We report the observation of the Autler-Townes absorption spectrum of a two-level atom in a 100% amplitude-modulated optical field. Two frequency-stabilized dye lasers interact with a three-level atomic cascade system. One of the two lasers is used to create the bichromatic field that saturates the lower transition, while the second one is tuned in the frequency between the second and third levels to probe the absorption line shape. We investigate the dependence of the spectrum on the detuning between the average frequency of the bichromatic field and the lower atomic resonance. In order to study the effects of saturation, for every value of the detuning we take a series of data for different intensity levels of the bichromatic field. There is very good agreement between theory and experiment and profound differences from the conventional Autler-Townes results with a monochromatic pump field. © 1996 The American Physical Society.

We have observed resonance structures in detached electron spectra arising from fast collisions of ${\mathrm{Li}}^{-}$ and ${\mathrm{B}}^{-}$ ions with gas targets. The structures are associated with the autodetaching decay of shape resonance states. Using Shore parametrization, we obtain resonance energies and widths of 50(6) and 64(25) meV, respectively, for the ${\mathrm{Li}}^{-}\left(2s2p{}_{}{}^3{}_{}{}^{}P\right)$ state and 104(8) and 68(25) meV, respectively, for the tentatively identified ${\mathrm{B}}^{-}\left(2\mathrm{}{p}^2{}_{}{}^1{}_{}{}^{}D\right)$ state. Other resonances in ${\mathrm{B}}^{-}$ remain unidentified.

A special type of multiphoton interaction of powerful laser radiation with fast truly free electrons is analyzed. The production of intense soft-x-ray emission is shown to arise due to the large differential cross section of the process and the Doppler shift of the simultaneously scattered photons.

The photoionization process is divided into (A) the absorption of the photon by one electron and (B) the correlated motion of the electron pair leading to singly or doubly ionized helium. We relate (A) to the total cross section to be calculated analytically in the quasiclassical reflection approximation as known from molecular problems. For (B) the two-electron wave function is propagated with the semiclassical version of Feynman's path integral to separate single and double ionizing events. A probe for (B) is the ratio between double and single ionization. The results for (A) and (B) and for absolute ionization cross sections obtained by combining (A) and (B) are in good agreement with different experiments that cover together a range of photon energy from the double-ionization threshold to several hundred eV.

The dissociative recombination of ground state O${\mathrm{H}}^{+}$ ions with electrons has been measured, using the heavy-ion storage ring technique. A comparison of the cross section to recent theoretical results reveals that several observed resonances are due to a type of indirect dissociative recombination involving Rydberg states with a bound excited ion core.

We use evanescent laser light to guide atoms through hollow-core optical fibers. The light, detuned to the blue side of rubidium's $D_2$ resonance lines, is launched into the glass region of a hollow capillary fiber and guided through the fiber by total internal reflection from the glass walls. Atoms interacting with the evanescent component of the field are repelled from the wall and guided through the fiber hollow. A second laser tuned to the red side of resonance is used to initially inject the atoms into the evanescent guide. An optical intensity threshold for guiding is observed as the evanescent-field-induced dipole forces exceed the van der Waals forces.

We report the demonstration of x-ray lasing in nickel-like tin at 12 nm by using the prepulse technique in combination with a curved target. A gain length of 4.8 was obtained with a 3-cm-long target of 1 m radius of curvature irradiated by the Asterix iodine laser at 1.315 μm with a total energy of 500 J and a 7.5% prepulse.

The hyperfine structures and constants of the 5d${\mathrm{}}^4$${\mathit{D}}_{7/2}$, 6p${\mathrm{}}^4$${\mathit{P}}_{5/2}$, 6p${\mathrm{}}^2$${\mathit{D}}_{5/2}$, and 6p${\mathrm{}}^4$${\mathit{D}}_{7/2}$ levels have been experimentally determined with the use of a fast ion-beam-laser technique. © 1996 The American Physical Society.

