Extracting generalized multiphoton ionization cross-sections from nonperturbative time-dependent calculations: An application in positronium

Ab initio time-dependent (TD) calculations of the behavior of positronium (Ps) under strong subpicosecond laser pulses are presented. The results are compared with results in H through scaling. It is found that a substantial amount of the population can be found in excited states after the pulse. In the perturbative regime, generalized multiphoton ionization cross-sections are extracted from the results of the time-dependent calculations. The generalized cross-sections are used to predict the response of Ps to nanosecond laser pulses at wavelengths of current experimental interest. Beyond the application to Ps, the generality of the method for extracting generalized cross-sections from TD nonperturbative calculations is discussed. Received 8 June 1999     

Momentum transfer theory of ion transport under the influence of resonant charge transfer collisions: the case of argon and neon ions in parent gases

Transport properties of ion swarms in presence of Resonant Charge Transfer (RCT) collisions are studied using Momentum Transfer Theory (MTT). It was shown that, not surprisingly, RCT collisions may be represented as a special case of elastic scattering. Using the developed MTT we tested a previously available anisotropic set of cross-sections for Ar+Ar ⁺ collisions by making the comparisons with the available data for the transverse diffusion coefficient. We also developed an anisotropic set of Ne+Ne ⁺ integral cross-sections based on the available data for mobility, longitudinal and transverse diffusion. Anisotropic sets of cross-sections are needed for Monte Carlo simulations of ion transport and plasma models. Received 16 June 2002 / Received in final form 2nd August 2002 Published online 24 September 2002 RID="a" ID="a"e-mail: vrhovac@phy.bg.ac.yu RID="b" ID="b"e-mail: zoran@phy.bg.ac.yu     

Two-color pump-probe experiments in helium using high-order harmonics

A pump-probe technique has been applied for measuring the lifetimes and absolute photoionization cross-sections of excited He states. The 1s2p ¹P and 1s3p ¹P states of He are excited by using the 13th and the 14th harmonic, respectively, of a tunable 70 ps dye laser generated in a Kr gas jet. The states are ionized after a varying time delay, by absorption of probe photons with energies between 1.6 and 4.5 eV. Lifetimes of τ(1s2p) = 0.57 ns and τ(1s3p) = 1.76 ns are determined with a precision of about 15%. A significant enhancement of the number of ions present in the lifetime curves at zero time delay for pressures above 6×10^-5 mbar is attributed to direct two-photon ionization of He in combination with AC Stark broadening of the excited state and absorption of the XUV light in the medium. Absolute photoionization cross-sections from the He 1s2p ¹P and He 1s3p ¹P states in the threshold region are determined by measuring the saturation of the ionization process with a precision of ∼ 25%. In addition, the variation of the relative orientation between the polarization vectors of the pump and probe beams enables the determination of partial photoionization cross-sections. Received 3 June 2002 / Received in final form 14 August 2002 Published online 22 October 2002 RID="a" ID="a"Present address: Department of Biophysics, Leiden University, 2333 CA Leiden, The Netherlands. RID="b" ID="b"Present address: Continuum Electro-Optics Inc., 3150 Central Expressway, Santa Clara, CA 95051, USA. RID="c" ID="c"Present address: CEA/DRECAM/SPAM, CEN Saclay, 91105 Gif-sur-Yvette, France. RID="d" ID="d"Present address: Department of Physics, National University of Rwanda, Butare, Rwanda. RID="e" ID="e"Present address: CELIA, Université Bordeaux 1, 33405 Talence, France. RID="f" ID="f"Present address: MAX-Lab, Lund University, Box 118, 221 00 Lund, Sweden. RID="g" ID="g"e-mail: anne.lhuillier@fysik.lth.se     

Registration of hydrogen-like leptonic bound states ( e-μ+) and ( e+μ-) in reactions of high-energy scattering of polarized electrons and positrons by nuclei with Z ～ 100 and analysis of CPT invariance

The cross-sections for the reactions of muonium (anti-muonium) production in the high-energy electron (positron) scattering by nuclei e ^-(e ⁺) + Z↦Z + M ⁰(ˉM) + μ^-(μ⁺) are calculated in dependence on energy and polarization of the initial electron (positron) and polarization of the final μ^-(μ⁺)-meson. Since this is a coherent phenomenon the cross-sections are proportional to Z². For Z ∼ 100, due to the factor Z², the cross-sections are large enough to be measured at the energies available for the HERA Collider at DESY. The results are discussed in connection with a test of CPT invariance. Received: 24 September 2002 / Accepted: 12 March 2003 / Published online: 27 May 2003     

