physics.

We have developed a theoretical approach to the study of field-fluctuation effects on nonlinear resonant transitions based on a Monte Carlo simultation. In this paper, we present a description of the model, as well as results from its application to ac Stark splitting in double-optical-resonance driven by a two-photon transition.