Sixteen fine-structure intervals separating n=10 neon Rydberg levels with L=5, 6, 7, and 8 were measured with the precision of about ±00.2 MHz, using fast beam microwave-optical techniques. The measured fine-structure pattern is in good agreement with the predictions of an effective potential model, and this comparison leads to precise determinations of the quadrupole moment and dipole polarizabilities of the ${}_{}{}^2{}_{}{}^{}$${\mathit{P}}_{3/2}$ ground state of the ${\mathrm{Ne}}^{+}$ ion: Q=-0.204 03(5)${\mathrm{ea}}_0^2$, ${\mathrm{\alpha }}_{\mathit{s}}$=1.3028(13)${\mathit{a}}_0^3$, ${\mathrm{\alpha }}_{\mathit{T}}$=-0.026(5)${\mathit{a}}_0^3$. The contributions of magnetic interactions to the Rydberg fine structure lead to a determination of the ${\mathrm{Ne}}^{+}$ ion’s g value, ${\mathit{g}}_{\mathit{J}}$=1.354(21). All of these core parameters provide challenging tests of atomic structure calculations. The measurements also set a limit on the possible contributions to the fine structure due to a predicted geometric phase effect that is about an order of magnitude smaller than expected. © 1996 The American Physical Society.

Cadmium ions (${\mathrm{Cd}}^{+}$) have been cooled sympathetically below 1 K by means of Coulomb interactions with laser-cooled ${\mathrm{Be}}^{+}$ ions that are trapped simultaneously in a Penning trap. The ultraviolet resonance lines (5${\mathit{s}}^{\mathrm{}2}$${\mathit{S}}_{1/2}$⇆5${\mathit{p}}^{\mathrm{}2}$${\mathit{P}}_{3/2}$, λ=214.5 nm) of the cooled ${\mathrm{Cd}}^{+}$ ions were observed using a frequency-doubled cw dye laser. This radiation is based on efficient frequency doubling in a nonlinear crystal [β-${\mathrm{BaB}}_2$${\mathrm{O}}_4$ (BBO)] with an external enhancement cavity. The spectra of the six isotopes (A=110–114, and 116) were obtained using a natural isotope mixture, and the reliability of the measured isotope shifts is discussed. © 1996 The American Physical Society.

We observe Ramsey fringes in the absorption line shape for a weak optical field probing the transition between an unperturbed quantum state and one that is dynamically Stark shifted by a strong pump field. © 1996 The American Physical Society.

A negative-pion capture process for several kinds of hydrocarbons was studied by the simultaneous observation of low-energy pionic x rays and ${\mathrm{\pi }}^0$ decays following the nuclear capture of pions by hydrogen. We measured the pionic x-ray intensity pattern and the pion-capture probabilities of constituent carbon and hydrogen. The x-ray intensity pattern depends on the chemical environment; a cascade calculation showed that this can be qualitatively attributed to the effects of a pion transfer from hydrogen. The pion-capture probabilities on hydrogen could be explained by a combined model involving a modified large mesomolecular model and an external transfer process. The external transfer parameter of the pion from a pionic hydrogen atom to carbon atoms was deduced to be ${\mathrm{\Lambda }}_{\mathrm{C}}$=1.7±0.2 based on the results for a series of alkanes in the condensed phase. The large difference between the pion-capture probabilities on hydrogen observed in alkanes and that in benzene can be well understood based on the proposed model. © 1996 The American Physical Society.

The relativistic many-body perturbation theory in a multiconfigurational model space has been extended to include the pseudoconfigurations. Using this approach, we have calculated the beryllium ground-state energy and obtained an accurate theoretical result, -14.669 329 hartrees, which agrees well with the ‘‘experimental energy’’ -14.669 332 4 hartrees. © 1996 The American Physical Society.

A perturbation approach to the Dirac equation starting from a pair of the Galilei-invariant four-component Levy-Leblond-type equations [J. M. Levy-Leblond, Commun. Math. Phys. 6, 286 (1967)] for a nonrelativistic electron or positron with spin is formulated. The perturbation expansion is obtained by contour integration of matrix elements of the Dirac resolvent expanded into appropriate power series of nonrelativistic resolvents. The expressions for the low-order energy corrections coincide with the well-known formulas for the Rayleigh-Schrödinger coefficients but with the sums going over both nonrelativistic electron and nonrelativistic positron states. To illustrate the method, all-order calculations are performed in the case of a quasifree particle. © 1996 The American Physical Society.

