Table of contents

In this letter we discuss the double ionization of helium in intense, elliptically polarized laser fields. We extend a model previously used for linear polarizations to the case of a two-dimensional model atom subject to a laser field of arbitrary ellipticity. Our results show a decrease in the efficiency of the non-sequential double ionization as the ellipticity of the driving field is increased, consistent with the recollision picture of double ionization. Our results are in qualitative agreement with previous experimental measurements.

We present a study of the sub-shell populations of 13.6 MeV/u Ar¹⁷⁺ions after transmission through thin carbon foils. We show that the combined effect of the wake field induced by the ion in the solid and multiple collisions leads to a strongly enhanced population of high angular momentum states. These results explain new experimental data for absolute total line emission intensities.

The concepts developed for direct photo-double ionization are deployed to analyse recent experimental data for indirect or resonant photo-double ionization of Ca in the region of the 3p-3d resonance given by Ross et al . As in the direct process the features in the triple differential cross sections can be understood in terms of selection rules, final-state electron correlation and interference effects. For most directions of the two-electron momenta strong destructive interference between various partial waves is found.

We demonstrate experimentally the difference between a sequential interaction of a femtosecond laser field with two electrons and a nonsequential process of double ionization mediated by electron-electron correlation. This is possible by observing the momentum distribution of doubly charged argon ions created in the laser field. In the regime of laser intensities where the nonsequential process dominates, an increase in laser power leads to an increase in the observed ion momenta. At the onset of the sequential process, however, a higher laser power leads to colder ions. The momentum distributions of the ions from the sequential process can be modelled by convolving the single-ionization distribution with itself.

We have explored experimentally a novel possibility to study exoergic cold atomic collisions. Trapping of small countable atom numbers in a shallow magneto-optical trap and monitoring of their temporal dynamics allows us to directly observe isolated two-body atomic collisions and provides detailed information on loss statistics. A substantial fraction of such cold collisional events has been found to result in the loss of one atom only. We have also observed for the first time a strong optical suppression of ground-state hyperfine-changing collisions in the trap by its repump laser field.

We compare the form of the Casimir-Polder interaction between a ground-state atom and a reflecting surface with the form obtained from a semiclassical calculation based on the method of images. The difference between these potentials is attributable to operator ordering and to the fundamentally different role played by the positive- and negative-frequency parts of the electric field in classical and quantum electrodynamics. Only with normal ordering does the images method lead to the fully quantum result.

Effective collision strengths are evaluated from electron-impact excitation collision strengths of doubly ionized atomic neon (Ne III) calculated in the close-coupling approximation using the ab initioR -matrix method. The 56 fine-structure levels arising from the 28 LStarget states; 2s² 2p⁴ , 2s2p⁵ , 2s² 2p³ 3 ( = s,p,d) and 2s² 2p³ 4s are retained in the present calculation. The effective collision strengths ( ) for all forbidden transitions within the 1s² 2s² 2p⁴³ P_2,1,0 , ¹ D₂ , ¹ S₀ground configuration levels are obtained by averaging the electron collision strengths ( ), for a wide range of incident electron energies, over a Maxwellian distribution of velocities. Results are presented for electron temperatures in the range 1000-1 000 000 K, applicable to many laboratory and astrophysical plasmas. We believe that the results are the most accurate to date and so should be used in preference to previous values available in the literature.

This paper discusses the use of quantum scattering methods for the evaluation of the elastic (rotationally summed) integral and differential cross sections produced in collisions at low energies (E_coll 20 eV) of electrons and positrons with polyatomic gaseous targets. The calculations use close-coupled equations and a parameter-free model of electron and positron interactions with the molecules in a single-collision situation. Comparison with existing experiments on the collision processes with the two projectiles helps us to know more detail about the features of the forces in play during the scattering events. Computed results are also found to be in good accord with experiments.

The ejected-electron spectrum for H⁺incident on H₂and He at 40 keV is examined experimentally and theoretically. The continuum-distorted-wave eikonal-initial-state (CDW-EIS) model is adopted to study the ionization process. Experimentally a 260 mm diameter hemispherical analyser with half-angle acceptance of 1.5° and energy resolution E /Eof 0.012, is used to measure the velocity distribution of the emitted electrons. The analyser and the surrounding interaction region are double shielded against the Earth's magnetic field by use of µ-metal enclosures. The observations and calculations, for double differential cross sections (differential with respect to electron momentum and emission angle), are in very good agreement and show a spectrum dominated by a well defined electron capture to the continuum (ECC) peak although existence of saddle-point electrons at 0° is not confirmed.

