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E-Verhandlungen 2001
Programm und Abstracts der Sitzung A 5

High resolution measurements and multichannel quantum defect analysis of the Kr(nd¢ J = 2,3) autoionizing resonances

•M.-W. Ruf¹, D. Klar¹, M. Aslam², M. A. Baig², K. Ueda³ und H. Hotop^1
1Fachbereich Physik, Universität Kaiserslautern, Germany
²Atomic and Molecular Physics Laboratory, Quaid-i-Azam University, Islamabad, Pakistan
³Research Institute for Scientific Measurements, Tohoku University, Sendai, Japan

Using resonant two-photon laser excitation of metastable Kr(5s) atoms via selected Kr(5p) levels, we have recorded the low-lying autoionizing resonances Kr(nd¢, J = 2, 3) (n = 6, 7) at high resolution in two different experiments (atomic beam spectroscopy and optogalvanic spectroscopy in a discharge). Accurate values for the resonance positions (quantum defects) and the (reduced) widths have been determined and profile indeces are reported. For comparison, multichannel quantum defect theory (MQDT) analyses, using energy dependent MQDT parameters, have been carried out for the odd Kr(J = 1,2,3) levels, yielding good agreement in the quantum defects and partial agreement in the predicted reduced widths for the nd¢ resonances with the experimental values. Moreover, simplified phase-shifted quantum defect theory analyses of the measured bound odd Kr(J = 2 and 3) Rydberg levels were carried out with the aim to provide improved insight into the dominant decay mechanism of the resonances. This work has been supported by the Deutsche Forschungsgemeinschaft (Forschergruppe Niederenergetische Elektronenstreuprozesse) and through the Graduiertenkolleg Laser- und Teilchenspektroskopie.

Photoelectron Diffraction Mapping: Molecules Illuminated from Within

•Reinhard Dörner¹, Allen Landers², Thorsten Weber³, Imad Ali³, Amine Cassimi⁴, Mirko Hattass³, Ottmar Jagutzki³, Andreas Nauert³, Timur Osipov⁵, Andre Staudte³, Mike Prior⁶, Lew Cocke⁵ und Horst Schmidt-Böcking^3
5Kansas State University, Manahttan Kansas, USA
⁶LBNL, Berkeley CA, USA
¹University Freiburg, Germany
²Western Michigan University., Kalamazoo, Mi, USA
³University Frankfurt, Germany
⁴CIRIL/CEA/CNRS/ISMR CAEN, Frannce

Much of our knowledge of the internal structure of matter results from the scattering and diffraction of electrons or X-rays. In most cases, the matter under investigation is in the solid form and can therefore be easily oriented in the laboratory. However, because much of the interesting matter found in nature is in the gas phase, it is important that we seek methods which allow the detailed exploration of randomly oriented systems. We demonstrate the use of a multiparticle coincidence technique to image the diffraction of an electron wave whose source is a specific site in a free molecule, i.e. core-level photoelectrons are used to illuminate the molecule from within. We illustrate this technique for CO, by creating a photoelectron from the C(1s) shell. A richly structured electron diffraction pattern is obtained in a body-fixed frame of the randomly oriented molecule in the gas phase.

Cross-section ratio of double to single ionization of Helium

•Andreas Muthig¹, L. Spielberger¹, R. Dörner¹, O. Jagutzki¹, H. Bräuning¹, Th. Weber¹, J. Wang², Y. Qiu³, J. Burgdörfer³ und H. Schmidt-Böcking^1
1Insitut für Kernphysik, Universität Frankfurt, August Euler Str.6, 60486 Frankfurt
²Department of Physics, Tulane University, New Orleans, Louisiana, 70118-5698
³Department of Physics, University of Tennesee 37831-6377

We have measured the ratio of double to single ionization of helium by high energy photons R_C = s⁺⁺/s+ at photon energies of 40, 75 and 100 keV. The technique used was recoil ion momentum spectroscopy. The ratio was found to be slightly energy-dependent. The theoretical calculations underline the importance of final-state-correlations which are still not negligible at these photon energies.

Apart from the ratio of total cross sections, the momentum-distribution of the singly-charged He-Ions was measured and is in agreement with the theoretical prediction.

ON THE SHORT LIVED RADIOISOTOP IONIZATION

•Viorica Gheorghe^1,2 und G. Huber^1
1Institut für Physik, Johannes Gutenberg Universität, 55099 Mainz
²National Institute for Laser, Plasma and Radiation, Bucharest, Romania

In this paper a simulation based on analytical models [1,2] describing the step-wise ionization of Beryllium exposed to two-colour pulsed laser excitations is presented. We displayed the evolution of the population in the autoionizing state of Beryllium, but also on the different levels which are intermediate steps of the studied ionization process. The experimental parameters for which the ionization process of Beryllium is optimal were distinguished.

This simulation is particularly important in order to identify the conditions for an efficient ionization of a short lived radioisotope from the resonance ionization laser ion source (RILIS) at CERN-ISOLDE [3].

[1] V. N. Gheorghe, Appl. Phys. 46, 185-190 (1988)

[2] D. Boudreau, P. Ljungberg, O. Axner, Spectrochimica Acta Part 51, 413-428 (1996)

[3] V. Sebastian, R. Catherall, V. N. Fedoseyev, U. Georg, G. Huber, Y. Jadin, O. Jonsson, U. Köster, M. Koizumi, K. -L. Kratz, E. Kugler, J. Lettry, V. I. Mishin, H. L. Ravn, C. Tamburella, A. Wöhr and the ISOLDE Collaboration, AIP Conf. Proceedings 455, ENAM'98, Bellaire, Michigan, USA, 126 (1998)

Orientation and alignment of photoionized atoms

U. Kleiman und B. Lohmann
Universität Münster, Institut für Theoretische Physik, Wilhelm-Klemm-Straße 9, D-48149 Münster, Germany

The knowledge of orientation and alignment of excited or ionized atoms is essential for determining the properties of the emitted Auger electrons. The angular distribution and dynamic spin polarization depend on the alignment, which has already been calculated for various ions within the Herman-Skillman approximation [1]. The transferred spin polarization of the emitted Auger electrons, however, originates from the orientation of the excited or ionized atoms. Though the transferred spin polarization is in general large and numerous experimental studies have been done in recent years, hardly any numerical values of the orientation are available.

