Showing papers in "Journal of Physics B in 1991"

Theoretical aspects of intense field harmonic generation

Abstract: The authors present theoretical studies of high-order harmonic generation in a rare-gas medium. The experimental results obtained at Saclay with a 1064 nm Nd-YAG laser in the 1013 W cm-2 intensity range are summarized. The harmonic emission strengths, first decrease rather steeply for the first orders, then form a long plateau up to the 21st harmonic in xenon, or up to the 33rd harmonic in argon, before decreasing again rather abruptly. The theoretical description of these experiments consists first in the calculation of the photoemission spectra emitted by a single atom. The spectra are obtained by numerically integrating a time dependent Schrodinger equation for the laser-excited rare-gas atom. Second, one must account for collective effects in the medium, described by Maxwell's equations. A theoretical framework for describing the generation and propagation of harmonics in strong laser fields is developed. An numerical solution of the propagation equations for the harmonic fields in xenon at 1064 nm provides results which agree well with experimental data.

Two-centre effects in ionization by ion impact

Abstract: When an atom is ionized by ion impact, the electron is ejected into a final continuum state of a two-centre potential due to the Coulomb fields of the projectile and ionized atom. The related effects on the electron yield or energy and angular distributions are referred to as two-centre electron emission (TCEE). The present report is devoted to a discussion of experimental and theoretical evidence of this TCEE. The use of heavy ions or antiprotons as projectiles allows to unravel these effects by monitoring the two centre potential. On the theoretical side, the continuum distorted wave-eikonal initial state theory (CDW-EIS) accounts for the TCEE thus allowing a detailed interpretation of the experimental findings.

R-Matrix-Floquet Theory of Multiphoton Processes

Abstract: Since its first formulation [1, 2], the R-matrix-Floquet (RMF) method has now become a useful tool in a wide variety of non-perturbative calculations of atomic multiphoton processes in intense laser fields. The R-matrix approach allows an ab initio, accurate description of atomic structure, while the Floquet ansatz constitutes a fully non-perturbative description of multiphoton processes. These processes can be classified into the categories laser-assisted scattering, ionization and detachment, and harmonic generation in a laser field. The R-matrix-Floquet theory has been successfully applied to the calculation of all of these processes. The development of the methods and the implementation of the RMF theory has relied heavily on previous work which used the R-matrix approach for perturbative calculations of photoionization for a wide variety of atomic and ionic species [3]. Other authors have performed nonperturbative Floquet calculations, however so far mostly for single-active-electron systems [4].

Recent developments and new trends in (e, 2e) and (e, 3e) studies

Abstract: The latest, most important developments in (e,2e) studies are reviewed from an experimental point of view. These include ionization processes near threshold, the symmetric energy sharing collisions involving double scattering mechanisms, core states ionization, (e,2e) measurements for autoionizing states and for the simultaneous excitation and ionization of the target. Double ionizing processes with coincidence detection of two or three electrons in the final state are also reviewed, including (e,e'-Auger) experiments and the new born (e,(3-1)e) and (e,3e) domain.

Direct Observation of Macroscopic Charge Quantization

Abstract: The circuit formed by a nanoscale tunnel junction in series with a capacitance and a voltage source is the building block of most multi-junction circuits of single electronics. The state of this “single electron box” is entirely determined by the numbern of extra electrons on the intermediate “island” electrode between the junction and the capacitance. We have fabricated such a system and measured the charge on the junction capacitance, which is directly related to the average value ofn, as a function of the bias voltage using a Fulton-Dolan electrometer. At low temperature, the junction charge followed thee-periodic sawtooth function expected from the theory of macroscopic charge quantization. Strikingly,e-periodic variations were also observed when the box was superconducting. The thermal rounding of the sawtooth function is well explained by a simple model, except at the lowest temperatures.

