Bulletin of the American Physical Society
2008 APS March Meeting
Volume 53, Number 2
Monday–Friday, March 10–14, 2008; New Orleans, Louisiana
Session B14: Casimir Forces, Collisions and Atomic Structure |
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Sponsoring Units: DAMOP Chair: Ho Bun Chan, University of Florida Room: Morial Convention Center 205 |
Monday, March 10, 2008 11:15AM - 11:27AM |
B14.00001: Measurement of near-field radiative heat transfer and implications for Casimir force measurements Arvind Narayanaswamy, Sheng Shen, Gang Chen Near--field force and energy exchange between two objects due to quantum electrodynamic fluctuations give rise to interesting phenomena such as Casimir and van der Waals forces, and thermal radiative transfer exceeding Planck's theory of blackbody radiation. Although significant progress has been made in the past on the precise measurement of Casimir force related to zero-point energy, experimental demonstration of near-field enhancement of radiative heat transfer is difficult. In this work, we present a sensitive technique of measuring near--field radiative transfer between a microsphere and a substrate using a bi--material atomic force microscope (AFM) cantilever, resulting in ``heat transfer-distance'' curves. Measurements of radiative transfer between a sphere and a flat substrate show the presence of strong near--field effects resulting in enhancement of heat transfer over the predictions of the Planck blackbody radiation theory. The implications of measurement of near-field radiative heat transfer for determining of the magnitude of the thermal component of the Casimir force will be discussed. [Preview Abstract] |
Monday, March 10, 2008 11:27AM - 11:39AM |
B14.00002: Casimir interactions between cold atoms and corrugated surfaces Diego A.R. Dalvit, Paulo A. Maia Neto, Astrid Lambrecht, Serge Reynaud The lateral Casimir-Polder force between an atom and a corrugated surface should allow the experimental study of non trivial geometrical effects in quantum vacuum. We apply the scattering approach to compute this force for an atom (or a BEC) above a corrugated surface, and compare our exact results with two commonly used approximations, the proximity approximation and the pairwise summation technique. We show that large corrections to these approximations could be measured using present-day technology with a BEC used as a vacuum field sensor. For details, see D.A.R. Dalvit et al, arXiv:0710.5249, 0709:2095. [Preview Abstract] |
Monday, March 10, 2008 11:39AM - 11:51AM |
B14.00003: Orientation dependence of Casimir force between uniaxial crystals Mark Romanowsky, Federico Capasso We present calculations showing that the Casimir force between uniaxial crystals depends on the orientation of the optical axes. For strongly anisotropic crystals, the Casimir force can be substantially different when the optical axes are perpendicular to the crystal surfaces versus the case in which they are parallel to the surfaces. We compute the force numerically for cases of experimental interest, and discuss the prospects for observing this effect. [Preview Abstract] |
Monday, March 10, 2008 11:51AM - 12:03PM |
B14.00004: Measurements of the Casimir interaction between a sphere and a rectangular corrugated plate Yiliang Bao, H. B. Chan We present measurements of the Casimir force gradient between a gold-coated sphere and a highly doped silicon plate with an array of nanoscale, high-aspect-ratio trenches. The Casimir force arises from quantum fluctuation of electromagnetic fields in vacuum and is strongly dependent on the boundary condition. While the majority of the precise measurements have been performed on the simple arrangement of plate-sphere or two parallel plates, few experiments have been done in geometries with interactions that deviate significantly form the pair-wise summation of two-body potentials. We choose one of the interacting surfaces to be an array of trenches with widths ranging from 200 nm to 500 nm and depth of 1um. Theoretical analysis predict that for perfectly conducting surfaces with such geometry, the Casimir force differs by up to 70 {\%} from pairwise summation at separation of 0.4 um. In our experiment, the force gradient is measured by the frequency shifts caused by Casimir force in the response of a periodically driven micromachined torsional oscillator, yielding a highly sensitive measurement of the force gradient for separations between 80nm and 500nm. [Preview Abstract] |
