We measure photoion spectra and angular distributions of photoelectrons from the two-color, two-photon resonant ionization of barium atoms in order to probe the effect of the photoionization pathway on autoionization processes. The (6s6p${)}^1$P${\mathrm{°}}_1$ and the (5d6p${)}^1$P${\mathrm{°}}_1$ states are used as intermediates in the photoexcitation of barium to energies in the range of a series of 5dnd autoionizing states. Dramatic differences in the profiles of autoionizing lines are observed for excitation pathways involving different intermediate states. Angular distributions of photoelectrons collected across the range of autoionizing resonances demonstrate strong variations that follow constraints imposed by the angular momentum of the autoionizing peaks and also suggest significant spin-orbit interactions in the photoionization step.

We have measured (e,2e) energy spectra in cadmium for the 4${\mathit{d}}^9$5${\mathit{s}}^2$5p J=1 autoionizing region. The ejected-electron energy range investigated was from 2.6 to 4.8 eV with an incident electron energy of 150 eV and a scattering angle of 3°. Small differences were found in spectra taken at ejected-electron directions 180° apart. We ascribe these differences to interference terms arising from coherent excitation of the J=1 and overlapping 5pnp J=0,2 autoionizing levels. A calculation of these effects is presented that agrees quite well with the experimental data. We obtain experimental excitation amplitude ratios and assign a previously undetected autoionizing level to 5p6p ${}_{}{}^3{}_{}{}^{}$${\mathit{D}}_2$.

The role of target-electron–projectile-electron interactions in projectile-electron loss is examined in the framework of the plane-wave-Born-approximation. The summation over target-electron states, which typically uses the closure approximation, is refined by using a sum rule for stopping power due to Bethe. The resulting expression for the cross section is compared with other modifications of the closure approximation and with the experimental ${\mathrm{H}}^0$+${\mathrm{H}}^0$ electron-loss section.

Previous time-dependent Hartree-Fock (TDHF) calculations of collisions between He ions and He atoms have been extended with improved accuracy. New features include using a Hermitian Coriolis term and installing classical quantum coupling, which allows completely self-consistent treatment of both the nuclear and electronic coordinates. Increased computer capabilities have allowed us to eliminate previous numerical limitations. We have calculated the single- and double-capture cross sections in the energy range of 1 to 250 keV. Comparison with experiments allows us to investigate the strengths and weaknesses of TDHF theory. Since correlation is best defined to be that which is not included in single-particle descriptions, we discuss converged results with regard to what they reveal about correlation.

A measurement of the doubly differential cross sections of secondary electrons ejected from molecular oxygen by electron impact has been made. A modulated crossed-beam method was used with incident electron energies from 25 to 250 eV. The energy and angular range of secondary electrons covered from 1.0 eV to one-half of the difference between the incident energy and ionization potential and from 12° to 156°, respectively. The present results have been compared with the previous measurements and considerable discrepancies were found.

We argue that a three-step transfer-excitation (capture and target excitation) mechanism enhances (depletes) the cross section for one- (two-) electron removal from a two-electron target. This mechanism, which becomes very important at high projectile charge (q), is mediated through quasimolecular couplings between diabatic double-capture and transfer-excitation channels, in which the inner of two active electrons is transferred to an excited target state. The inclusion of this process in the extended classical over-barrier model gives excellent agreement with unexplained experimental cross sections for one- and two-electron removal from He in slow collisions with Xe ions of high charge (q≤31).

The results of an experiment on the associative ionization collision between sodium and cesium excited atoms are reported. The atoms are excited by two resonant cw lasers to the 3${\mathit{P}}_{3/2}$ and 6${\mathit{P}}_{3/2}$ levels, respectively, and the formation of the ${\mathrm{NaCs}}^{+}$ molecular ion is observed. The rate constant of such process has been measured relative to the known rate constant of sodium associative ionization and its value resulted in K=(5.4±3.2)×${10}^{\mathrm{-}11}$ ${\mathrm{cm}}^3$ ${\mathrm{s}}^{\mathrm{-}1}$.