Wannier theory for double photoionization of noble gases

Abstract: The Wannier theory not only gives the threshold laws for processes such as near-threshold double photoionization, but also predicts an angular correlation between the two escaping electrons. Within the frame of this theory and using an LS scheme the authors derive the triple, double differential and integral cross sections, for noble gases ions left in the ground or first excited states. The approach should allow a direct comparison with forthcoming experiments, as it uses the individual angles of the two electrons. In the case of unfavoured final states, which have a node when the angle between these electrons is 180', the triple differential cross sections depend critically on the angular correlation. The authors give their detailed structure in space for double ionization by both polarized and unpolarized photons. They also show that a particular choice of Euler angles allows a straightforward integration which leads to the double differential cross sections.

Investigations of above-threshold ionization using subpicosecond laser pulses

Abstract: At very high light intensities, the electron energy spectrum in multiphoton ionization (MPI) spectroscopy of even the simplest atoms changes from a single, well defined threshold peak into multiple peaks, separated from one another by the photon energy. This phenomenon is generally referred to as 'above-threshold ionization' (ATI). The original experiments investigating ATI used relatively long laser pulses, with the result that amplitudes, energy widths and angular distributions of the individual photoelectron peaks depended on the laser intensity. In addition, the widths of the peaks, as well as their absolute energy positions, changed according to the temporal width of the laser pulse. These dependencies were not intrinsic to the ionization process, but rather were all eventually ascribed to ponderomotive forces exerted on free photoelectrons by the laser focus. The ponderomotive effects frustrated comparisons between theoretical calculations and experimental data.

Semiclassical cycle expansion for the helium atom

Abstract: We analyse the classical dynamics of near-collinear electron configurations in helium. The dynamics turns out to be fully chaotic. An application of periodic orbit quantization techniques yields the energy of doubly excited states with high accuracy. The analysis shows that near-collinear intra-shell resonances are associated with an asymmetric stretch like molion of the electron pair rather than the symmetric stretch motion along the Wannier ridge. The purpose of this letter is threefold. We first analyse the classical dynamics for near-collinear arrangements of the two electrons in the helium atom. We find strong evidence that the resulting motion is fully chaotic in the corresponding symmetry plane whereas the linearized motion of the plane is stable. We then apply modern semi- classical techniques to quantize the chaotic dynamics and obtain the energies of certain doubly excited states. Finally, our results (together with numerically highly accurate quantum mechanical calculations) unambiguously show that the widely accepted viewpoint of electron pair propagation along the Wannier ridge for doubly excited 11 Prrmanent address: Max-Planck lnrlitut Tir Kcmphyrik. Postfach 103980. 6900 Heidelberg. Federal Republic of Gemany.

Excitation of the fine-structure transitions of C in collisions with ortho- and para-H2

Abstract: The authors present the results of ab initio (SCF and CEPA) calculations of the interaction potentials for the C(3P)-H2(1 Sigma g+) system. The five potential curves corresponding to collinear (3 Sigma -,3 Pi ) and orthogonal (3A2,3B1,3B2) approaches of the C atom to the midpoint of the H2 molecule have been computed, as well as the curve of 3A' symmetry for an intermediate geometry. These results are analysed and used in quantum mechanical (close-coupling) calculations of the cross sections for excitation of the fine structure transitions in C(3PJ) by ortho- and para-H2. The rate coefficients for these transitions are evaluated for a range of temperatures encompassing those relevant to studies of the interstellar medium.

An atomic trap based on evanescent light waves

Abstract: A new type of gradient light trap is suggested for neutral atoms, based on the evanescent fields of two laser beams experiencing total internal reflection at a dielectric- vacuum interface A distinctive feature ofthe trap is that its bottom is close to the interface: at a distance of the order of the wavelength of light

Bound states using the R-matrix method: Rydberg states of HeH

Abstract: A method is presented for adapting scattering calculations performed with the molecular R-matrix method to find bound states based on the atomic method of Seaton. Quantum defect theory is used to determine initial energy grids and to determine whether all the bound states have been located. This method is particularly suited to the Rydberg states of electron plus molecular ion systems. The authors calculate and assign the lowest 33 electronic states of the HeH molecule. Previously on 14 of the lowest (n<5) bound states have been fully characterized, with several states omitted. They suggest that the omitted states give rise to some of the observed but previously unexplained weak transitions. Vibrational motion is included in their calculations within the adiabatic approximations. Effects arising from short-range correlations and nuclear motion are shown to be very significant for the lowest electronic states. Transition energies amongst the excited states agree with accurate spectroscopic determinations to better than 50 cm.