In view of this we have calculated both, alignment and orientation. The calculation has been performed within the Hartree-Fock approximation and considers all singly photoionized atoms with a ¹S₀ ground state configuration where all sorts of ionic hole states from 2p_1/2 up to 6p_3/2 have been taken into account. The kinetic energy of the emitted photoelectron has been varied between zero and at least 20 Ry. Within the considered energy region the dipole approximation is applicable.

Our results are compared with Herman-Skillman calculations [1] and different experimental data.

[1] E. G. Berezhko, N. M. Kabachnik and V. S. Rostovsky: J. Phys. B: At. Mol. Phys. 11, 1749 (1978)

Spin polarisation in the resonant Auger decay of argon

•Burkhard Langer¹, György Snell^2,3, Sophie Canton^2,3 und Nora Berrah^2
3Advanced Light Source, LBL, Berkeley, CA, USA
¹Max-Born-Institut, Berlin, Germany
²Western Michigan University, Kalamazoo, MI, USA

The dynamics of the photoionization and Auger decay processes can be fully described by the corresponding dipole and Coulomb transition matrix elements, respectively. A complete set of such matrix elements, amplitudes and phase shifts, enables the prediction of all parameters of the emission process, such as the angular distribution and spin polarisation of the electrons. In recent years, significant progress was achieved in the field of complete photoionization experiments. In contrast, only very few spin polarisation studies have been carried out for Auger decay processes [1]. Following the first measurements of the spin polarisation of resonant Auger electrons in Xe [1] we have measured the spin polarisation of resonant Auger electrons emitted from the decay of the Ar 2p^-1® 4s and 2p^-1 ® 3d excited states for the first time. We used monochromatized, circularly and linearly polarised light of 245 eV photon energy from the new elliptical polarization undulator (EPU) beamline at the Advanced Light Source (ALS) to carry out the experiment. Some of the Auger lines show a high degree of polarization, which will possibly enable the determination of Coulomb matrix elements.

[1] U. Hergenhahn, G. Snell, M. Drescher, B. Schmidtke, N. Müller, U. Heinzmann, M. Wiedenhöft, and U. Becker, Phys. Rev. Lett. 82, 5020 (1999).

Absorption of Circularly Polarized VUV Radiation in Polarized Iron Vapor

•Georg Prümper¹, Oliver Geßner¹, Björn Zimmermann¹, Jens Viefhaus¹, Rainer Hentges¹, Hans Kleinpoppen^1,2 und Uwe Becker^1
1Fritz-Haber-Institut der Max-Planck-Gesellschaft, Berlin, Germany
²University of Stirling, Scotland, UK

The effect of magnetic dichroism in the total ion yield of iron i.e. in the absorption of polarized iron vapor was observed for the first time far beyond the ionization threshold or autoionizing resonances. These observations can be explained by the orientation of orbital angular momentum of the 3d-electrons. This effect has not yet been observed in absorption measurements of magnetized iron films due to the smallness of the expectation value of the orbital angular momentum in the solid target. The explanation of the details of this dynamical behavior go beyond the single electron approximation.

A New Setup for Electron-Electron and Electron-Ion TOF Coincidence Spectroscopy of Atoms and Molecules

•Jens Viefhaus¹, Oliver Geßner¹, Henrik Haak¹, Rainer Hentges¹, Daniel Rolles¹, Georg Prümper¹, Uwe Becker¹, Burkhard Langer², Oliver Kugeler³ und Uwe Hergenhahn^3
1Fritz-Haber-Institut der Max-Planck-Gesellschaft, Berlin, Germany
²Max-Born-Institut, Berlin, Germany
³Max-Planck-Institut für Plasmaphysik, Garching, Germany

A substantial amount of progress in the understanding of photoionisation properties of atoms and molecules has been achieved by testing the most sensitive predictions of theoretical descriptions with coincidence experiments. We have designed and built a new end station dedicated to angle resolved multi-coincidence photoionisation experiments of gas-phase targets. This will allow us to conduct highly resolved molecular photoelectron-Auger-electron coincidence studies and to investigate dichroic effects in the molecular photoionisation of fixed-in-space molecules. In addition, double photoionisation studies will allow for obtaining a complete picture on the involved three-body Coulomb problem. Future directions will be discussed in the light of recent measurements.

Investigation of the 3p-3d resonances of Ti by photoion spectroscopy

•Kai Godehusen¹, Sandra Brünken¹, Birgit Kanngieß er¹, Peter Zimmermann¹ und Michael Martins^2
1Institut für Atomare und Analytische Physik, Technische Universität Berlin, Hardenbergstr. 36, D-10623 Berlin, Germany
²Institut für Experimentalphysik, Freie Universität Berlin, Arnimallee 14, D-14195 Berlin, Germany

The 3p-3d resonances of free Ti atoms have been studied using synchrotron radiation of the storage ring BESSY II in Berlin.

Using a time-of-flight spectrometer the ion-yield for the different ionic states of Ti has been recorded in the energy region of the 3p-3d resonances (30 - 55 eV). The high energy resolution of the U125/1-PGM beamline (10 meV) makes it possible to reveal the complex structure of these resonances. The experimental results are compared to Hartree-Fock calculations.

2p photoemission of 3d transition metal atoms: manganese and chromium

•Michael Martins¹, Philippe Wernet², Joachim Schulz², Bernd Sonntag², Kai Godehusen³, Bernhard Obst³ und Peter Zimmermann^3
3Institut für Atomare und Analytische Physik, Technische Universität Berlin, Hardenbergstr. 36, D-10623 Berlin, Germany
¹Institut für Experimentalphysik, Freie Universität Berlin, Arnimallee 14, D-14195 Berlin, Germany
²II. Institut für Experimentalphysik, Universität Hamburg, Luruper Chaussee 149, D-22761 Hamburg, Germany

The 2p photoemission of the 3d transition metals manganese and chromium has been studied using synchrotron radiation and Hartree-Fock calculations including correlation effects and relativistic corrections.