Electron correlation has been studied in the singly ionized 5p^-1 , 5s^-1and 4d^-1states and the double ionized (5s, 5p, valence)^-2states of Ba by carrying out extensive configuration interaction Hartree-Fock calculations including the mixing of configurations with outermost valence electrons (nl= 6s,5d,6p,4f, ... ). The results of calculations have been compared with existing experimental photoelectron, ion-yield, electron-ion coincidence, and Auger electron spectra.

Relativistic effective core potential (RECP) and all-electron Dirac-Coulomb calculations of transition energies for low-lying states of the mercury atom and its ions are carried out with equivalent basis sets by the relativistic coupled-cluster method. RECP results are compared with corresponding all-electron values to estimate the accuracy of different RECP versions. Contributions from correlations of different electron shells to the calculated transition energies are studied. Effects of different nuclear models and of basis set truncation at different orbital angular momenta as well as errors of the Gaussian approximation of the generalized RECP components are reported. We show that at least 34 external electrons of the mercury atom should be correlated and the one-electron basis set should contain up to h-type functions in order to attain consistent agreement within 200 cm^-1with experimental data.

We present absolute normalized differential cross sections (DCSs) for electron-impact excitation of the three individual electronic states of argon: 4s´[½]₁ , 4p[½]₁and 4p´[½]₀ . The measurements were performed with incident electron energies of 16, 20, 30, 40, 50 and 80 eV, scattering angles between 5° and 150° and an angular resolution of better than 2°. The absolute scale for the reference state (4s´[½]₁ ) was established by experimental determination of inelastic-to-elastic intensity ratios for the 4s´[½]₁state at the above impact energies. Absolute inelastic DCSs for the other two states were determined using the intensity ratios of the peaks in the energy-loss spectra. The absolute DCSs were extrapolated to 0° and 180° and integrated numerically to yield integral, momentum transfer and viscosity cross sections.

Excitation energies and oscillator strengths are calculated for all electric dipole transitions between the individual levels of the 1s² 2s² , 1s² 2s2p and 1s² 2p²states for a range of Be-like ions from B II to Kr XXXIII. The configuration interaction wavefunctions used in these calculation were generated by the CIV3 code. Relativistic effects were included by using the Breit-Pauli approximation. The results are compared with recent calculations and are found to be in good agreement with more elaborate calculations.

Electronic energy loss straggling is treated within the sudden approximation. Sum rules are derived for the first and the second energy moments of the transition probability function. On this basis an expression is found which relates the energy loss straggling of fast multiply charged ions to the two-electron position correlation function of the target. This spatial correlation effect leads to a reduction of straggling compared with the result of the binary collision model. The conditions /₀ >>1 and Ze² / >1 ( is the ion velocity, ₀is the Bohr velocity, Zis the ion charge) are required for applicability of the theory.

We have studied the mechanisms of giant resonance in 4d photoionization of Eu atoms by density functional theory with the optimized effective potential and self-interaction correction method. The dynamic electron correlation is taken into account by the linear density response method. Our calculated photoionization cross sections in the giant resonance region are in good agreement with experimental measurements. Since Eu is a highly spin polarized system, by decomposing the contribution of 4d electrons in spin-up and spin-down states, we clearly identified that the strong asymmetry line profile of the giant resonance is due to a broad resonance of 4d spin-down electrons interaction with a sharp resonance of 4d spin-up electrons.

We have measured the two-electron contribution of the ground state energy of helium-like argon ions using an electron beam ion trap (EBIT). A two-dimensional map was measured showing the intensity of x-rays from the trap passing through a krypton-filled absorption cell. The independent axes of this map were electron beam energy and x-ray energy. From this map, we deduced the two-electron contribution of the ground state of helium-like argon. This experimentally determined value (312.4±9.5 eV) was found to be in good agreement with our calculated values (about 303.35 eV) and previous calculations of the same quantity. Based on these measurements, we have shown that a ten-day absorption spectroscopy run with a super-EBIT should be sufficient to provide a new benchmark value for the two-electron contribution to the ground state of helium-like krypton. Such a measurement would then constitute a test of quantum electrodynamics to second order.

