Bulletin of the American Physical Society
2006 37th Meeting of the Division of Atomic, Molecular and Optical Physics
Tuesday–Saturday, May 16–20, 2006; Knoxville, TN
Session T3: Strong Field (Intense Laser) Processes II |
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Chair: Stephen Southworth, Argonne National Laboratory Room: Knoxville Convention Center 301D |
Friday, May 19, 2006 10:30AM - 10:42AM |
T3.00001: Observation of Enhanced Excitation of $I_2^{2+}$ by Strong Laser Fields George Gibson, Ryan Coffee, Li Fang Using pump-probe spectroscopy with ultrashort laser pulses, we see an enhancement of the charge-asymmetric dissociation (CAD) channel, $(I_2^{2+})^* \rightarrow I^{2+} + I$, over a narrow range of internuclear separation. The enhancement of the CAD channel appears to come from excitation of the symmetric ground state dissociation channel ($I^{+} + I^{+}$) for two reasons. First, there is a depletion in the symmetric channel at approximately the same pump-probe delay as the asymmetric enhancement. Second, for a fixed delay, the asymmetric channel increases as a function of probe intensity while the symmetric channel decreases. In addition, we find that the kinetic energy of the extra $I ^{2+} + I$ ions decreases for increasing delay. To explain this dependence of the kinetic energy release on delay, we introduce model potential energy curves. Based on these curves, we conclude that the excitation is produced by a resonant 3-photon transition within $I_2^{2+}$ rather than by ionization of $I_2^+$. [Preview Abstract] |
Friday, May 19, 2006 10:42AM - 10:54AM |
T3.00002: Resonant-Like~Enhancement~of~the High-Energy Plateau in ATD Katarzyna Krajewska, Ilya I. Fabrikant, Anthony F. Starace We calculate the intensity variation of above-threshold detachment (ATD) rates for H$^{-}$ and F$^{-}$ in the high-energy plateau region, by solving the time-dependent Schr\"odinger equation within the Sturmian-Floquet approach [1]. We find pronounced enhancement of the ATD spectra as the laser-field intensity induces ATD channel closings. We confirm previous results on negative ions with an active s-electron [2], and extend them to the case of p-symmetry of the initial state. Depending on the symmetry of the initial state, we show that the enhancement is most pronounced for even- or odd-channel closures. Similar resonant-like enhancements have been observed experimentally in above-threshold ionization spectra for rare gases [3], calculations for which are in progress. [1] R. M. Potvliege, {\it Comp. Phys. Comm.} {\bf 114}, 42 (1998); [2] B. Borca {\it et al.}, {\it Phys. Rev. Lett.} {\bf 88}, 193001 (2002); [3] See, e.g., F. Grasbon {\it et al.}, {\it Phys. Rev. Lett.} {\bf 91}, 173003 (2003). [Preview Abstract] |
Friday, May 19, 2006 10:54AM - 11:06AM |
T3.00003: Two-photon excitation/ionization of inner shell atoms: Non-dipole approximation S.A. Novikov, S.S. Tayal The absolute values and shapes of cross sections for the processes of two-photon one-electron excitation/ionization of the 1s-shell of neon atom, including the effect of relaxation of the atomic residue in the field of the creating vacancies, are calculated. We used velocity form for matrix elements of electromagnetic field interaction operator in three approximations: dipole, non-dipole (one channel) and non-dipole (many channels). The cross sections have been calculated for both linearly and circularly polarized photons. [Preview Abstract] |
Friday, May 19, 2006 11:06AM - 11:18AM |
T3.00004: Electron-Positron Pair Production by the Impact of a High-Power Laser Pulse on Relativistic Ions Katarzyna Krajewska, Jerzy Z. Kami\'{n}ski, Fritz Ehlotzky With presently available laser sources, which can yield powers such that the ponderomotive energy of an electron can be much larger than twice the energy of an electron at rest, it has become of interest to reconsider fundamental processes of quantum electrodynamics in such fields, in particular, electron-positron pair production in collisions of laser pulses and highly charged ions. We calculate highly nonlinear production rates for that process, and investigate the most favorable conditions for pair creation, either along the direction of linear polarization, or in the propagation direction of the laser pulse. [Preview Abstract] |
