Bulletin of the American Physical Society
43rd Annual Meeting of the APS Division of Atomic, Molecular and Optical Physics
Volume 57, Number 5
Monday–Friday, June 4–8, 2012; Orange County, California
Session P3: Experimental Probes of Atoms, Molecules, Clusters and Surfaces |
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Chair: Frank Narducci Room: Grand Ballroom E |
Thursday, June 7, 2012 2:00PM - 2:12PM |
P3.00001: Electric dipole polarizabilities of atomic clusters of Sodium Anthony Liang, John Bowlan, Walter deHeer A new discussion of the electronic shell structure of simple metal clusters is presented.~Due to size quantization, cluster valence electrons order into energy shells as in atoms. We show that the oscillation of electric dipole polarizability as a function of size for sodium clusters (both in amplitude and shell closing numbers) can be explained by spherical well filling of electron wavefunctions. The shell closing numbers are closely examined.~Interestingly, most theories involving cluster shape deformations do not yield the measured amplitude and closing numbers, while an existing simple spherical shape theory has correctly predicts both.~This may hint at the occurrence of proposed resonant shape coexistence in nanoclusters.~ We also discuss the trend of oscillations (again, both in amplitude and shell closing numbers) in measurements of atomic separation energy of sodium clusters, the magnetic moments of nickel clusters, the magnetic moment of the sodium cluster Na69, and photoabsorption of sodium clusters, and point out interesting similarities.~ It appears that there may be more universal properties originating from shell filling in simple metal clusters than previously observed.~ The~electric and magnetic field deflection measurements were carried out with a 20 K sodium cluster molecular beam apparatus. [Preview Abstract] |
Thursday, June 7, 2012 2:12PM - 2:24PM |
P3.00002: Double and triple ionization of silver clusters by electron impact Avik Halder, Anthony Liang, Chunrong Yin, Vitaly Kresin Metal clusters are finite droplets of delocalized electrons with discrete energy levels, due to the small confinement volume, often referred to as ``artificial atoms'' or ``superatoms.'' Ionization processes involving clusters provide insight into the energetics of charging of finite quantum systems. While the evolution of the single-ionization energy with cluster size has been extensively studied, less is known about multiple ionization thresholds and efficiencies. We probed the production of several selected silver cluster cations Ag$_{n}^{2+}$ and Ag$_{n}^{3+}$ by electron impact ionization. The scaling of ionization thresholds with particle radius follows the metallic droplet model, but, curiously, with a slope which is significantly different from the previous literature values for single ionization. Another observation is that as the electron energy increases, the yield of high-charge cations grows faster than that of singly-charged Ag$_{n}^{+}$. This behavior is consistent with the power-law dependence of post-threshold ionization known for atomic systems. Mechanisms involved in multiple ionization phenomena in metal clusters appear far from completely understood and call for further experimental and theoretical examination. [Preview Abstract] |
Thursday, June 7, 2012 2:24PM - 2:36PM |
P3.00003: Spectrally encoded optical/x-ray relative delay with $\sim$10 fs RMS resolution Ryan Coffee, Mina Bionta, Christoph Bostedt, Matthieu Chollet, David Fritz, Nick Hartmann, Henrik Lemke, Marc Messerschmidt, Daniel Ratner, Sebastian Schorb, James Cryan, James Glownia, Mariano Trigo, Marion Harmand, Sven Toleikis, Marco Cammarata, Doug French, Daniel Kane We present a spectral encoding technique that measures the single-shot relative delay between optical and x-ray laser pulses at the Linac Coherent Light Source. The technique has now been shown capable of resolving relative delays with an RMS accuracy down to 10 fs for both soft and hard x-rays. We sort the single-shot measurements into time-ordered traces and construct a scanning spectrogram representation of the x-ray/optical cross-correlation reminiscent of frequency resolved optical gating. We will discuss how such measurements can be used to reconstruct the ultrafast material response to the x-ray pulses. Once the material response is known, it may be possible to reverse the algorithm to reconstruct the average temporal shape of the x-ray pulses. [Preview Abstract] |
Thursday, June 7, 2012 2:36PM - 2:48PM |
P3.00004: Spectroscopy of the Cs 6s-5d quadrupole transition for qubit measurements Tian Xia, Alex Carr, Gang Li, Siyuan Zhang, Mark Saffman We have performed spectroscopy of the Cs $6s_{1/2} - 5d_{5/2}$ quadrupole transition using a heated thermal cell and a sample of cold trapped atoms. Measurements of the excited state hyperfine splittings have been used to determine values for the hyperfine constants. This transition is of interest for background free measurements on Cs qubits, as well as narrow line cooling since the Doppler tmeperature is only about $3 ~\mu\rm K$. We will report on experimental progress towards laser cooling and hyperfine qubit state detection using the quadrupole transition. [Preview Abstract] |