Electron impact single detachment on the F - ions using the heavy ion storage ring CRYRING: Cross-section determination

Electron Impact Single Detachment (EISD) of F^- has been studied using the heavy ion storage ring CRYRING at the Manne Siegbahn Laboratory, Stockholm, Sweden. F^- ions stored in the ring were merged with an electron beam in one of the ring sections. Neutral F atoms produced in the EISD process were detected in the zero-degree direction using a surface barrier detector. The threshold for the detachment process was found to be around 7.6 eV, thus more than twice the binding energy of F^-. The cross-sections increased smoothly up to 55 eV where it reached a maximum of 1.9×10 ^-16 cm ² . At higher energies a slow decrease of the cross-section was observed, which follows the energy dependence predicted by the Bethe-Born approximation. The experiment showed that CRYRING can be used favourably for studies of anions, and several experiments are forthcoming. Received 14 June 2000 and Received in final form 11 September 2000     

Cross-section for collisions of ultracold Li with Na

In a magneto-optical trap (MOT) we are able to simultaneously trap and cool ⁷Li and Na. We investigated the loading behavior of the cloud of Li atoms in presence of the overlapped cloud of cold Na atoms, and, by blocking the weak repumping beam for Na, compared it with the loading curve for Li atoms only. Out of these loading curves we calculated the collision cross-section of Na on Li to be 10^-11 cm ³ /s. Received 11 January 2002 / Received in final form 5 April 2002 Published online 24 September 2002     

Merged beam study of the associative ionisation ( C + , N + ? and ? O + )+ O -

Total cross-sections have been measured for the associative ionisation of C ⁺ + O ^- , N ⁺ + O ^- and O ⁺ + O ^- by means of a merged-beam set-up operating with keV beams. These original measurements might be relevant to the understanding of some astrophysical objects or laboratory-made plasmas (flames and etching plasmas). The magnitude of these cross-sections is particularly large whatever the associating system, as these are in the range of 1×10 ^-14 cm² at thermal energies. Their behaviour as a function of energy significantly differs from one system to another, and is characterised by the Wigner law at low energy, and a rapid fall-off at higher energy due to competition with non-associative ionisation processes. Received 10 December 2001 and Received in final form 12 March 2002     

Spin-polarized Dirac-Slater calculations on the energies of hollow argon atom

In this work, the multiplet splitting in terms of a spin-dependent model is analyzed. The spin-polarized and unpolarized single configuration Dirac-Fock-Slater wavefunctions have been used in the evaluation of the total energies of highly ionized argon with different L shell population The transition energies of hollow argon atom with initial configurations 1s ⁰ _1/22s ^m _1/22p ⁿ _1/22p ^l _3/2 with m = 0 to 2 and n + l varying from 6 to 1 are reported in this work. The calculations have been carried out by taking into account a relativistic exchange potential in the Dirac-Slater potential. To account for the correlation effects, a correction term has also been considered perturbatively. The present calculations show that the spin-polarized technique which is mainly applied to the ground states of atoms may also be applied to atoms ionized in the inner shells with a good degree of accuracy. Received 5 December 2000 and Received in final form 9 April 2001     

Scattering of light exotic atoms in excited states

The differential and total cross-sections for the scattering of muonic, pionic, kaonic and antiprotonic hydrogen in excited states from atomic hydrogen have been calculated for the purpose of atomic cascade calculations. The scattering problem is treated in a fully quantum mechanical framework which takes the energy shifts and, in the case of the hadronic atoms, the widths of the ns states into account. The validity of semiclassical approximations is critically examined. Received 4 December 2001 and Received in final form 4 February 2002     

Properties of doubly excited states of H- and He associated with the manifolds from N = 6 up to N = 25