Friday, May 19, 2006 11:18AM - 11:30AM |
T3.00005: Charges in an electromagnetic wave directed along a constant magnetic field: quantum and classical solutions William E. Baylis, Renan Cabrera Analytical quantum and classical solutions are derived for the relativistic dynamics of charges in classical electromagnetic fields. They are found to be closely related when the classical one is expressed in terms of spinors. This is the natural approach of the Clifford (or geometric) algebra of physical space. The field configurations explicitly treated include that of the autoresonant laser accelerator\footnote{W. E. Baylis and Y. Yao, Phys. Rev. A \textbf{60}, 785 (1999).}, and although the electromagnetic wave is taken to be a plane wave, it may be pulsed or continuous. The quantum solutions generalize solutions to the Dirac equation found by Bergou and Ehlotzky\footnote{J. Bergou and F. Ehlotzky, Phys. Rev. A \textbf{27}, 2291 (1983).} and by Volkov\footnote{D. M. Volkov, Z. Phys. \textbf{94}, 250 (1935).}. [Preview Abstract] |
Friday, May 19, 2006 11:30AM - 11:42AM |
T3.00006: Optimizing dynamic alignment in diatomic molecules by pulse shaping Daniel Pinkham, Karen Mooney, Robert Jones We have used intense laser pulses in combination with a genetic algorithm and laser pulse shaper to explore transient laser alignment of room temperature CO. A molecule with an anisotropic polarizability may undergo periodic, transient alignment following its exposure to an intense ultra-short laser pulse. The laser populates a coherent superposition of rotational states via a sequence of Raman transitions. As this rotational wavepacket evolves, the angular probability distribution periodically aligns parallel and perpendicular to the linear laser polarization. In the impulsive limit, the laser bandwidth exceeds the energy separation between rotational levels, and the degree of alignment depends only on the time-integrated intensity in the pulse. We have used shaped laser pulses in an attempt to optimize the degree of alignment at fixed pulse energy and explore transient laser alignment outside the impulsive limit. In particular, we use phase-only shaping and, using a 30 fsec Coulomb explosion pulse, probe molecular alignment at a delay corresponding to one of the alignment ``revivals.'' By inspecting the time-of-flight of the exploded fragments, we obtain rapid experimental feedback which enables us to test the effectiveness of hundreds of different pulse shapes. In the experiment and corresponding theoretical simulations, we find complex, non-impulsive shapes which align the molecules at least as well as a transform-limited pulse. This work has been supported by DOE BES. [Preview Abstract] |
Friday, May 19, 2006 11:42AM - 11:54AM |
T3.00007: X-ray probe of Alignment by Strong-field Ionization C. H\"{o}hr, E. Peterson, D.A. Arms, E.M. Dufresne, R.W. Dunford, D.L. Ederer, E.P. Kanter, B. Kr\"{a}ssig, E.C. Landahl, J. Rudati, R. Santra, S.H. Southworth, L. Young The study of atoms in strong laser fields has led to several discoveries with fundamental and technological interest. While experiments so far have focused on the detection of ejected particles from the focal region of the laser beam, we present results of an experiment on Kr atoms probing within the strong-field volume ($4 \times 10^{14}$ W/cm$^2$) using micro-focused, tunable x-ray radiation from the APS (14.3 keV). The polarization of the x rays enables determination of magnetic sublevel alignment in the residual krypton ion after tunneling ionization in the laser field. We detect the x-ray fluorescence following K-shell excitation as a function of target pressure, time, laser intensity and polarization, and external magnetic fields. We demonstrate methods both to destroy and preserve the alignment created in the tunnel ionization process. This experiment develops methodology for future studies using x-ray free electron lasers, where intensities of up to $10^{20}$ W/cm$^2$ are expected. [Preview Abstract] |