Monday, March 10, 2008 12:03PM - 12:15PM |
B14.00005: Numerical Evaluation of the Casimir Force in Concave Piston Geometries Martin Schaden Using a modified wordline approach[1] for a massless scalar field satisfying Dirichlet boundary conditions, I numerically calculate the Casimir force on a piston in a \textit{closed} \textit{concave} cavity of various shapes. There are no contributions from arbitrarily short paths and the Casimir \textit{force} on the piston is finite for all systems considered. I relate the Casimir interaction energy of concave cavities to a probability measure on the convex hulls of Brownian bridges. The resulting algorithm for Casimir forces in concave geometries is numerically stable, fast and very accurate. The results depend only on the number of hulls and points used and extrapolate readily to the continuum limit. I compare some of these numerical results to semiclassical estimates of the force[2] in similar piston geometries. [1] H. Gies, K. Langfeld and L. Moyaerts, JHEP 0306, 018 (2003); H. Gies and K. Klingmuller, Phys. Rev. D74, 045002 (2006). [2] L Mateescu and M. Schaden, [quant-ph/0705.3435]. [Preview Abstract] |
Monday, March 10, 2008 12:15PM - 12:27PM |
B14.00006: Measurements of the Casimir-Lifshitz force between a metal and a dielectric in fluid Jeremy Munday, Federico Capasso The Casimir force arises from the confinement of quantum fluctuations of the electromagnetic fields. The boundary conditions on these fields are dependent upon the electromagnetic properties of the interacting materials and affect both the magnitude and sign of the resulting force. We will discuss our experiments for measuring the Casimir-Lifshitz force between a metal and a dielectric immersed within a fluid and will describe situations which can give rise to a repulsive interaction. [Preview Abstract] |
Monday, March 10, 2008 12:27PM - 12:39PM |
B14.00007: On the repulsive Casimir force using metamaterials. Felipe Da Rosa, Diego Dalvit, Peter Milonni It is known for quite some time that Casimir repulsion between a dielectric and a magnetodielectric plate is possible, and the development of metamaterials brought this phenomenon closer to experimental possibilities. The purpose of this work is to discuss as realistically as possible the role that metamaterials play in the Casimir force and bring to the surface some aspects of this issue that were previously rarely mentioned, such as the typical anisotropy of metamaterials and the presence of a Drude background in its electric permittivity. We also study the Casimir and Polder force between an atom and a metamaterial, since this setup may be more suitable to the detection of repulsion forces. [Preview Abstract] |
Monday, March 10, 2008 12:39PM - 12:51PM |
B14.00008: Spin depolarisation of N$_{2}^{+}$ ($^{2}\Sigma ^{+})$ in collisions with $^{3}$He and $^{4}$He in a magnetic field Thierry Stoecklin, Gr\'egoire Guillon, Anatoli Voronin The possibility of tuning the interactions between atoms and molecules $^{[}$\footnote{ R. V. Krems, A. Dalgarno, J. Chem. Phys. 120, 2296 (2004)}$^{]}$ using a magnetic field has open new perspectives of controlling collisional energy transfer at very low temperature. In a first study dedicated to He-N$_{2}^{+}$ inelastic collisions we found that spin free collisions of N$_{2}^{+}$ with $^{3}$He and $^{4}$He exhibit a strong isotope effect in the ultra cold regime $^{[}$\footnote{ T.Stoecklin and A. Voronin, Phys Rev A. (2005), 72 : 042714.}$^{,}$\footnote{ G. Guillon, T. Stoecklin and A. Voronin, Phys. Rev. A. \textbf{75,} 052722 (2007).