The Mg 3pns (n=10–18) and 3pnd (n=9–40) doubly excited J=1 autoionizing states have been observed experimentally using an isolated-core excitation method with stepwise laser excitation. Linearly polarized laser light excites the 3pnd J=1 and J=3 states, as well as the 3pns J=1 states. Our previous study of the J=3 states enables the J=1 states in the 3pnd spectra to be identified. We have also calculated the spectra from a reaction matrix, and the agreement between theory and experiment is generally excellent. Differences between the J=1 and J=3 features are discussed and qualitatively explained in terms of semiclassical arguments. Additionally, energies of the 3sns ${}_{}{}^1{}_{}{}^{}$${\mathit{S}}_0$ (n=11–22) Rydberg levels are reported.

Iodine atoms, produced by laser-induced dissociation of molecular iodine, have been photoionized with synchrotron radiation in the region of the 4d→εf shape resonance. By using photoelectron spectroscopy in conjunction with ionic spectroscopy, we have measured the 4d, 5p, 5s, and valence satellites’ absolute photoionization cross sections, as well as the relative oscillator strengths for the production of ${\mathrm{I}}^{+}$, ${\mathrm{I}}^{2+}$, and ${\mathrm{I}}^{3+}$. Some specific open-shell effects have been observed such as the strong 4d-5p hole electrostatic coupling, as revealed by the 4d photoelectron spectrum, and the weak enhancement of the outer-shell cross sections, especially the 5p one, in the region of the 4d→εf shape resonance, as compared with xenon. Multiple-electron processes associated with 4d photoemission have been studied and account for about 20% of the photoabsorption cross section. Moreover, as already pointed out in iodine compounds, the total oscillator strength of atomic iodine is found, in the studied spectral region, to be surprisingly weak.

Decay of autoionizing resonances in atoms often leads to preferential production of excited states of the resulting ion. The population of these states can exceed the population of the ground state by more than an order of magnitude. We report measurements of the branching ratios for ${\mathrm{Mg}}^{+}$ ions resulting from the decay of selected autoionizing levels. These derive either from single excitation of a 2p electron or from two-electron excitations. For excited states of the type 2${\mathit{p}}^5$${(}^2$${\mathit{P}}_{1/2,3/2}$)3${\mathit{s}}^2$ns, n=4 or 5, production of ${\mathrm{Mg}}^{+}$ ions having the final configuration 2${\mathit{p}}^6$ns is the predominant decay feature. For n>5, the predominant decay feature is the 2${\mathit{p}}^6$(n+1)s configuration, indicating that shakeup is very strong in the decay of the excited state. For excited states of the type 2${\mathit{p}}^5$${(}^2$${\mathit{P}}_{1/2}$)5d and 2${\mathit{p}}^5$${(}^2$${\mathit{P}}_{3/2}$)6d, the decay behaves in a manner similar to that of the ns states. However, for the 2${\mathit{p}}^5$${(}^2$${\mathit{P}}_{1/2}$)6d state, the most prominent decay channel appears to be the 2${\mathit{p}}^6$11d state, which leads to the emission of an electron with the same kinetic energy as the ${\mathit{L}}_2$-${\mathit{M}}_1$${\mathit{M}}_1$ Auger line.

A new technique sensitive to molecular orientation and geometry, and based on measuring the polarization of x-ray emission, has been applied to the Cl-containing molecules methyl chloride (${\mathrm{CH}}_3$Cl) and the chlorofluoromethanes (${\mathrm{CF}}_3$Cl, ${\mathrm{CF}}_2$${\mathrm{Cl}}_2$, and ${\mathrm{CFCl}}_3$) in the gas phase. Upon selective excitation using monochromatic synchrotron radiation in the Cl K-edge (Cl 1s) near-threshold region, polarization-selective x-ray emission studies reveal highly polarized molecular valence x-ray fluorescence for all four molecules. The degree and the orientation of the polarized emission are observed to be sensitive to the incident excitation energy near the Cl K edge. In some cases, the polarization direction for x-ray emission reverses for small changes in incident excitation energy (a few eV). It is shown that the polarized x-ray emission technique can be used to infer, directly from experiment, symmetries of occupied and unoccupied valence molecular orbitals, an- isotropies in absorption and emission, and orientational and geometrical information. It is suggested that the x-ray polarized-fluorescence phenomenon, reported here for simple molecules, can be used as a new approach to study more complicated systems in a variety of environments.