This paper discusses theory and results on ¹P⁰ doubly excited states (DES) in He and in H^- of very high excitation, up to the N = 25 manifold. Our calculations employed full configuration interaction (CI) with large hydrogenic basis sets and produced correlated wavefunctions for the four lowest roots at each hydrogenic manifold by excluding open channels and the small contribution of series belonging to lower thresholds. The suitability of the hydrogenic basis sets for such calculations is justified, apart from their practicality, by the fact that, by computing from them natural orbitals, the results were shown to be the same with those of earlier multiconfigurational Hartree-Fock (MCHF) calculations on low-lying DES. In total, 160 states were computed, most of them for the first time. Their energy spectrum should be of use to possible future photoabsorption experiments. For certain low-lying DES up to N = 13, for which previous reliable results are available, comparison of the calculated energies shows good agreement. The correlated wavefunctions contain systematically chosen single and double excitations from each hydrogenic manifold of interest. From their analysis, we determined the “goodness" of different quantum numbers and the geometry (average angles and radii) as a function of excitation. For the Sinano lu-Herrick ( K , T ) classification scheme, whose basis is a restricted CI with hydrogenic functions and which has thus far been tested only on low-lying DES, we established that, whereas T remains a good index as energy increases, K does not. Consequently, a more flexible than K quantum number is needed in order to account for most of the additional correlation. This number, represented by F = N - K - 1, where N and K are not good numbers anymore, produces consistently a much higher degree of purity than the ( K , T ) scheme does, especially as N increases and as the relative significance of various virtual excitations due to electron correlation increases. Among the four states of each manifold, in all cases in H^- and in most cases in He, the three are of the intrashell type and one is of the intershell type with ( F , T ) = (0, 0). The lowest intrashell states and the lowest intershell states exhibit a wide angle geometry tending to 180 ^° as N ↦∞. Received 10 September 2001 and Received in final form 12 November 2001     

Photoionization cross-sections of the first excited states of sodium and lithium in a magneto-optical trap

A two element magneto-optical trap (MOT) for Na and ⁷Li or ⁶Li is used to cool and trap each of them separately. A fraction of the cold atoms is maintained in the first ²P_3/2 excited state by the cooling laser. These excited state atoms are ionized by laser light in the near-UV region, giving rise to a smaller number of trapped atoms and to different loading parameters. Photoionization cross-sections were derived out of these data. They are in reasonable agreement with data previously obtained using thermal samples and with theoretical predictions. Received 21 March 2001 and Received in final form 3 August 2001     

Cold rubidium molecule formation through photoassociation: A spectroscopic study of the 0 g - long-range state of 87 Rb 2

We report the detailed analysis of translationally cold rubidium molecule formation through photoassociation. Cold molecules are formed after spontaneous decay of photoexcited molecules from a laser cooled atomic sample, and are detected by selective mass spectroscopy after two-photon ionization into Rb ₂ ⁺ ions. A spectroscopic study of the 0 _g ^- (5 S + 5 P _3/2 ) pure long-range state of ⁸⁷Rb₂ is performed by detecting the ion yield as a function of the photoassociation laser frequency; the spectral data are theoretically analyzed within the semiclassical RKR approach. Molecular ionization is resonantly enhanced through either the 2 ³ Π _g or the 2 ³ Σ ⁺ _g intermediate molecular states. Some vibrational levels of the latter electronic state are observed and assigned here for the first time. Finally, cold molecules formation rates are calculated and compared to the experimentally measured ones, and the vibrational distribution of the formed molecules in the a ³ Σ ⁺ _u ground triplet state is discussed. Received 18 January 2001 and Received in final form 10 April 2001     

Low energy electron attachment to formic acid

Using a high resolution electron energy monochromator low energy electron attachment to formic acid is studied for the first time by means of mass spectrometric detection of the product anions. The largest dissociative electron attachment (DA) cross-section produces HCOO ^-+H with weaker channels for OH^- and O^- becoming apparent at higher incident energies. Received 23 January 2002 and Received in final form 9 February 2002 Published online 13 September 2002     

Universality in diffusion front growth dynamics

We have studied the scaling properties of diffusion fronts by numerical calculations based on the mean field approach in the context of a lattice gas model, performed in a triangular lattice. We find that the height-height correlation function scales with time t and length l as C(l, t) ≈l ^α f (t/l ^α/β) with α = 0.62±0.01 and β = 0.39±0.02. These exponent values are identical to those characterising the roughness of the diffusion fronts evolving through a square lattice [1,2], thus confirming their universality. Received 14 November 2001 / Received in final form 20 April 2002 Published online 31 July 2002     

Investigation of α-nucleus interaction in the 27Al (α,α)27Al scattering and 27Al (α, d )29Si reaction