}$^{]}$. and recently found a similar effect for another ionic system: the He-CH$^{+} \quad ^{[}$\footnote{ T. Stoecklin and A. Voronin, Eur. Phys. J. D. (2007) DOI: 10.1140/epjd/e2007-00293-3}$^{] }$collision. In the present work, we compare first in the absence of an applied magnetic field the cross sections for the transitions changing the projection of the total angular momentum of N$_{2}^{+}(^{2}\Sigma )$ in collisions with $^{3}$He and $^{4}$He at very low collision energy$^{[}$\footnote{ G.guillon, T.Stoecklin and A. Voronin, Eur. Phys. J. D. (in press)}$^{]}$.In the second part of this contribution we investigate the effect of an applied magnetic field and compare the results obtained for the fundamental states of the two nuclear spin isomers of N$_{2}^{+}$. As a result of the different mechanisms of action of the spin rotation interaction for these two rotational levels we find a great difference of sensitivity to the applied magnetic field. Whereas even moderate values of the applied magnetic field (10 Gauss) completely modify the very low collision energy behaviour of the spin depolarisation cross section of the fundamental ortho level, we find that one has to apply magnetic fields two orders of magnitude larger to obtain similar effects for the fundamental para level. [Preview Abstract] |
Monday, March 10, 2008 12:51PM - 1:03PM |
B14.00009: Depolarization of Ne*(2p$_{i}$ [J=1]) atoms induced by collisions with He(1s$^{2}$) at 10 K $<$ T $<$ 3,000 K Cristian Bahrim, Vaibhav Khadilkar Depolarization mechanisms of Ne$^{\ast }$(2p$^{5}$ 3p; 2p$_{i}$ [J=1]) atoms induced by collisions with helium in a gas mixture at thermal equilibrium are analyzed using a close-coupling quantum method within a model potential approach [1]. Our goal is to explain measurements of alignment destruction (which is a mechamism that includes the disalignment and the depopulation of atoms) [2], disorientation [3], and disalignment [4] of Ne$^{\ast }$(2p$_{i }$[J = 1]) atoms induced by He-Ne collisions at 10 K $<$ T $<$ 3,000 K using the density matrix formalism. Such analysis offers accurate information about anisotropic atom-atom potentials and improves our model potential for the HeNe$^{\ast }$(2p$^{5 }$3p) system [1] by adding the polarizability of Ne$^{\ast }$ atoms on various 2p$_{i}$ states to the long-range potentials. Our results for neon excited on the 2p$_{2}$, 2p$_{5}$, 2p$_{7}$ and 2p$_{10}$ states are reported, and excellent agreement with available measurements [2, 3, 4] is found. Also the atomic polarizabilities for Ne$^{\ast }$on 2p$_{2}$, 2p$_{5}$, 2p$_{7}$ and 2p$_{10}$ states are reported. [1] Bahrim C, Kucal H and Masnou-Seeuws F 1997 \textit{Phys. Rev. A} \textbf{56} 1305. [2] Carrington C G and Corney A 1971 \textit{J. Phys. B }\textbf{4} 869. [3] Seo M, Shimamura T, Furatani T, Hasuo M, Bahrim C and Fujimoto T 2003 \textit{J. Phys. B }\textbf{36} 1885. [4] Nimura M, Hasuo M and Fujimoto T 2004\textit{ J. Phys. B. }\textbf{37} 4647. [Preview Abstract] |
Monday, March 10, 2008 1:03PM - 1:15PM |
B14.00010: Observation of the n($^3$He,t)p Reaction by Detection of Far-Ultraviolet Radiation Charles W. Clark, Alan K. Thompson, Michael A. Coplan, John W. Cooper, Patrick Hughes, Robert E. Vest We have detected Lyman alpha radiation as a product of the n($^3 $He,t)p nuclear reaction, induced in a $^3$He gas cell irradiated by a cold neutron beam at the NIST Center for Neutron Research. The predominant source of this radiation appears to be decay of the 2p state of tritium produced by charge transfer and excitation collisions with the background $^3$He gas. For atmospheric pressure and room temperature in the $^3$He cell, we find yields of tens of Lyman alpha photons for every neutron reaction. These results suggest a method of cold neutron detection that is complementary to existing technologies that use proportional counters. In particular, this approach may provide single neutron sensitivity with wide dynamic range capability, and a class of neutron detectors that are compact and operate at relatively low voltages. [Preview Abstract] |
Monday, March 10, 2008 1:15PM - 1:27PM |