Multiplet splitting and valence shell re-coupling in the core excited state give rise to numerous main and satellite lines. The theoretical spectra shows excellent agreement with the experimental data and recent theoretical calculations using the MCDF method by Bagus et al. [1] for MnO. The atomic photoelectron spectra of the metals exhibit the same structure as the compounds and thin films and present the importance of intra-atomic effects on the electronic structure of solid state systems. Furthermore it can be shown by comparing the atomic and ionic photoelectron spectra, that the 4s electrons in manganese and chromium have a neglectable effect on the electronic structure of the atoms and condensed systems.

[1] P. S. Bagus et al., Phys. Rev. Lett. 84, 2259 (2000)

Single photoionization of helium close to the double-ionization threshold

•Tobias Schneider¹, Chien-Nan Liu² und Jan-Michael Rost^1
1Max-Planck-Institut für Physik komplexer Systeme, Dresden, Germany
²Kansas State University, Manhattan, Kansas, USA

We investigate the ¹P ^o double-excitation resonances of the helium atom close to the double-ionization threshold. Eigenchannel R-matrix calculations for total and partial photoionization cross sections are presented.

The low-lying Rydberg series of resonance states can be classified by means of approximate quantum numbers [1]. However, in the higher energy region, Rydberg series associated with different thresholds overlap, leading to the breakdown of this classification scheme. Therefore, a spectroscopic analysis of the cross section on the basis of individual resonances is no longer valid. It was estimated [2] that already above the He⁺(N = 15) threshold the cross section is dominated by irregular, random-like fluctuations, so-called Ericson fluctuations. Our calculations up to the eleventh threshold enable us to analyze the transitional regime from regular to irregular behavior.

[1] G. Tanner, K. Richter, and J.-M. Rost, Rev. Mod. Phys. 72, 497 (2000)

[2] J. Burgdörfer, Y. Qiu, and J. Müller, in Classical, Semiclassical and Quantum Dynamics in Atoms, edited by H. Friedrich and B. Eckhardt (Springer-Verlag, Berlin) 1997, p. 304

Line shapes in photo-excitation of triply excited hollow states

•Lars Bojer Madsen¹ und Peter Lambropoulos^2
1Institute of Physics and Astronomy,University of Aarhus, DK-8000 Aarhus C, Denmark
²Foundation for Research and Technology Hellas, Institute of Electronic Structure and laser, P.O. Box 1527, Heraklion 71110 Crete, and Department of Physics, University of Crete, Greece

Taking as point of departure a frequently used experimental detection scheme, we present a derivation of the analytical expressions of the line shapes in partial decay channels pertaining to photo-excitation of triply excited hollow states in atomic lithium with subsequent autoionization into a specific doubly excited state in the singly charged ion. The line shapes represent useful generalizations of the Fano parameterization of the profile of an autoionizing resonance. The line shapes give the positions and widths of the triply excited autoionizing resonances, and carry information about specific partial autoionization widths, and partial asymmetry parameters. It is also shown, that introducing a laser-induced transition from the triply excited state to another lower lying one, provides a knob that can be used to gain direct information about partial widths. The schemes are readily implemented experimentally, and would allow for the first measurements of such quantities.

Absolute cross section for photoionization of CO⁺

•Torkild Andersen, Henrik Kjeldsen, Helge Knudsen und Finn Folkmann
Institute of Physics and Astronomy, University of Aarhus, Denmark

The absolute cross section for photoionization of CO⁺ ions in their ground vibrational state, leading to longlived metastable CO²⁺ ions, has been measured from threshold to 55 eV utilizing a merged ion-photon setup with synchrotron radiation from an undulator. The continuum cross section near threshold has been determined to be 2.5(4)Mb, which is in agreement with an estimated cross section based on the absolute photoabsorption cross section for CO and the branching ratios for the decay of highly excited CO⁺ ions. Superimposed on the continuum cross section discrete structures are observed, which can be assigned to a specific CO⁺Rydberg series.

Symmetric Rotor of Hollow States

•Lars Bojer Madsen und Klaus Mølmer
Insitute of Physics and Astronomy, University of Aarhus, DK-8000, Aarhus C

A triply excited state in lithium, also referred to as hollow state, represents a fundamental case of highly correlated three-electron dynamics in a purely Coloumbic four-body system.

Based on the dynamical O(4) symmetry of the Coulomb problem, we present an analytical scheme for the explicit construction of highly correlated triply excited hollow states in Coulombic three- electron systems. The Ansatz for the three-electron state is physically well justified and it produces configuration- interaction mixing coefficients in qunatitative agreement with data obtained from ab initio calculations.

Absolute photoionisation cross sections for ions of astrophysical relevance

•H Kjeldsen¹, P A Andersen¹, F Folkmann¹, H Knudsen¹, B Kristensen¹, R L Brooks², J B West³ und T Andersen^1
1Institute of Physics and Astronomy, Univesity of Aarhus, Aarhus, Denmark
²Department of Physics, University of Guelph, Guelph, Ontario N1G 2W1, Canada
³CLRC Daresbury Laboratory, Warrington WA4 4AD, United Kingdom

Absolute photoionisation cross sections have been measured for singly-charged ions of the astrophysically abundant elements C, N, O and Fe. The measurements were performed by merging a beam of the respective target ion with an intense beam of VUV synchrotron radiation from an undulator. In the case of O⁺ separate data sets were obtained for the ⁴S ground state and for the longlived ²D and ²P metastable states, since the fraction of metastable O⁺ ions could be changed by O⁺ + N₂ charge-exchange reactions. The data have been compared with the most recent theoretical predictions using the R-matrix method. For C⁺, N⁺ and O⁺ the agreement between theory and experiment is rather good, however, very significant deviations are observed for the more complicated Fe⁺ cross section.

COHERENT CONTROL OF IONIZATION PRODUCTS IN ALKALINE-EARTH ATOMS

•Mireille Aymar, Jean-Marie Lecomte, Eliane Luc-Koenig und Martial Millet
Laboratoire Aimé Cotton CNRS II Bât. 505 91405 Orsay Cedex France

Two recent experiments in alkaline-earth atoms concerning the dynamics of coherent ionization processes and coherent control of photoionization products are interpreted. Combination of jj-coupled eigenchannel R-matrix and multichannel quantum defect theory was used to calculate the spectroscopic parameters.