Ab initiocalculations are performed for the two species involved in the attachment of an electron to Cl₂ : the neutral ground state (¹_g⁺ ) and the four anionic states (²_u⁺ , ²_g , ²_u , ²_g⁺ ). Four approaches are used: CASPT2, CASPT3, AQCC and CCSD(T). The good agreement with the known spectroscopic constants for Cl₂gives confidence in the present results. The neutral-anionic crossings have been carefully located. For the most reliable method, CCSD(T), we found the crossings at 95, 441, 955 and 1276 meV above the neutral ground vibrational level, for ²_u⁺ , ²_g , ²_uand ²_g⁺anionic curves respectively. The recently observed resonance around 100 meV in electron scattering is thus assigned to the lowest crossing only, in contradiction with the previously suggested mechanism which also involved the states of Cl₂^- .

The Coulomb explosion of carbon dioxide in a 55 fs laser pulse of intensity 1.5-3 × 10¹⁶W cm^-2has been studied using a variety of techniques based on time-of-flight mass spectroscopy. Covariance mapping has been used to identify Coulomb explosion channels and to measure the associated kinetic energy release. By comparing time-of-flight spectra taken with linearly and circularly polarized light, a clear signature of laser-induced reorientation is found, which is strongest for the lowest Coulombic channel. Ion-momentum imaging coupled with Monte Carlo simulation shows that the zero-point bend-angle distribution is better preserved than for longer laser pulses. However, some residue of the sequential processes dominant in much longer pulses is found.

We present a theoretical model which for the first time includes a metallic surface, a periodic potential, a strong laser field, and the transportof the electronic charge in the medium. We show that the photoemission process is strongly enhanced by the presence of localized impurities close to the surface and that the generation of high-order harmonics is affected by the dynamics of the wavepacket in the presence of the screened electric field.

Differential cross sections for 18 excited electronic states and 4 composite excited electronic states in nitric oxide (NO) have been determined for incident electron energies of 15, 20, 30, 40 and 50 eV. These differential cross sections were obtained for the scattered electron angle range 10°-90° by analysing electron energy-loss spectra in NO at a number of fixed scattering angles within that range. This study appears to be the first comprehensive investigation of electron-impact excitation of the electronic states of NO to be reported in the literature.

Integral cross sections (ICSs) for the excitation of 18 excited electronic states, and four composite excited electronic states, in nitric oxide (NO) have been determined for incident electron energies of 15, 20, 30, 40 and 50 eV. These ICSs were derived by extrapolating the respective measured differential cross sections (M J Brunger et al2000 J. Phys. B: At. Mol. Opt. Phys.33783) to 0° and 180° and by performing the appropriate integration. Comparison of the present ICSs with the results of those determined in earlier optical emission measurements, and from theoretical calculations is made. At each incident energy considered, the current ICSs are also summed along with the corresponding elastic and rovibrational excitation ICSs from B Mojarrabi et al(1995 J. Phys. B: At. Mol. Opt. Phys.28487) and the ionization cross sections from Rapp and Englander-Golden (1965 J. Chem. Phys.431464), to derive an estimate of the grand total cross sections (GTSs) for e^- + NO scattering. The GTSs derived in this manner are compared with the results from independent linear transmission experiments and are found to be entirely consistent with them. The present excited electronic state ICS, and those for elastic and rovibrational excitation from Mojarrabi et al , appear to represent the first set of self-consistent cross sections for electron impact scattering from NO.

Absolute three-photon detachment cross sections are calculated for the fluorine negative ion within the lowest-order perturbation theory. The Dyson equation of the atomic many-body theory is used to obtain the ground-state 2p wavefunction with correct asymptotic behaviour, corresponding to the true (experimental) binding energy. We show that in accordance with the adiabatic theory (Gribakin and Kuchiev 1997 Phys. Rev.A 553760) this is crucial for obtaining absolute values of the multiphoton cross sections. Comparisons with other calculations and experimental data are presented.

The problem of the Thomas-Fermi atom in strong static homogeneous electric fields is revisited. The static field situation is a reasonable approximation to the otherwise fully time-dependent problem of an atom in a laser pulse, provided that the laser frequency is not too high. Exact results for the ionization degree versus the applied electric field, obtained numerically, are compared with those from an approximate model which is much easier to solve. Very good agreement is found. Although the approximate model was published ten years ago, an unexplained discrepancy with both experimental results and the exact numerical solution of about one order of magnitude in the ionization degree had remained. In this paper it is shown that this discrepancy does not exist. A comparison with the simpler `over the barrier ionization' model and available experimental data for rare gases is made. A very appealing feature of the Thomas-Fermi model is its universalitythanks to the scaling behaviour with respect to the nuclear charge Z .

The applicability of the Born-Bethe approximation to electron-atom (molecule) total cross sections in the energy range below 10 keV is discussed. The need for further more precise measurements is stressed.