Bulletin of the American Physical Society
39th Annual Meeting of the APS Division of Atomic, Molecular, and Optical Physics
Volume 53, Number 7
Tuesday–Saturday, May 27–31, 2008; State College, Pennsylvania
Session B6: Photon Interactions with Atoms, Ions, and Molecules |
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Chair: Tom Killian, Rice University Room: Nittany Lion Inn Boardroom II |
Wednesday, May 28, 2008 11:00AM - 11:12AM |
B6.00001: Resonant X-Ray Attenuation by Highly Ionized Ions of High-Z Elements Anil Pradhan, Sultana Nahar, Yan Yu, C. Sur, M. Montenegro, M. Mrozik, R. Pitzer Heavy elements interact very efficiently with X-rays with large attenuation coefficients. Ionization and excitation of extended electronic shells have large photoabsorption cross sections up to very high energies. It is shown that X-ray absorption in gold ions is considerably enhanced by factors of up to 1000 or more at energies of K-shell resonances from the K-alpha excitation energy to the K-ionization edge at $\sim$80 keV. Such large enhancements may be realized by X-ray irradiation in the 67-80 keV range creating inner-shell vacancies below the K-edge. We calculate the Auger resonant probabilities and cross sections to obtain total mass attenuation coefficients with detailed resonance structures for the K ---$>$ L,M,N,O,P shell transitions. This work may be potentially useful in the calculation of resonant plus non-resonant attenuation coefficients by high-Z elements in plasmas created with high-intensity lasers, monochromatic synchrotron light sources, and electron-beam-ion-traps. Spectral models for X-ray absorption and transmission, and properties of chemical compounds of high-Z elements for applications in medical and nanotechnology research are also under investigation. [Preview Abstract] |
Wednesday, May 28, 2008 11:12AM - 11:24AM |
B6.00002: Characterization of the relativistic contributions to Compton doubly differential cross sections L.A. LaJohn, R.H. Pratt Illustrations and a detailed analysis will be provoded on how relativistic factors enter into the predictions of Compton doubly differential cross sections (DDCS), using the impulse approximation (IA) theory as a model. Within IA theory it was found that one can analyze the relativistic contributions to DDCS in terms of two components, a kinematic factor $K(p_z)$ and the Compton profile $J(p_z,\rho_{rel} )$, both functions of $p_z$, the z component of the ejected electron. $J(p_z,\rho_{rel} )$ is also a function of the relativistic charge density $\rho_{rel} $. It will be shown by taking the nonrelativistic limit of $p_{min}$ (the relativistic version of $p_z$), which accounts for most of the relativistic effects on DDCS, that the relativistic shift of the Compton peak in DDCS to higher energy, determined by $J(p_z,\rho_{rel})$, as well as most of the relativistic increase in the peak amplitude determined by $K^{rel}(p_{min})$, is due to the only term in $p_{min}$ that differs from $p_z$, that is $\omega_1 \omega_2$ in the former and ${\omega_1^2 + \omega_2^2 \over 2}$ in the latter, the two terms becoming equal as $\omega_2 \rightarrow \omega_1$, ($\omega_1$ and $\omega_2$ are the incident and scattered photon energies). However the true nonrelativistic limit of $K(p_{min})$ is not obtained unless $\omega_2(1-cos \theta )$ is neglected, which is valid if $\omega_2 << mc^2$, also if $\theta $ is small even at high energies. Finally the much smaller relativistic contribution due to $\rho_{rel} $, which decreases the Compton peak height with increasing nuclear charge Z will be examined. [Preview Abstract] |
Wednesday, May 28, 2008 11:24AM - 11:36AM |
B6.00003: Characterization of laser-induced recolliding wave packet. Application to non-sequential double ionization. Samuel Micheau, Zhangjin Chen, Anh-Thu Le, Chii-Dong Lin The concept of electronic recollision in intense laser-atom interaction is of particular interest since it lies at the core of many strong field phenomena such as above-threshold ionization electrons, high-harmonic generation, or non-sequential double ionization. Underlying this concept is the generation of an electron wave packet which is driven back to its parent ion by the oscillating laser field, inducing further rescattering processes. However, this recolliding wave packet (RWP) can not be directly measured since it evolves in the presence of the laser field. We will show at the Conference how we retrieve properties of the RWP from the 2D photoelectron momentum distribution. Then, to demonstrate the physical reality of the extracted electronic wave packet, we will show that the non-sequential double ionization of atoms by a laser pulse can be interpreted as the impact ionization of the ionic core by this recolliding wave packet. [Preview Abstract] |