The spectrum of four-times ionized cerium Ce v has been observed with a sliding-spark discharge and a 10.7-m normal-incidence vacuum spectrograph. A total of 107 lines have been classified as transitions between 51 energy levels. All levels of the 5${\mathit{p}}^6$, ${5\mathrm{p}}^5$4f, 5${\mathit{p}}^5$5d, 5${\mathit{p}}^5$6s, 5${\mathit{p}}^5$6p, and 5${\mathit{p}}^5$6d configurations have been determined. Because of collapse of the 4f shell, the 5${\mathit{p}}^5$4f configuration is situated well below 5${\mathit{p}}^5$5d. Ab initio calculations and least-squares fits of the energy parameters to the experimental energy levels are reported. The ionization energy is revised to 527 100±2000 ${\mathrm{cm}}^{\mathrm{-}1}$ (65.35±0.25 eV).

Optical pumping of the electronic ground state of molecules has been investigated under conditions of plane, elliptically, and circularly polarized light broadband excitation. The analysis has been performed assuming arbitrary angular momentum values, and applying the apparatus of polarization moments, the latter forming the coefficients in the expansion of the density matrix over irreducible tensor operators, including simplifications due to asymptotic limits for large momentum values. In addition to accounting for the external magnetic field, attention has been also given to the dynamical Stark effect arising from the absence of coincidence between the centers of the exciting line and the absorption line. An alternative classical description is also proposed using multiple moments as coefficients of the expansion of a classical quasidensity of states over spherical harmonics. The equivalence of both approaches of description in the limit of infinitely large angular moments is demonstrated, and the meaning of the origin of coherence is clarified, as well as its destruction by the magnetic field in the classical description of the system.

In experiments using targets consisting of many thin metal foils, we have demonstrated that a narrow, forward-directed cone of transition radiation in the 8- to 60-keV spectral range can be generated by electron beams with moderate energies (between 100 and 500 MeV). The theory suggests that high-density, moderate-atomic-number metals are the optimum foil materials and that the foil thickness can be chosen to maximize photon production within a desired spectral range. The three targets used in the experiments consisted of 10 foils of 1-μm-thick gold, 40 foils of 8.5-μm stainless steel, and 20 foils of 7.9-μm copper. The efficiency with which hard x rays are generated, and the fact that the requisite electron-beam energies are lower by a factor of 5 to 10, make such a radiation source an attractive alternative to synchrotron radiation for applications such as medical imaging, spectroscopy, and microscopy.

The paper deals with the interaction of strong and pulsed radiation fields with a two-electron-state molecule. The light pulses under discussion are shorter than a vibrational period. The molecule is treated within the framework of the Born-Oppenheimer approximation. The nuclear degree of freedom is modeled as displaced harmonic potentials for the ground and the excited electronic states, respectively. Using coherent states as expansion tools for the nuclear motion, analytical expressions are given for the short-pulse region defined by the duration of the light pulse and the displacement of the potentials. The resonance condition is derived for this region. It is shown that within the short-pulse region the two-electron-state molecule mimics the behavior of a two-electron-state atom. Longer pulses are treated with numerical methods. The concept of resonance is discussed for the longer-pulse case. Limitations on population inversion are considered.

Stark spectra of barium Rydberg states above the classical field-ionization limit have been recorded at high density of states employing high-resolution laser spectroscopy. The spectra, taken at a binding energy of about ɛ≊-55 ${\mathrm{cm}}^{\mathrm{-}1}$ with respect to zero-field threshold and at electric-field strengths F≊100 V/cm exhibit dramatic variations in line profiles with increasing external field strength. The WKB quantum-defect theory of the Stark effect of nonhydrogenic atoms was used to analyze the recorded spectra, and excellent agreement between measured and calculated line profiles was achieved. Core-induced coupling of closed and open hydrogenic Stark channels was found to contribute dominantly to the linewidth of the recorded Stark resonances. In addition, a reformulation of this theory by McNicholl, Ivri, and Bergeman (unpublished) in terms of a configuration-interaction model was employed to analyze a particular avoided crossing of several Stark components. In this way recorded line profiles were parametrized within a limited model to obtain resonance energies, linewidths, and Fano q parameters.