The first example concerns control of photoionization yields into different ionic channels 6s_1/2, 5d_3/2, 5d_5/2 of barium by means of two coherent interfering photoionization routes, which are both two-photon, two-colour pathways resonantly enhanced by an intermediate state [1,2].

The second example deals with wave packets in calcium created by two short coherent pulses (Ramsey interferometry) [3]. Isolated-core excitation of autoionizing Rydberg states by the two delayed pulses (0.4ps) creates two nonstationary two-electron wave packets. Variation of the delay between the coherent pulses allows to control the energy and angular distributions of electrons ejected into the 4sel and 3d_3/2,5/2e¢l¢ continua.

[1] F. Wang, and D.S. Elliott, Phys. Rev. Lett. 77, 2416, 1996.

[2] E. Luc-Koenig, M. Aymar, M. Millet, J-M. Lecomte and A. Lyras, Eur. Phys. J. D 10, 205, 2000.

[3] R. van Leeuwen, M.L. Bajema and R.R. Jones, Phys. Rev. Lett. 53, 2852, 1999.

Laser Plasma Sourced VUV Photoabsorption Imaging System: A Progress Report

•John T. Costello¹, Eugene T. Kennedy¹, John S. Hirsch¹, Luca Poletto² und Piergiorgio Nicolosi^2
1NCPST, School of Physical Sciences, Dublin City University, Dublin 9, Ireland
²Department of Electronics and Informatics, University of Padua,Via Gradenigo 6/a, 35131 Padova, Italy

We have recently completed the design phase of a system based on a 1m normal incidence vacuum monochromator with corrected (toroidal) optics and intended to produce a wavelength tuned and collimated Vacuum-UV beam (30 - 100 nm). The VUV source is a laser generated rare earth plasma. The primary function of this system (under construction at the time of writing) will be the measurement of time resolved 'images' or spatial distributions of photo-absorption/ionization. Our new system design is significantly improved over that used in [1]. We will present the results of ray tracing calculations for various options, e.g., collimated beam and projection imaging of the sample plasma. We also expect to present first measurements from the commissioning phase of the project and if as expected at the time of writing, data are available we will present first photoasorption images of laser produced calcium plumes. These data can be compared with the results obtained in [1] from which an assessment of the system performance enhancement can be made. 1. Hirsch J S, van Kampen P, Meighan O, Mosnier J-P, Costello J T, Lewis C L S, MacPhee A, Hirst G, Westhall J and Shaikh W, J.Appl.Phys 88, 4953 (2000)

On shapes of the single- and double-excitation autoionization resonances in the photoabsorption of outer shells of Kr.

•V.L. Sukhorukov¹, I.D. Petrov¹, B.M. Lagutin¹, Ph.V. Demekhin¹, K.-H. Schartner², H. Liebel³, A. Ehresmann³ und H. Schmoranzer^3
1Rostov State University of TC, 344038 Rostov-on-Don, Russia
²I. Phys. Inst., Justus-Liebig-Universität, D-35392 Giessen, Germany
³FB Physik, Universität Kaiserslautern, D-67653 Kaiserslautern, Germany

Measurements of the total and partial photoionization cross sections near the 4s-threshold of the Kr atom exhibited a well-pronounced resonance structure connected with the doubly-excited states [1,2]. From the calculations performed in [2] the majority of the observed features were interpreted. However, discrepancies between the computed and measured shapes of the resonances remained in many cases. Thus we applied the K-matrix technique to compute the shapes of the first peaks 4s ® 5p observable in the 4p-photoabsorption. This allowed to include the interchannel mixing caused by the spin-orbit interaction of 4p-electrons and the interaction between resonances via continua under consideration. A substantially better agreement of the calculated and measured shapes of the resonances was obtained. A similar technique is planned to be applied for computing the resonances in the 4s- and corresponding satellite states photoionization.

[1] M.G. Flemming et al., Phys. Rev. A 44 (1991) 1733.

[2] H. Schmoranzer et al., J. Phys. B: At. Mol. Opt. Phys. 30 (1997) 4463.

An Experiment for Studies of Ionisation in Negative Ions

•jason greenwood und gerry collins
Department of Pure and Applied Physics, Queen's University Belfast, Belfast, BT7 1NN, UK

A versatile experimental apparatus is currently being set up for the investigation of inner shell ionisation in negative ions. Negative ions produced from a cesium sputter ion source will undergo interactions with a crossed beam. Initially an electron beam will be used but future experiments will use synchrotron soft X-ray radiation for single photon ionisation and intense lasers for multi-photon ionisation. The processes will be studied by final charge state analysis of the ion beam and by energy analysis of emitted Auger electrons using a cylindrical mirror analyser.

Transition probabilities from Na(3p_3/2) to excited discrete and continuum states

•Kaspars Miculis¹ und Wilfried Meyer^2
1Universität Kaisrslautern, Fachbereich Physik, Erwin Schrödinger Str., D-67663 Kaiserslautern, Germany
²Universität Kaisrslautern, Fachbereich Chemie, Erwin Schrödinger Str., D-67663 Kaiserslautern, Germany

We present a calculation of photoabsorption cross sections from the Na(3p_3/2) state. The Coulomb and non-local exchange potentials were calculated using the Hartree-Fock method. For the core polarisation potential an empirical form was adopted. Transition moments were obtained from the wavefunctions and energies determined by the solution of the Schrödinger equation using the Numerov method. Probabilities of the 3p_3/2 ® ns_1/2, nd_5/2,3/2 transition are tabulated for n = 3 - 50, and the photoionisation cross sections for photoelectron energy 0 - 2 eV are presented by an analytical formula. The results are compared with various experimental ant theoretical results for the lifetimes and cross sections. The accuracy of our results is estimated as 2 - 3 % in the lower energy range (from bound states up to 0.2 eV in continuum), and up to 5 % for the energies where the ionisation cross section becomes rather small. This estimate is corroborated by calculations with a variety of local exchange and core polarisation potentials each of which yields results within the given margin.