E1 transitions of astrophysical interest in neutral oxygen

Abstract: Atomic transition rates and f values have been calculated in the triplet and quintet systems and for some intercombination lines of neutral oxygen. Configuration interaction was included in the calculations performed with the CIV3 code of Hibbert (1975) for all the transitions connecting the n=3 and n=4 energy levels. In order to reproduce the observed energy splittings between the energy states, small empirical adjustments were introduced to the diagonal matrix elements. Comparisons with previously published f values and radiative lifetimes are discussed and the subsequent astrophysical applications of the results are briefly mentioned.

Enclosed quantum systems: use of the direct variational method

Abstract: A variational boundary perturbation method for solving Schrodinger's equation for enclosed quantum systems with infinite potential walls has been recently proposed by Gorecki and Byers Brown. Their treatment was based on the use of an input wavefunction chi =Gc0 f, where chi 0 is the wavefunction of the free system and f a non-singular function that vanishes at the boundary. The class of functions f was further used to minimize the total energy. In the present work the direct variational method is applied to the ansatz function chi by assigning the variational parameter to the free-system wavefunction, once an adequate cut-off function, f, is defined. This is in contrast to the previous authors' work. The procedure is applied to hydrogen and helium atoms within hard spherical boxes and also to a hydrogen atom confined between two impenetrable parallel walls. The results are in fair agreement with exact calculations.

Saddle structure of the three-body Coulomb problem; symmetries of doubly-excited states and propensity rules for transitions

Abstract: The separable two-centre Coulomb problem is investigated with emphasis on the saddle structure of its potential. A new quasi-degeneracy between molecular orbitals (MO) is identified. The authors demonstrate that the results have threefold relevance for the formation and character of symmetric doubly-excited states: (i) it is shown that all features of Herrick's various multiplet classifications can be explained in terms of MO saddle dynamics; (ii) for the first time quantitative results of resonance energies from the adiabatic MO approach are reported and compared to other theoretical and experimental data; (iii) the propensity rules for radiative and nonradiative transitions are summarized and details of their derivation within the framework of motion in the vicinity of the potential saddle are given.

Measurement of post-collision interaction effects for p on Ne collisions

Abstract: The authors have studied the post-collision interaction distortion of the Ne K-L2.32(1D2) Auger electron line excited by proton bombardment in the energy range 0.7-2.0 MeV. A systematic study of both the bombarding energy and the observation angle dependence has been performed for the first time. They have found the effect to be most pronounced at 0' observation angle and at the bombarding energy where the projectile and the outgoing Auger electron have about the same velocity. The results are in a reasonable agreement with theories which assume PCI between the Auger electron and the outgoing projectile.

Relativistic distorted-wave calculation of electron impact excitation of xenon

Abstract: A relativistic formulation of the distorted-wave method is presented for electron impact excitation of xenon. Calculations are performed for xenon 6s(3/2)01 and 6s(1/2)01 states with both the target atom and scattered electron treated fully relativistically. The wavefunctions representing the distorted waves are fully antisymmetrized to account for electron exchange. Differential cross sections and collisional alignment and orientation parameters are presented and compared with recent experimental data. The overall agreement with experiment is very satisfactory. The occurrence of changes in the spin orientation of the scattered electron due to both exchange and relativistic effects is predicted. The possibility of experimentally observing these spin changes in the collision frame through electron-photon coincidence measurements is pointed out.

Generalizations of the virial relations for the Dirac equation in a central field and their applications to the Coulomb field

Abstract: The virial relations for the Dirac equation are generalized for the case of the non-diagonal radial matrix elements and for the case when the author integrates over an arbitrary finite interval of the radial variable. The case of the Coulomb field is considered separately. Recurrence relations between the solutions of some class of inhomogeneous equations appearing in the relativistic calculations of atoms according to perturbation theory are derived. Some applications of the obtained relations are discussed. In particular, a possibility of their application to the calculations of hyperfine structure of multiply charged ions in the first and higher orders of perturbation theory is demonstrated. The corresponding non-relativistic analogues of the relations obtained are also considered.