B14.00011: Photo-Fragmentation of Lithium Atoms in FEL Radiation Fields Matthew Foster, J. Colgan, M.S. Pindzola, Alexander Dorn Multi-electron ejection from lithium induced by absorption of a single photon is a fundamental few-body reaction that tests the correlated interaction dynamics between atomic constituents. Experiments have been proposed using intense FEL radiation at FLASH in combination with reaction microscopes to measure four-body dynamics. These proposed experiments will first measure the double ionization dynamics from both the even parity Li ground state and the laser excited odd parity Li(2p $^2P^o$) state. We will present theoretical calculations using the time-dependent close-coupling method (TDCC) to assist in the experimental search for interesting correlation effects for both double and triple ionization of lithium. [Preview Abstract] |
Monday, March 10, 2008 1:27PM - 1:39PM |
B14.00012: Long range anions:diatomic, triatomic and polyatomic molecules Edward C. Chen, Edward S. Chen The observation of both long range and valence anions of O$_{2}$, NO, CS$_{2}$, N$_{2}$O, O$_{3}$, SF$_{6}$ , C$_{6}$F$_{6}$ CH$_{3}$NO$_{2}$, tetracene, anthracene, acridine, perylene, pyrene, naphthalene and the nucleobases is reported in negative ion mass spectrometry, photoionization, electron impact in nanodroplets, electron swarm experiments at low temperature and alkali metal beam studies.. New polarization or quadrupole bound electron affinities less than 0.1 eV are reported for the aromatic hydrocarbons, O$_{2}$, SF$_{6}$, CS$_{2}$ , C$_{6}$F$_{6}$ while dipole bound values less than 0.15 eV are reported for the nucleobases, N$_{2}$O and NO. The long range states act as gateways to valence states. This relationship is illustrated by Morse potential energy curves in single bond dissociations and in reaction coordinates analogous to Marcus parabolas. New adiabatic electron affinities are reported for some of these molecules, including (in eV) guanine, 1.64(2) adenine, 1.09(2); C$_{6}$F$_{6}$, 1.26(5); acridine, 1.09(2); perylene ,1.09(2); tetracene, 1.10(2), naphthalene, 0.19(2) eV. [Preview Abstract] |
Monday, March 10, 2008 1:39PM - 1:51PM |
B14.00013: The Effect of Birth Location in an Intense Laser Focus on the Non-Sequential Double Ionization Yield Jay Paquette, Jan Chaloupka Atoms in an intense laser field can become doubly ionized through a direct process known as rescattering, where a single electron is liberated through tunnel ionization and is driven back to the ion core by the laser field, leading to impact ionization and release of a second electron. In this quasi-classical description, the trajectory of the first electron will have a strong influence on the probability of release of a second electron, as evidenced by the reduction in yield with elliptical laser polarization. Even with linear polarization, the first electron can avoid a reencounter with the ion due to the v$\times $B term or the longitudinal electric field component (E$_{z})$ in the laser focus. Since the E$_{z}$ term is given from the requirement of zero divergence of the electric field, its magnitude will vary as a function of position within the laser focus. Using completely classical 3-D simulations, we demonstrate how longitudinal electric field variations affect electron trajectories, how the ion yields from various regions within the focus are affected, and the likelihood of observing this effect experimentally. [Preview Abstract] |
Monday, March 10, 2008 1:51PM - 2:03PM |
B14.00014: Modeling Plasma Induced Dispersion in a Modified Nonlinear Schr\"{o}dinger Equation Jeremy Gulley, William Dennis Ultrafast laser pulse propagation in dielectrics is often modeled by a modified nonlinear Schr\"{o}dinger equation (NLSE). At laser intensities sufficient to cause ionization, a plasma term can be included in the modified NLSE to account for free-carrier optical effects. This term is linear in the field and may be calculated from the classical Drude theory. We explore the consequences of a newly developed method for including dispersion relations of the plasma term as predicted by the Drude theory into the framework of the modified NLSE. The plasma induced dispersion terms can be shown to strongly effect ultrashort pulse evolution through regions of high plasma density after propagation distances as short as the skin depth. Our results suggest that, in regions high plasma density, plasma induced dispersion may be more significant than other linear or nonlinear dispersion corrections to the NLSE. [Preview Abstract] |
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