Above-threshold double ionization of helium

•Manfred Lein^1,2, Volker Engel¹ und E.K.U. Gross^2
2Institut für Theoretische Physik, Universität Würzburg
¹Institut für Physikalische Chemie, Universität Würzburg

We use a reduced-dimensional model of the helium atom to study double ionization under the influence of intense laser pulses. We calculate the correlated two-electron momentum distribution and the momentum spectrum of the He²⁺ recoil ion. We find excellent agreement with experiment. We use the Wigner transformation to analyze the phase-space motion of the electronic center of mass. For long wavelengths (780 nm), the phase-space analysis confirms that rescattering leads to non-sequential double ionization. At short wavelengths, we discover the two-electron analogue to ordinary above-threshold ionization. The total-energy spectrum of the ejected electrons consists of peaks separated by the photon energy. We propose an approximate method to reconstruct the total-energy spectrum from the recoil-ion momentum distribution.

Relativistic electron dynamics in intense crossed laser beams

•Guido R. Mocken, Yousef I. Salamin und Christoph H. Keitel
Theoretische Quantendynamik, Fakultät für Physik, Universität Freiburg, Hermann-Herder-Straße 3, D-79104 Freiburg

We investigate analytically and numerically the dynamics of a single electron, in vacuum, in the presence of a pair of laser beams [1] crossing at an arbitrary angle [2]. Our analysis evolves from exact solutions to the relativistic equation of motion and predicts average acceleration gradients of several TeV/m over short distances, using laser field intensities of up to 10²²Wcm^-2. Plane-wave fields are employed in our analytic calculations and pulsed fields are used in our numerical work. The radiation losses of the accelerated electron are shown to be small but not negligible and sample emission spectra are given. Effects on the electron trajectories and electron energy gain due to departure from the ideal initial conditions, like the existence of a distribution of initial velocities, off-axis injection, and realistic beam representation, are studied numerically. We point out the regime of stability for off-axis injection and find out that the electron leaves the interaction region with spatially confined laser beams with energy gain in the GeV range [3].

[1] E. Esarey, P. Sprangle, and J. Krall, Phys. Rev. E 52, 5443 (1995). [2] Y. I. Salamin and C. H. Keitel, Appl. Phys. Lett. 77, 1082 (2000). [3] Y. I. Salamin, G. R. Mocken, and C. H. Keitel, Phys. Rev. E (submitted).

Multiply Charged Ions in Strong Laser Fields

•M. Casu, S.X. Hu und C.H. Keitel
Theoretische Quantendynamik, Fakultät für Physik, Universität Freiburg, Hermann-Herder-Str.3, 79104 Freiburg

Multiply charged ions [1] in strong laser fields have recently been pointed out as a possible source of coherent radiation in the x-ray regime [2]. Especially strong Coulomb interaction between the electron and the ion in highly charged ions [1] are noticeable and lead to evident relativistic effects. Therefore a relativistic investigation is appropriate and as a first step we have realized this with a lower dimensional Dirac code. We have examined highly charged, hydrogen-like ions up to an atomic number of Z=92 in laser fields with intensities up to the highest realized so far. In particular the characteristic lines of the excited states of the resonance spectra have been examined in detail. The relativistic theory has then been compared with the Schrödinger theory, so that relativistic signatures could be identified.
[1] P.H. Kokler und Th. Stoehlker, Adv.At.Mol.Opt.Phys. 37, 297 (1996)
[2] M. Casu, C. Szymanowski, S.X. Hu und C.H. Keitel, J.Phys. B 33, L411 (2000)

Excess Photon Detachment of H^- in a Strong Infrared Laser Field

•Igor Yu. Kiyan, Rainer Reichle und Hanspeter Helm
Fakultät für Physik, Freiburg University, Stefan-Meier-Str.19, 79104 Freiburg

Angular resolved momentum spectra of photoelectrons formed in photodetachment of H^- ions are recorded using an electron imaging technique. The negative ions are exposed to a laser pulse of 130 fs duration, generated from an OPA at the wavelength of 2150 nm. Saturation of the photodetachment process at the leading front of the laser pulse restricts laser intensities that negative ions experience before their depletion. At least three excess photon detachment (EPD) peaks are observed in the energy spectrum of the photoelectrons. The ponderomotive broadening of the energy spectrum is of the order of 150 meV. This corresponds to the maximum intensity experienced by the negative ion of ~ 3×10¹¹  W/cm². The analysis of the angular distribution of the photoelectrons shows high angular momentum contributions which arise from from EPD channels with reemission of photons.

Supported by DFG (SFB 276, TP C14)

Momentum analysis of strong field fragmentation of H₂

•H. Rottke¹, C. Trump¹, M. Wittmann¹, G. Korn¹, W. Sandner¹, R. Moshammer², A. Dorn², C.D. Schröter², D. Fischer², J. Crespo², B. Feuerstein², J. Ullrich² und P. Neumeier^3
1Max-Born-Institut, Max-Born-Str. 2a, D-12489 Berlin
²Fak. für Physik, Univ. Freiburg, Hermann-Herder-Str. 3, D-79104 Freiburg
³Gesellschaft für Schwerionenforschung, Planckstr. 1, D-64291 Darmstadt

We used COLd Target Recoil Ion Momentum Spectroscopy (COLTRIMS) to investigate the fragmentation of molecular hydrogen exposed to high intensity (I > 10¹⁴ W/cm²), linearly polarized, ultra-short light pulses (t_p = 25 fs). The final momenta of the charged particles were analysed for the reaction channels: H₂ ® H₂⁺ + e^- (1), H₂® H(1s) + H⁺ + e^- (2), and H₂ ® H⁺ +H⁺ + e^- + e^- (3). The momentum distribution f([p\vec]₁,[p\vec]₂) of the two photoelectrons emitted in reaction (3) indicates that the momenta they gain in the double ionization process are not correlated. For reaction (2) the momentum distribution of H⁺ ions emitted along the light polarization vector is independent of the kinetic energy of the photoelectron. This indicates that the momentum the ion gains does not depend significantly on the intensity level in the light pulse, measured by the electron kinetic energy, when reaction (2) is initiated.