Full finite-range macroscopic calculations in the distorted-wave Born approximation have been performed using the molecular and Michel α-nucleus potentials to analyze the angular distributions of cross-sections of the ²⁷Al(α, d)²⁹Si reaction, at 26.5 and 27.2 MeV incident energies, leading to seven transitions up to the excitation energy E _X = 4.08 MeV of the final nucleus. The parameters of the two types of the α-nucleus potentials are determined from the elastic-scattering data. Both the molecular and Michel potentials, without any adjustment to the parameters needed to fit the elastic-scattering data, are able in most cases to reproduce, simultaneously, the absolute cross-sections particularly at large angles, where the previous calculations failed to reproduce by orders of magnitude, and the gross pattern of angular distributions of the reaction. The deuteron-cluster spectroscopic factors for most of the seven transitions, deduced using the two α-²⁷Al potentials, differ from those obtained in earlier works. The spectroscopic factor for the ground-state transition, deduced in the present work for the 25.8 MeV data, agrees well with the shell model prediction. Received: 15 July 2002 / Accepted: 8 August 2002 / Published online: 10 December 2002 RID="a" ID="a"e-mail: akbasak2001@Yahoo.com Communicated by G. Orlandini     

Σ- atoms and the ΣN interaction

It is shown that among four models of the Nijmegen baryon-baryon interaction only model F --which leads to a repulsive potential felt by the Σ hyperon inside the nucleus-- is consistent both with the analysis of Σ^- atoms and of the (K ^-,π) reactions. The Nijmegen models are used to determine the strong complex single-particle (s.p.) potential of Σ^-, and to calculate the strong-interaction shifts and widths of the lowest observed levels of Σ^- atoms. The results obtained with model F are in best agreement with the experimental data. Received: 15 January 2002 / Accepted: 14 March 2002     

Influence of the helium autoionization structures on the single/double ionization signal

Calculations of intense field (around 10¹⁶ W/cm²) single- and double-ionization processes in helium at XUV wavelengths are presented. The laser wavelength is chosen near the | 2s2p ¹ P autoionization structure and the dynamics are explored. Single and double ionization yields, as well as the photoelectron energy spectrum for photon energies around the autoionization structure are calculated. In the case of a pulse of few femtoseconds duration, no significant enhancement of the double ionization yield has been found in tuning the photon frequency around the peak of the resonance. It is also shown that in the case of a long pulse (and hence narrow compared with the relevant autoionization width), the branching ratio of double to single ionization yield can be relatively enhanced by tuning to the absorption minimum of the resonance. Received 19 February 2002 / Received in final form 2 May 2002 Published online 19 July 2002     

Measurement of the 24Mg(p, t)22Mg reaction for the states near the 21Na + p threshold

Differential cross-sections of the ²⁴Mg(p, t)²²Mg reaction were measured at 34.68 MeV for the states near the proton threshold at 5.502 MeV in ²²Mg. Among them, the new states at 5.962, 6.046, 6.246 and 6.323 MeV, which were reported previously, have been confirmed. Angular distributions for these states were analyzed by distorted-wave Born-approximation calculations to deduce the spins and parities. The angular distribution for the 5.714 MeV state, which is considered to be most crucial for the stellar reaction ²¹Na(p, γ)²²Mg, has been found to be consistent with J ^π = 2⁺ assignment. The 6.046 MeV state is newly assigned to have J ^π = 0⁺, and the 5.962 MeV state is tentatively assigned to have J ^π = (1^-). These two states will also play an important role for ²²Mg production in novae. Received: 7 March 2002 / Accepted: 7 May 2002     

Nonlinear Hanle effect in Cs vapor under strong laser excitation

We report results of a theoretical and experimental study of the ground state nonlinear Hanle effect under strong laser excitation. It is shown that besides the well-known zero-magnetic field suppression of absorption on F _g = F→F _e = F - 1 transitions caused by population trapping, an optical pumping induced enhanced absorption occurs on F _g = F→F _e = F + 1 transitions for small B-fields. The latter effect becomes more pronounced for high F values. The experiment with atomic vapor of Cs (D₂ line, F _g = 4) confirms an increase of the spectrally unresolved fluorescence yield at zero magnetic field and 600 mW/cm² laser intensity by 9% or 42%, when excitation occurs with linearly or circularly polarized light, respectively. The results of the experiment agree with numerical simulation studies using equations of motion for a density matrix. Received 24 November 2001 / Received in final form 25 March 2002 Published online 24 September 2002