Thursday, June 7, 2012 2:48PM - 3:00PM |
P3.00005: Time-Dependent Electron Interactions in Double-Rydberg Wavepackets F. Robicheaux, X. Zhang, R.R. Jones We consider the evolution of double-Rydberg wavepackets in which the two valence electrons in Ba are radially localized via sequential short-pulse laser excitation. Following the launch of the wavepackets, the atoms are exposed to a subpicosecond, half-cycle electric field pulse (HCP). The atoms either directly ionize during the HCP or are further excited by it, autoionizing to a distribution of Ba$^{+}$ Rydberg states. These ions are then subjected to a field ramp which ionizes the most highly excited Ba$^{+}$ to Ba$^{2+}$. The Ba$^{2+}$ signal is recorded as a function of the relative launch times of the two electrons and as a function of the HCP delay. The data show a dramatic reduction in the Ba$^{2+}$ yield when the HCP arrives after the excitation of the second wavepacket, suggesting that autoionization occurs almost instantly. This conclusion is supported by both full quantum and classical Monte Carlo calculations. The quantum calculations numerically solve the time dependent Schrodinger equation using a radial grid of points for each electron and coupled spherical harmonics. These calculations do not include the HCP but clearly show that more than 80\% of the atoms ionize within one Rydberg period and the character of the ionization depends on the relative launch times. [Preview Abstract] |
Thursday, June 7, 2012 3:00PM - 3:12PM |
P3.00006: Manipulation of Raman Resonances Using Magnetic Fields Sara A. DeSavage, Jon P. Davis, Frank A. Narducci We have theoretically and experimentally studied Raman resonances in multi-level atoms (specifically $^{85}Rb$). Our emphasis has been on varying the relative orientation of the magnetic field with respect to the propagation direction of the Raman fields. We find that, in general, the spectrum consists of up to 11 peaks. By considering selection rules, we show that it is possible to orient the magnetic field so that either a 6 peak spectrum or 5 peak spectrum results, depending on whether the Raman fields contain a polarization component along the magnetic field direction or not. Furthermore, we find that the spectrum is not always symmetric with respect to the magnetically insensitive transition (clock transition). We explore the origins of the asymmetry and the overall shape of the spectra. We will discuss applications to magnetically sensitive atom interferometry.\\ [Preview Abstract] |
Thursday, June 7, 2012 3:12PM - 3:24PM |
P3.00007: Formation of hollow atoms above a surface Jean Pierre Briand, Ronald Phaneuf, Stephane Terracol, Zuqi Xie Slow highly stripped ions approaching or penetrating surfaces are known to capture electrons into outer shells of the ions, leaving the innermost shells empty, and forming hollow atoms. Electron capture occurs above and below the surfaces. The existence of hollow atoms below surfaces e.g. Ar atoms whose K and L shells are empty, with all electrons lying in the M and N shells, was demonstrated in 1990 [1]. At nm above surfaces, the excited ions may not have enough time to decay before hitting the surfaces, and the formation of hollow atoms above surfaces has even been questioned [2]. To observe it, one must increase the time above the surface by decelerating the ions. We have for the first time decelerated O$^{7+}$ ions to energies as low as 1 eV/q, below the minimum energy gained by the ions due to the acceleration by their image charge. As expected, no ion backscattering (trampoline effect) above dielectric (Ge) was observed and at the lowest ion kinetic energies, most of the observed x-rays were found to be emitted by the ions after surface contact. \\[4pt] [1] J. P. Briand et al., Phys.Rev.Lett\textbf{. 65}(1990)159.\\[0pt] [2] J.P. Briand, AIP Conference Proceedings \textbf{215 }(1990) 513. [Preview Abstract] |
Thursday, June 7, 2012 3:24PM - 3:36PM |
P3.00008: Surface enhanced quantum control of a two-level system Chitra Rangan, Somayeh M.A. Mirzaee We demonstrate the enhanced purification of the quantum state of a two-level system subject to a near-resonant driving field when in proximity to a gold nanoparticle. The quantum dynamics of the driven two-level system in the presence of decay is modelled by the Lindblad Master equation. The electrodynamics of the gold nanoparticle illuminated by the driving field and the field radiated by the atomic dipole is solved using a finite-difference time-domain method. We discover that the presence of a proximate gold nanoparticle enhances the purity of a driven two-level system even at short times. [Preview Abstract] |
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