Strong field double ionization of Ne and Ar

•H. Rottke¹, C. Trump¹, M. Wittmann¹, G. Korn¹, W. Sandner¹, R. Moshammer², A. Dorn², C.D. Schröter², D. Fischer², J. Crespo², B. Feuerstein², J. Ullrich² und P. Neumeier^3
1Max-Born-Institut, Max-Born-Str. 2a, D-12489 Berlin
²Fak. für Physik, Univ. Freiburg, Hermann-Herder-Str. 3, D-79104 Freiburg
³Gesellschaft für Schwerionenforschung, Planckstr. 1, D-64291 Darmstadt

COLd Target Recoil Ion Momentum Spectroscopy (COTRIMS) was used to investigate atomic double ionization in high intensity ( I > 10¹⁴ W/cm²) ultra-short infrared ( l = 790 nm) light pulses (t_p = 25 fs). The complete kinematical analysis of the motion of the charged particles in the final state along the polarization vector of the light beam allowed to identify electronic rescattering as the main mechanism responsible for non-sequential atomic double ionization which is found in a certain range of intensities. The results for Ne and Ar show distinct differences which indicate pure rescattering as mechanism for Ne. In case of Ar mechanisms other than rescattering also seem to contribute to non-sequential ionization. A transition from non-sequential to sequential double ionization with rising light intensity is clearly visible in the data.

Atomic ionisation dynamics in strong laser fields

•M. Dammasch, U. Eichmann und W. Sandner
Max Born Institut, Berlin, Germany

Experimental investigations of atomic ionisation dynamics at high laser intensities above 10¹⁶W/cm² are only scarce. Using the MBI high intensity laser facility we have investigated the ionisation dynamics of atoms at weakly relativistic laser intensities. We present laser intensity dependent total ion yield measurements on quasi He-like ions and rare gas atoms. We will discuss the influence of the relativistic continuum on the ionization process. Furthermore we will discuss the possibility to determine high laser intensities directly from ion yield measurements.

Simultaneous investigation of the three most important effects of atoms in strong laser fields

•Felix Grasbon, Gerhard G. Paulus und Herbert Walther
MPI für Quantenoptik, D-85748 Garching

Atoms interacting with intense laser fields show three distinctive phenomena: Above-threshold ionization (ATI), non-sequential double ionization (NSDI), and high-harmonic generation (HHG). Theoretical models show that all of them are governed by much the same physics. Similarities and differences can easily be detected by studying their yields as a function of the ellipticity of the laser polarization.

We present the first experiment that investigates ATI, NSDI, and HHG under identical conditions. The measurements were done with a femtosecond laser system with an extremely high repetition rate (100kHz). The photoelectrons and the ions are detected by time-of-flight spectroscopy and the harmonics in a XUV spectrometer with a novel CCD detector. The results are compared to a semi-classical model. They confirm the overall validity of that theory, however, we also observe deviations between theory and experiment.

Heuristisches Model zur Ionisation von Atomen und Molekülen in starken Laserfeldern

•C.J.G.J. Uiterwaal, C.R. Gebhardt und H. Schröder
Max-Planck-Institut für Quantenoptik, Hans-Kopfermann-Straße 1, D-85748 Garching

Multiphotonen-Prozesse setzen voraus, dass eine genügende Anzahl von Photonen innerhalb eines ausreichend kurzen Zeitintervalls auf einen effektiven Absorptionsquerschnitt treffen. Ausgehend von diesem heuristischen Ansatz entwickeln wir ein statistisches Model zur Beschreibung der Ionisation von Atomen und Molekülen in intensiven Laserfeldern. Das Model enthält keine freien Parameter und ermöglicht z.B. die Voraussage der Erscheinungsintensitäten für eine Vielzahl von Atomen und Molekülen. Insbesondere wird das unterschiedliche Ionisierungsverhalten von Xe und O₂ trotz ihres annähernd gleichen Ionisierungspotentiales richtig beschrieben.

Threshold laws for four-particle fragmentation

•Valentin Ostrovsky
Institute of Physics, The University of St Petersburg, 198904 St Petersburg, Russia

Break-up of atomic particle into several charged fragments can be achieved by multiple photoionization or by collision processes. The threshold behaviour of fragmentation cross sections generally is described by power laws, s_frag ~ E^m, where E is excess energy above break-up threshold. For three charged fragments the threshold law of this type was revealed in the famous paper by Wannier who considered two electrons receding from charged core. The general universal approach to calculation of threshold indices m was developed recently by Kuchiev and Ostrovsky [1]. In the present contribution we consider in more detail break up into four fragments. The threshold behaviour of these processes remains object of active experimental and theoretical studies. We presume that masses of all fragments are finite and thus fully account for recoil effects. For the fragment charges we consider two possibilities available: (i) two particles with the positive charge and two particles with negative charge; (ii) one particle with positive (negative) charge and three particles with negative (positive) charge. All the fragments have different masses and charges with only one restriction imposed to limit the choice systems considered: it is presumed that two fragments represent identical particles. The threshold indices are evaluated for large number of systems, including systems that contain exotic particles. Some previously suggested threshold laws are revised.

[1] M. Yu. Kuchiev and V. N. Ostrovsky, Phys. Rev. A 58, 321 (1998).

STIMULATED VARIATION OF THE MECHANISM OF Ba²⁺ IONS FORMATION DURING MULTIPHOTON IONISATION OF ATOMS IN VISIBLE SPECTRAL RANGE

•Vasyl Suran und Ivan Bondar
Physical Department of Uzhgorod National University, Voloshin str., 54, Uzhgorod 88000, Ukraine

The main goal of our work is the creation of conditions for realization of direct two-electron mechanism of Ba²⁺ ions formation in visible spectral range. The idea of experimental setup consists in following: during ionization of Ba atoms by visible laser radiation we realized the strong additional perturbation of atoms by laser radiation of other frequency range. The radiation of the second harmonic of YAG laser was used for ionization and the radiation of laser on colour centres CCL (8500-8950 cm^-1) was used for perturbation of Ba atoms.