Elastic DCS for e+CH4 collisions, 1.5-100 eV

Abstract: Absolute differential, integral and momentum transfer cross sections for electron-methane collisions were determined using a crossed beam spectrometer for impact energies from 1.5-100 eV and angles from 10'-130'. Comparison with published total cross sections shows good agreement. At the shape resonance (7.5 eV) excitation measurements of the two vibrational peaks were taken and the angular distribution of their DCS was determined. The elastic DCS are compared with recent measurements and theoretical calculations.

Determination of the electron-ion dissociative recombination coefficients for several molecular ions at 300 K

Abstract: The electron-ion dissociative recombination rate coefficients, alpha e, have been determined for ten molecular ions, seven of which have not previously been measured. The data were obtained under truly thermal conditions at 300 K using a flowing afterglow/Langmuir probe (FALP) apparatus. Recombination coefficients were obtained for the following ions: CO2+ ( alpha e=3.1*10-7 cm3 s-1), N2+ (2.0), CO+ (1.6), KrH+ (<0.2), XeH+ (<0.4), HSO2+ (2.7), CH3CNH+ (3.3), CH3CHOH+ (3.9), C2H5CNH+ (4.7) and ((CH3)2CO)2H+ (14). The dissociative recombination of many of the ions studied may be important in planetary, cometary and terrestrial atmospheres and be involved in the synthesis of interstellar molecules.

Elastic scattering and rovibrational excitation of H2 by low-energy electrons

Abstract: The authors report differential and integral cross sections for elastic scattering and rovibrational (v=0 to 1) excitation of H2 by electrons at seven energies in the range 1-5 eV. The measurements were conducted on a fine energy grid in an attempt to resolve an impasse that had arisen between theory and 'swarm' derived cross sections and to enable a detailed comparison to be made with the best available theoretical calculations.

Triply differential cross sections for ionization of hydrogen atoms by electrons: the intermediate and threshold energy regions

Abstract: Triply differential cross sections in coplanar geometry for the ionization of atomic hydrogen by electron impact have been measured and calculated at incident electron energies of 54.4, 27.2, 16.5 and 15.6 eV. The theory is the same as that used successfully at higher energies and is shown to reproduce nearly all features of the cross sections in this low-energy regime as well. Moreover, the physical origin of peaks and dips in the triply differential cross sections has been explained. The authors conclude that kinematical effects, exchange effects, correlation and the coherence between electron-electron and electron-proton scattering are all important at these energies.

Some aspects of total scattering cross sections of electrons for simple hydrocarbon molecules

Abstract: Total electron scattering cross sections for C2H4, C2H6, C3H6 (propene and cyclopropane) and C3H8 have been measured in the energy range between 4 and 500 eV. Measured results show a remarkable isomer effect and dependence on the geometrical dimension of the molecule.

Nitrogen atom recombination in high pressure Ar-N2 flowing post-discharges

Abstract: The afterglow resulting from the nitrogen atom recombination in a flowing Ar-N2 microwave discharge (915 MHz and 100 W) is characterized by the first positive emission from N2 (B,V'). This emission occurs with a vibrational enhancement which is shifted from V'=11 in pure nitrogen to V'=8 in an Ar-2%N2 mixture. The N-N recombination rates producing N2(B,V') in Ar-x%N2 mixtures (x varies from 2-100%) were calculated from the measurement of the first positive emission intensity by spectrometry and the determination of the nitrogen atom concentration by a NO titration. Normalized values were obtained from previous published data in pure nitrogen.

The role of Rydberg states in H 2 + dissociative recombination with slow electrons

Abstract: The cross section for H+2 dissociative recombination with slow electrons is calculated using the multichannel quantum defect theory, with the set of molecular data used for the recent study of H+H* associative ionization by Urbain et al. Excited vibrational levels of s sigma g and d sigma g Rydberg states show up as broad resonances, mostly dips, in satisfying agreement with the vibrationally selected measurements of Mitchell et al. The authors analyse the mechanisms for resonance formation and discuss the adequacy of the theoretical treatment.