Mechanisms of intermolecular dephasings revealed in the time-resolved coherent anti-Stokes Raman scattering (CARS) spectra of a molecular ensemble are theoretically studied by deriving the microscopical expression for the scattered photon intensity averaged over the heat-bath variables. The intermolecular dephasing means the decay of the intermolecular coherence created by the pumping process, i.e., a linear combination of the vibrational Raman transitions of the molecules at different sites. The intensity of the time-resolved CARS is expressed in terms of time-development matrix elements of the intermolecular coherence. An equation of motion for the intermolecular coherence is derived by using the projection operator method. The structure of the intermolecular-dephasing constant is clarified. The intermolecular-dephasing constant consists of the sum of population decay constants and the intermolecular pure dephasing constant that originates from an elastic interaction between the relevant molecules and a common heat-bath mode. The intermolecular pure dephasing constant is expressed in terms of the intramolecular pure dephasing constants and destructive interference of the intramolecular pure dephasings between the relevant two molecules. The intermolecular-dephasing constant is expressed as the sum of the intramolecular-dephasing constants of the relevant molecules when there is no common heat-bath mode. Inhomogeneous effects on the time-resolved CARS spectra are also investigated. A degree of the structural order is introduced to qualitatively express the inhomogeneity of local structures in the molecular ensemble. The microscopic expression derived in this paper has taken into account both incoherent and coherent optical processes in the thermally distributed molecular ensemble.

The interaction of a nonrelativistic, ‘‘two-level’’ atom with a single-frequency, quantized electromagnetic field is usually analyzed in the dipole approximation by applying the rotating-wave approximation (RWA) to obtain the exactly solvable Jaynes-Cummings model (JCM). In this paper, it is shown that for a spin-1/2 particle, at rest in a constant magnetic field and for a system that obeys the electric dipole Δm=±1 selection rules, a choice of a perturbing field that consists of circularly polarized photons can make the RWA unnecessary for the solution of the quantum-electrodynamic (QED) problem. For these cases, the only approximation necessary to obtain an exact solution of the QED problem is to neglect the spontaneous emission of photons with a sense of polarization opposite that of the perturbing field. The matrix elements corresponding to these ‘‘virtual’’ processes are independent of the number of photons n in the applied field and become relatively small, compared with those that correspond to energy-conserving processes, as n becomes large.

However, a two-level system that obeys the Δm=0 selection rule will require the RWA in order to reduce it to the JCM, regardless of the choice of polarization of the perturbing field. For this case, the emission of the photons that do not conserve energy is an induced process that corresponds to matrix elements that increase as √n+1 . It is thus concluded that, provided the right choice of selection rules and polarization of the applied field is made, the JCM provides a more accurate description of some systems than the accuracy of the RWA. Furthermore, it is concluded that an experimental test of the validity of the RWA can be made by performing the same experiments on an electric dipole Δm=±1 transition, first with circularly polarized light and then with linearly polarized light of the same intensity. Any difference in response of the atomic system for these two experiments can be identified with terms that are neglected in the RWA. This experimental technique may prove useful for observing the optical Bloch-Siegert shift and for investigating the failure of the RWA and the chaotic quantum behavior predicted by Milonni, Ackerhalt, and Galbraith [Phys. Rev. Lett. 50, 966 (1983)].

According to the exact solution of a two-level atomic system interacting with a single-mode field in a cavity and including a pumping mechanism, sub-Poissonian photon statistics can be generated either by regular pumping or by resonance absorption of the lower level. When the atomic lifetime is much smaller than the interaction time (${\gamma }_{\mathit{a}\mathit{b}}$τ≫1), which corresponds to a usual laser system, the sub-Poissonian photon distribution can be obtained only by regular pumping. On the other hand, if the atomic lifetime is comparable with the interaction time (${\gamma }_{\mathit{a}\mathit{b}}$τ≤1), the sub-Poissonian photon distribution can be generated by both mechanisms.

A series of multifrequency solutions of a standing-wave laser are presented. The parameter condition under which the solutions are valid can be satisfied in dye lasers and semiconductor lasers if the cavity length is on the order of 100 μm and the reflectivity of the mirrors is about 99%. The physical origin of the multifrequency operations is the spatial hole burning of population inversion by standing waves. These solutions consist of (2N+1) constant mode intensities, where N=0,1,..., and may go into operation one after another as the pump parameter is increased. If the pump parameter is further increased to above a certain threshold, then another kind of instability occurs and the originally constant mode intensities begin temporal oscillation. The pump thresholds of these two kinds of instabilities are determined by linear stability analysis.