Wednesday, May 28, 2008 11:36AM - 11:48AM |
B6.00004: Strong-Field Control of Laser Filamentation Mechanisms Robert Levis, Dmitri Romanov, Aleskey Filin, Ryan Compton The propagation of short strong-file laser pulses in gas and solution phases often result in formation of filaments. This phenomenon involves many nonlinear processes including Kerr lensing, group velocity dispersion, multi-photon ionization, plasma defocusing, intensity clamping, and self-steepening. Of these, formation and dynamics of pencil-shape plasma areas plays a crucial role. The fundamental understanding of these laser-induced plasmas requires additional effort, because the process is highly nonlinear and complex. We studied the ultrafast laser-generated plasma dynamics both experimentally and theoretically. Ultrafast plasma dynamics was probed using Coherent Anti-Stokes Raman Scattering. The measurements were made in a room temperature gas maintained at 1 atm in a flowing cell. The time dependent scattering was measured by delaying the CARS probe with respect to the intense laser excitation pulse. A general trend is observed between the spacing of the ground state and the first allowed excited state with the rise time for the noble gas series and the molecular gases. This trend is consistent with our theoretical model, which considers the ultrafast dynamics of the strong field generated plasma as a three-step process; (i) strong-field ionization followed by the electron gaining considerable kinetic energy during the pulse; (ii) immediate post-pulse dynamics: fast thermalization, impact-ionization-driven electron multiplication and cooling; (iii) ensuing relaxation: evolution to electron-ion equilibrium and eventual recombination. [Preview Abstract] |
Wednesday, May 28, 2008 11:48AM - 12:00PM |
B6.00005: Structure of the Xe 6s and 6s' Rydberg states in supercritical Ar Luxi Li, Xianbo Shi, C.M. Evans, G.L. Findley In this paper, we present new absorption measurements and complete lineshape simulations (including all blue satellite bands) of the Xe 6s and 6s' Rydberg states doped into Ar from low density to the density of the triple point liquid, at both noncritical temperatures and on an isotherm near (+0.5$^{\circ}$C) the critical temperature of Ar. Using these simulations, as well as the known Ar induced shift of the ionization energy of a dopant\footnote{C. M. Evans and G. L. Findley, Phys. Rev. A 72, 022717 (2005).}, the Ar induced shift in the Xe 6s and 6s' term energies is determined and is shown to exhibit a large critical point effect. The nature of this critical point effect is discussed. \\ \emph{The experimental measurements reported here were performed at the University of Wisconsin Synchrotron Radiation Center (NSF DMR-0537588). This work was supported by grants from the Petroleum Research Fund, from the Professional Staff Congress--City University of New York, and from the Louisiana Board of Regents Support Fund.} [Preview Abstract] |
Wednesday, May 28, 2008 12:00PM - 12:12PM |
B6.00006: Double-resonance spectroscopy of interacting Rydberg-atom systems Aaron Reinhard, Kelly Younge, Tara Cubel-Liebisch, Brenton Knuffman, Paul Berman, Georg Raithel Systems of cold Rydberg atoms provide an excellent platform for the study of many-body, collective phenomena. In this work, the energy level spectrum of a many-body system containing two shared, collective Rydberg excitations is measured using cold atoms in an optical dipole trap. Two pairs of independently tunable laser pulses are employed to spectroscopically probe the spectrum in a double-resonance excitation scheme. Depending on the magnitude of an applied electric field, the Rydberg-atom interactions can vary from resonant dipole-dipole to attractive or repulsive van der Waals, leading to characteristic signatures in the measured spectra. Our results agree with theoretical estimates of the magnitude and sign of the interactions. [Preview Abstract] |