At the frequency 8880 cm^-1 one photon transition between 5d6p¹P⁰₁ and 6s7d¹D² states takes place. In the vicinity of this frequency the dynamic polarizability of these states has large absolute values and strongly depend on frequency of CCL radiation. That's why in the vicinity of this frequency the strong perturbation of these states conditioned by ac-Stark effect manifestation have to take place. The results of our experiments showes that excitation of strongly perturbed in result of ac-Stark effect of 6s7d¹D² and 5d6p¹P₁⁰ states leads to strong enhancement in the yield of Ba²⁺ ions during the ionization in visible spectral range as well. So, the obtained results show the realization of direct two-electron mechanism of doubly-charged ions formation under the circumstances of performed experiments.

Adiabatic evolution of atomic populations in gaussian light pulses

•Mohand Brouri, Vola M. Andrianarijaona, Jozo Jureta und Xavier Urbain
Département de Physique, Université Catholique de Louvain, Belgium

In a two-colour experiment, we plan to study the time evolution of the excited population of a three-level atomic system. A continuous wave Ar-ion laser operating on the 488nm line (singlemode) illuminates a fast beam of metastable atoms and is Doppler detuned in order to match the frequency of the 2s-4p transition in atomic hydrogen. The interaction region extends on about one centimetre, and is probed by a sharply focused infrared beam that selectively ionises the 4p population, giving an instantaneous picture of the system.

With this experimental scheme, we aim at demonstrating the deviation from the Rabi formula in the adiabatic excitation regime associated with gaussian pulses. The available power should be sufficient to observe, exactly on resonance, three to four complete Rabi cycles. The adiabatic evolution out of resonance will cause the populations to evolve in a monotonous and reversible manner throughout the interaction zone.

Collective Effects in Resonant Multiphoton Ionisation of Atomic Deuterium

•B J Shortt¹, F Giammanco², P J M van der Burgt¹ und R W O'Neill^1
1Dept. of Experimental Physics, National University of Ireland, Maynooth, Ireland
²Dipartimento di Fisica, Universita di Pisa, Pisa, Italy

A transient laser plasma is formed by 3-photon ionisation of deuterium with an excimer-pumped dye laser (pulsewidth 13 ns, wavelength 243 nm, maximum intensity 300 MW/cm2). Inside a vacuum chamber the laser beam is crossed with a deuterium beam produced by an RF discharge source. The photoionisation is resonant with the metastable 2s state. An extraction electric field accelerates the ions towards a detector, and quenches the metastable atoms. The emitted Lyman-alpha photons are also detected.

At high laser intensities collective effects due to the mutual interaction of the photo-electrons and the ions affect the time- evolution of the plasma. The resonant multiphoton ionisation is influenced by the Stark mixing of the 2s and 2p states in the collective field. Measurements of the ion and photon yields as a function of laser intensity and wavelength will be presented.

Quantum interference in the third-harmonic generated by two time-delayed coherent laser pulses.

•Lorenzo Fini¹, Stefano Cavalieri¹ und Roberto Buffa^2
1INFM, Dipartimento di Fisica and LENS, Universit\` a di Firenze, Largo E. Fermi 2, 50125, Firenze, Italy.
²INFM and Dipartimento di Fisica, Università di Siena, Via Banchi di Sotto 55, 53100 Siena, Italy

In a recent experiment on three-photon ionisation of atomic barium,performed with a sequence of two coherent nanosecond laser pulses,we demonstrated that the quantum-interference properties of the ionisation process can be investigated on a time-scale much shorter than the duration of the laser pulses. In the present work we extend our investigation to the quantum coherence properties of the third-harmonic generation process in an atomic vapour. We utilise an interferometric technique involving two delayed pulses of a multi-mode, nanosecond dye laser to investigate the resonance-enhanced third-harmonic generation in sodium vapour. The fringe pattern that is observed in the third-harmonic emission as a function of the time delay of the two pulses carries the information on the quantum coherence properties of the atomic polarisation. This kind of process opens new interesting possibilities in the field of time-delay spectroscopy.

ON THE SHORT LIVED RADIOISOTOPE IONIZATION

•Viorica Gheorghe¹ und G. Huber^2
1Institut für Physik, Johannes Gutenberg Universität, 55099 Mainz
²National Institute for Laser, Plasma and Radiation, Bucharest, Romania

In this paper a simulation based on analytical models [1,2] describing the step-wise ionization of Beryllium exposed to two-colour pulsed laser excitations is presented. We displayed the evolution of the population in the autoionizing state of Beryllium, but also on the different levels which are intermediate steps of the studied ionization process. The experimental parameters for which the ionization process of Beryllium is optimal were distinguished.

This simulation is particularly important in order to identify the conditions for an efficient ionization of a short lived radioisotope from the resonance ionization laser ion source (RILIS) at CERN-ISOLDE [3].

[1] V. N. Gheorghe, Appl. Phys. 46, 185-190 (1988)

[2] D. Boudreau, P. Ljungberg, O. Axner, Spectrochimica Acta Part 51, 413-428 (1996)

[3] V. Sebastian, R. Catherall, V. N. Fedoseyev, U. Georg, G. Huber, Y. Jadin, O. Jonsson, U. Köster, M. Koizumi, K. -L. Kratz, E. Kugler, J. Lettry, V. I. Mishin, H. L. Ravn, C. Tamburella, A. Wöhr and the ISOLDE Collaboration, AIP Conf. Proceedings 455, ENAM'98, Bellaire, Michigan, USA, 126 (1998)

Approximate treatment of the relativistic two-photon bremsstrahlung process.

•Igor Solovjev, Tatiana Fedorova und Andrei Korol
Department of Physics, St.Petersburg State Maritime Technical University, Leninskii prospect 101, St.Petersburg 198262, Russia

We propose an approximation for effective quantitative analysis of the processes the amplitude of which contains, as a constituent part, the radiative free-free matrix elements between two states one of which (or both) is a virtual state (e.g. two-photon bremsstrahlung). The exact relativistic evaluation of compound matrix elements is extremely cumbersome, and for many processes such a problem has not been solved so far. The approach we propose [1], although being approximate, allows to effectively evaluate the main parts of the free-free matrix element with much less analytical and computational efforts. It is based on the accurate analytical treatment of the diagonal singularities in the one-photon matrix elements which appear in the compound matrix element.