We study the energy levels of ${\mathrm{H}}_2$ molecules in a superstrong magnetic field (B≳${10}^{12}$ G), typically found on the surfaces of neutron stars. The interatomic interaction potentials are calculated by a Hartree-Fock method with multiconfigurations assuming electrons are in the ground Landau state. Both the aligned configurations and arbitrary orientations of the molecular axis with respect to the magnetic-field axis are considered. Different types of molecular excitations are then studied: electronic excitations, aligned (along the magnetic axis) vibrational excitations, and transverse vibrational excitations (a constrained rotation of the molecular axis around the magnetic-field line). Similar results for the molecular ion ${\mathrm{H}}_2^{+}$ are also obtained and compared with previous variational calculations. Both numerical results and analytical fitting formulas are given for a wide range of field strengths. In contrast to the zero-field case, it is found that the transverse vibrational excitation energies can be larger than the aligned vibration excitation, and they both can be comparable to or larger than the electronic excitations. For B≳${\mathit{B}}_{\mathit{crit}}$=4.23×${10}^{13}$ G, the Landau energy of the proton is appreciable and there is some controversy regarding the dissociation energy of ${\mathrm{H}}_2$. We show that ${\mathrm{H}}_2$ is bound even for B≫${\mathit{B}}_{\mathit{crit}}$ and that neither proton has a Landau excitation in the ground molecular state. © 1996 The American Physical Society.

A minimax formulation for the time-independent Dirac equation is extended to the two-particle problem. The question of solutions of the minimax formulation is discussed and related to the continuum dissolution problem. The approach is applied to the ground state and the n=2 complex of states of He-like ions. Variational calculations are made for bases with a small number of basis functions, varying the nonlinear parameters, as well as for large-dimensional configuration-interaction-type bases. © 1996 The American Physical Society.

We present a theoretical study of the connections between quantum and classical descriptions of lithium in a magnetic field. We find that the localized nature of the ionic core causes a breakdown in the generic connections between energy-level statistics and classical motion: classical chaos is observed in a regime where the energy-level distribution is Poissonian. The breakdown arises because the source of chaos is localized on a node of the odd-parity wave function. In addition, we employ the principles of periodic-orbit theory to identify classical trajectories with spectral periodicities. We identify a series of core-scattered recurrences in the Fourier transform of the spectrum and quantitatively describe them by a simple model. Finally, we introduce recurrence counting and demonstrate how it can help relate the classical dynamics to the quantum spectrum. © 1996 The American Physical Society.

We report on large-scale ab initio multiconfiguration Dirac-Fock calculations of hyperfine-structure parameters and oscillator strengths for the lowest few levels in Tl II and Tl III. The effect of both core-valence and core-core correlation on these properties is investigated, probing effects of all core subshells. The agreement with most experiments is excellent. These data are of vital interest for the interpretation of recent spectra of the chemically peculiar star χ Lupi. © 1996 The American Physical Society.

The depolarized, rototranslational light-scattering spectrum of gaseous nitrogen at 295 K has been computed in the far spectral wings, taking into account both the allowed Raman lines and collision induction spectra. Good agreement between our theoretical results and the experimental data [Y. Le Duff and V. Teboul, Phys. Lett. A 157, 44 (1991)] has been observed. Our findings indicate that most of the intensity at large frequency shifts is due to the wings of the pressure broadened Raman lines. © 1996 The American Physical Society.

Direct scattering (→${\mathrm{H}}^0$), stripping (→${\mathrm{H}}^{+}$), and electron capture (→${\mathrm{H}}^{\mathrm{-}}$) are studied at energies of 1.0, 2.0, and 3.0 keV in small-angle ${\mathrm{H}}^0$+${\mathrm{N}}_2$ collisions. Time-of-flight and electrostatic-energy analysis are used to identify the dominant collision processes. The electronically elastic channel is found to be weak beyond the very smallest scattering angles. As an example, at 1.0 keV, electronically inelastic processes dominate the collision for scattering angles larger than 0.2°. Beyond the smallest angles, both the stripping and electron-capture processes can also result in electronic excitation of the ${\mathrm{N}}_2$ or ${\mathrm{N}}_2^{+}$. The reduced cross sections, when plotted as a function of reduced scattering angle, are shown to scale reasonably well for each of the three processes studied. © 1996 The American Physical Society.