Wednesday, May 28, 2008 12:12PM - 12:24PM |
B6.00007: Velocity-Changing Collisions in Ultracold Neutral Plasma. Hong Gao, Jose Castro, Clayton Simien, Tom Killian We have studied the optical pumping by using fluorescence imaging in an ultracold neutral plasma (UNP). Velocity-changing collisions (VCC) have been observed during the optical pumping process. The collision causes the ions to quickly exchange momenta with their neighborhood and are optically pumped from ground state. We present our experimental data and discuss the VCC effect implications for laser cooling of a UNP. [Preview Abstract] |
Wednesday, May 28, 2008 12:24PM - 12:36PM |
B6.00008: \textit{In situ} measurements of three-dimensional ion densities in focused femtosecond radiation. James Strohaber, Cornelis Uiterwaal In conventional time-of-flight ion mass spectrometry, ionized targets are created from a distribution of intensities in the focus. In general, it is difficult to distinguish between ions created at these different intensities. Consequently, the usual course of action is to integrate ions over the entire focal region. The inevitable result of volumetric weighting is the loss of information (the volume effect). We circumvent this problem by collecting ions from a well-defined three-dimensional region of space (3 um by 10 um in the transverse direction) where the intensity is essentially constant[1]. What we have realized for use in intense field ionization experiments is a photodynamical test tube. Experimental results of spatially resolved ion densities of xenon using laser radiation having a central wavelength of 800 nm and pulse duration of 50 fs will be discussed. In addition, we present some preliminary data on the photoionization and photofragmentation of aryl halide molecules [1] J. Strohaber and C. J. G. J. Uiterwaal, Phys. Rev. Lett. \textbf{100,} 023002 (2008). [Preview Abstract] |
Wednesday, May 28, 2008 12:36PM - 12:48PM |
B6.00009: Photoelectric Effect Formula Must Include Rotational and Vibrational as well as Linear Kinetic Energies Stewart Brekke The incident photon energy in the Photoelectric Effect has traditionally thought to manifest itself only in the work function and the linear kinetic energy of the ejected electron. However, the ejected electron is also spinning and vibrating. Therefore, the incident photon energy must also manifest itself in the spin and vibrational kinetic energy of the ejected electron. It is thought that the electron loses its initial linear kinetic energy through collisions with other electrons in the material. Also, the spin and vibration could be lost in collisions with other electrons in the material. The resulting equation for the Photoelectric Effect must then be $hf = 1/2mv^2 + 1/2I\omega^2 + 1/2kx_0^2 +\phi$ where $ 1/2I\omega^2$ is the rotational kinetic energy of the electron,k is a constant of vibration and $x_0$ is the amplitude of vibration of the electron. The inverse photoelectric effect must also include the spin and vibration energies going into creation of the resulting photon. [Preview Abstract] |
Wednesday, May 28, 2008 12:48PM - 1:00PM |
B6.00010: Direct measurement of core-level relaxation dynamics on a surface- adsorbate system Jing Yin, Luis Miaja-Avila, Guido Saathoff, Chan La-o-Vorakiat, Margaret Murnane, Henry Kapteyn, Stefan Mathias, Martin Aeschlimann, Michael Bauer Electronic coupling between an adsorbate and the surface on which it resides is fundamental to the understanding of many surface interactions. However, the interaction of highly-excited adsorbate states is an area that has been explored only indirectly to-date. In this work, we present the first direct time-resolved observations of the lifetime of core-excited states of an atom adsorbed onto a surface. By implementing laser-assisted Auger decay on an adsorbate/surface system, we directly measure the lifetime of the 4d-1 core level of Xenon on Pt(111) to be 7.1 $\pm$ 1.1 fs. This result opens up time domain measurements of highly-excited state dynamics in materials systems where, because of complex interactions, energy-resolved measurements provide incomplete information. [Preview Abstract] |
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