We illustrate the applicability of the method by presenting the results of calculations of spectral and angular distributions for two-photon bremsstrahlung process and their comparison with available experimential data and more simpler approximations.

This work is partially supported by RFBR grant 99-02-18294a.

[1] T.A.Fedorova, A.V.Korol, I.A.Solovjev, J.Phys.B: At.Mol.Opt.Phys. 33 (22), 5007 (2000)

Atoms interacting with intense, high-frequency laser pulses: effect of the magnetic field component on atomic stabilization

•Niels Kylstra¹, Richard Worthington¹, Javier Vazquez de Aldana², Luis Roso², Peter Knight³ und Akshay Patel^3
1Department of Physics, University of Durham, Durham DH1 3LE, United Kingdom
²Departamento de Fisica, Universidad de Salamanca, E-37008 Salamanca, Spain
³Optics Section, Blackett Laboratory, Imperial College, London SW7 2BZ, United Kingdom

At sufficiently high laser intensities, the magnetic field component the laser field can strongly influence the stabilization of atoms in the high-frequency regime by inducing motion along the laser pulse propagation direction. Using a two-dimensional model atom, we investigate how the duration of the laser pulse affects stabilization in this non-dipole regime. Results obtained using a fully spatially dependent vector potential are compared to a long wavelength approximation. We also discuss non-dipole effects when the atom interacts with two counter-propagating pulses.

Photon emission by ions interacting with short intense laser pulses

•Niels Kylstra¹, Robert Potvliege^1,2 und Charles Joachain^2,3
1Department of Physics, University of Durham, Durham DH1 3LE, UK
²Physique Théorique CP227, Université Libre de Bruxelles, B-1050 Brussels, Belgium
³Max-Planck-Institut für Quantenoptik, Garching, Germany

Photon emission by light hydrogen-like ions interacting with intense, few-cycle Ti:Sapphire laser pulses is calculated within the strong-field approximation of Lewenstein and co-workers, modified so as to treat the coupling of the atom with the incident field beyond the dipole approximation. The approach is tested by comparing with results obtained by numerically solving the time-dependent Schrödinger equation for a two-dimensional model ion. The effect of the magnetic field component of the driving pulse is significant beyond 2 ×10¹⁷ W cm^-2 peak intensity. It leads, in particular, to a saturation in photon emission with increasing laser intensity. As compared to emission polarized along the same direction as the incident field, emission polarized along its propagation direction is weaker by about two orders of magnitude at the maximum peak intensity considered, 3.6 ×10¹⁷ W cm^-2.

Coherent radiation from the dense lithium vapor upon quasiresonant excitation

•Irena Labazan, Davorka Azinovic, Slobodan Milosevic und Goran Pichler
Institute of Physics, PO Box 304 HR-10000 Zagreb, Croatia

An intense red and near-infrared radiation from lithium vapor contained in the heat-pipe oven has been observed in forward direction when the pulsed nanosecond laser was tuned near the 2p-4d resonance at 460.3 nm. To characterize this emission we have measured time resolved intensities, excitation and emission spectra near 3d-2p, 3s-2p, 4s-2p transitions versus laser intensity and density of lithium vapor, both in forward and in perpendicular direction of the laser beam. The most intense emission near 610.4 nm is observed to be slightly off-axis indicating that the main excitation is due to four-wave mixing process, which involves 4d, 4p, 3d and 2p lithium atom levels. We believe that this is the first time that such process is observed from initial level, which is the excited state. This is achieved with quasiresonant excitation where initial 2p level population goes through various molecular channels involving the leading edge of laser pulse, while the rest of the pulse excites 4d level. Other nonlinear, parametric and cooperative emission processes will be discussed.

Tunneling Photoionization in a Strong Bichromatic Laser Field

•Vladimir Pazdzersky¹ und Vladimir Usachenko^2
1Physics Department, National University of Uzbekistan, 700174 Tashkent, Uzbekistan
²Institute of Applied Laser Physics, Nakashlyk str.,1, 700185 Tashkent, Uzbekistan

In the presented contribution the process of electron tunneling through one-dimensional non-stationary barrier created by atomic potential and external electromagnetic (EM) bichromatic field consisting of coherent superposition of the fundamental laser field of frequency w ( w << I_p, where I_p is the ionization potential of a field-exposed atomic system; the atomic system of units is used) and its second harmonic is considered and studied theoretically. Within frame of imaginary time method [1] the gauge-invariant analytical expression for probability of tunneling photoionization has been derived in standard exponential form. The derived analytical expression (which is an analogue of the famous Keldysh formula for photoionization probability generalized for a case of bichromatic EM field) is analysed for various limiting values of so called the adiabaticity parameter

g = (w·Ö{2·I_p})/E₁ and studied depending on the ratio a = E₂/E₁ of electric field strengths of incident bichromatic field harmonics.

The research described in this presentation was made possible in part by Award No.ZP2-2279 of the U.S. Civilian Research and Development Foundation (CRDF) for the Independent States of the Former Soviet Union.

[1] Perelomov A.M., Popov V.S. and Terent'ev M.V., 1966, Zh.Exp.Teor.Fiz., v.51, p.309.

Laser-Induced Plasma Spectroscopy (LIPS) in the VUV for the Quantitative Characterisation of Steel Alloys

•Mohamed Khater, Eugene Kennedy, John Costello, Jean-Paul Mosnier und Paul van Kampen
National Centre for Plasma Science and Technology (NCPST), Dublin City University, Dublin 9, Ireland

We demonstrate that spatially resolved spectroscopy, in the deep VUV region (40-160 nm), of laser induced plasmas provides a useful technique for quantifying the carbon content in steel target materials. Three ionisation stages of carbon have been investigated using a series of solid steel samples with carbon concentrations in the 0.001-1.32% range.

The original CEMA/PDA detector have been replaced by a VUV-sensitive CCD array and an extensive optimisation study for several experimental parameters and conditions have been carried out. Our latest results provide a significant reduction in the detection limit to 2 ppm of carbon.

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