Friday, May 19, 2006 11:54AM - 12:06PM |
T3.00008: Aligned Molecules for X-Ray Microprobe Studies E. Peterson, C. H\"{o}hr, S. Southworth, E. Landahl, B. Kr\"{a}ssig, E. Kanter, S. Pratt, L. Young We prepare a sample of aligned molecules for experiments with 100-ps x-ray pulses at the Advanced Photon Source. Established techniques of adiabatic alignment by 10-ns pulses are not best suited for this application because of the low laser repetition rates (10-100 Hz). Instead, we choose to align adiabatically with amplified chirped pulses from a Ti:sapphire laser system (800 nm, 1.3 mJ, 120 ps, 1 kHz). In these laser-only experiments, we investigate the alignment of cold $\rm{N_{2}}$ and $\rm{CF_{3}Br}$ molecules by detection of the charged fragments following ionization by a short (50fs) 800nm laser pulse. Molecules from a supersonic jet intersect an aligning pulse of variable linear polarization and variable delay relative to a near-colinear circularly-polarized dissociating probe pulse. The resulting ions are accelerated in a velocity-map imaging spectrometer toward a position-sensitive microchannel plate detector; the polarization dependence of this fragment distribution indicates alignment of the sample. [Preview Abstract] |
Friday, May 19, 2006 12:06PM - 12:18PM |
T3.00009: Dielectronic Recombination From a Continuum of Finite Bandwidth Above the Classical Ionization Limit Edward Shuman, Tom Gallagher Dielectronic recombination (DR) is the recombination of an energetic electron and an ion via autoionizing Rydberg states lying below an excited state of the ion. In the presence of a 38.7 GHz microwave field, we observe DR from a continuum of finite bandwidth at energies above as well as below the Ba$^+~6p_{1/2}$ limit. The observed DR signal exhibits periodic structure with an energy spacing equal to the microwave frequency. The periodic structure is observed above and below the limit and is due to the absorption or stimulated emission of microwaves during recombination, i.e. DR occurs via the $6p_{1/2}n\ell$ Rydberg series dressed by the microwave field. [Preview Abstract] |
Friday, May 19, 2006 12:18PM - 12:30PM |
T3.00010: High repetition rate saturated output lasers in nickel-like ions at wavelengths down to 13.2 nm M.A. Larotonda, Y. Wang, B.M. Luther, D. Alessi, M. Berrill, V.N. Shlyaptsev*, J.J. Rocca There is significant interest in the development of compact high-average-power soft-x-ray lasers for a variety of applications in imaging, spectroscopy, plasma diagnostics, and metrology. In particular, there is a need for the development of compact sources of coherent light at wavelengths within the bandwidth of the Mo--Si mirrors centered at 13.5 nm for at-wavelength metrology related to extreme ultraviolet lithography. Soft x-ray laser operation at this wavelength have been limited to one shot every several minutes due to inefficient plasma heating. In this work we report the first demonstration of high repetition rate operation of saturated lasers emitting in this spectral region. Operation at 5 Hz repetition rate generated an average power of 1-2 $\mu $W at 13.9 and 13.2 nm in the 4$d_{1}S_{0}$--4$p_{1}P_{1}$ transitions of Ni-like Ag and Ni-like Cd ions respectively. The results were obtained by efficiently heating a plasma an with 8 ps pulse of only 1 J energy impinging at grazing incidence. Laser amplification at wavelengths as short as 10.9 nm was also observed by isoelectronic scaling these results along the Ni-like ion sequence. These table-top lasers can be expected to enable numerous new applications of intense coherent soft x-ray light. Work supported by the NSF EUV ERC, Award EEC-0310717 and by the US Department of Energy. [Preview Abstract] |
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