Absolute vibrationally elastic cross sections for electrons scattered from water molecules between 6 eV and 50 eV

Abstract: The authors report absolute vibrationally elastic differential cross sections for electrons scattered from water in the energy range 6-50 eV. From an extrapolation of the differential cross section data, absolute integral and momentum transfer cross sections have also been obtained. For incident electron energies above 20 eV and below 100 eV there exists only one previous report of differential cross sections (Danjo and Nishimura (1985)). Significant discrepancies have been found in this region between the cross sections reported by Danjo and Nishimura and those obtained in the present work.

Absolute total cross sections for electron scattering on CH4 molecules in the 1-4000 eV energy range

Abstract: Absolute total cross sections for electron-methane scattering have been measured in the energy range from 0.9 to 4000eV in two separate experiments. The present results, the first available above 500 eV, are in good agreement with other recent experimental and theoretical data. The law energy scattering is dominated by a shape resonance; a broad hump has been detected centred around 100eV. A short critical review of previous measurements is given.

Valence Auger and X-ray participator and spectator processes for neon and argon atoms

Abstract: The rates of decay, both radiative and non-radiative, have been calculated for the valence shake-up states (vs, vp)nl of Ne(v = 2) and Ar(v = 3) for the five lowest n values. The various decay channels are discussed and trends for large n are examined. The calculations are performed at the single-configuration non-relativistic Hartree-Fock level. Inner-valence and valence-multiplet Auger channels are shown to be completely dominant, with maximum fluorescence yields of 2.7% in Ne and 0.2% in Ar. The total decay rates can be larger than the corresponding atomic K-shell rates for certain states, an extreme example being the Ar (3s, 3p)(1P)3d(2P) rate of 0.148 au. The calculated widths are shown to be consistent with the experimental results of Svensson et al (1988). The valence-multiplet participator Auger transition is a mode of decay involving a change of multiplet state for the same initial and final two-hole valence configuration, and has been shown to account for the difference in width between certain photoelectron satellites in Ne. The experimental spectrum of Becker et al (1989) is discussed in the light of present results. The valence decay mechanisms identified are expected to have general applicability to light atoms and to molecules containing first or second row atoms.

Resonance ionization spectroscopy on a fast atomic ytterbium beam

Abstract: Resonance ionization spectroscopy in collinear geometry has been successfully applied to a fast beam of ytterbium atoms. The atoms were excited stepwise into a Rydberg state by pulsed laser light, ionized in an electrical field and deflected onto a secondary electron detector. The efficiency was 1*10-5 detected ions per incoming atom on a background from collisional ionization of 1*10-8. The technique has been exploited for the measurement of hyperfine structures and isotope shifts of unstable ytterbium isotopes, in particular 157Yb, 159Yb and 175Yb.

Electron collision cross sections at low energies for all transitions between the n=1, 2, 3, 4 and 5 levels of atomic hydrogen

Abstract: Collision strengths for all transitions among states up to the n=5 level are reported in the energy range below 1.96 Ryd. The R-matrix and no-exchange programs have been used for partial waves with L 9 respectively. The results are analysed in detail highlighting the resonance structure. They are also compared with other results. The effective collision strengths obtained after averaging the collision strengths over a Maxwellian distribution of electron velocities are listed at temperatures below 50000 K. Though the results require an improvement especially in the intermediate energy region, nevertheless they are probably the best available for most of the transitions.

State-selective electron capture in collisions of He2+ with H

Abstract: Charge transfer into He+ (nl) states (n)5) in collisions (2-13 keV amu-1) of He2+ on atomic hydrogen has been studied by means of photon emission spectroscopy. State selective electron capture cross sections into the degenerate He+(4l) states have been determined by deconvoluting the intensity distribution of the He II (n=4 to n=3) emission along the total ion-beam path inside the collision chamber. They are compared with theoretical results, especially with the large-basis set extended atomic orbital calculations of Fritsch (1988). Generally good agreement with these extended calculations is found. However, the structures in the theoretical cross sections for capture into the non-dominantly populated n=4 states are not observed experimentally.