Bulletin of the American Physical Society
38th Annual Meeting of the Division of Atomic, Molecular, and Optical Physics
Volume 52, Number 7
Tuesday–Saturday, June 5–9, 2007; Calgary, Alberta, Canada
Session H2: Undergraduate Research Session |
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Chair: A. Landers, Auburn University Room: TELUS Convention Centre Macleod D |
Thursday, June 7, 2007 10:30AM - 11:06AM |
H2.00001: New Active Feedback Scheme for Minimization of Instrumental Asymmetries Invited Speaker: Chiral effects in nature exist but are generally difficult to detect. A Univeristy of Nebraska Lincoln experiment to detect electron circular dichroism by sending polarized electrons through a chiral medium has met with difficulties in reducing the instrumental asymmetry below the expected true asymmetry of $\sim$10$^{-4}$. In order to minimize this false asymmetry, a new optical apparatus designed for the production of polarized electrons as been built. It is based on the fast chopping of two spatially separated beams of light with orthogonal linear polarizations which are recombined and passed through a quarter wave plate to yield a single beam with rapidly flipping helicity. New methods for measuring the helicity-dependent intensity asymmetry in this apparatus have been developed. These methods show that the main reason for large instrumental asymmetries is drift due to laser polarization instability. Active electro-optical feedback has been successfully employed to maintain this asymmetry below 2x10$^{-5}$. [Preview Abstract] |
Thursday, June 7, 2007 11:06AM - 11:42AM |
H2.00002: Photodetachment Spectroscopy of Ce$^{-\ast }$ Invited Speaker: The lanthanide series of negative ions provides interesting challenges and rich spectroscopic results for experiment and theory alike. A 12keV Ce$^{-}$ beam has been used in perpendicular arrangement with tunable radiation from an Nd:YAG pumped OPPO laser to perform laser photodetachment spectroscopy. The quantity of photodetached neutrals has been measured as a function of photon energy in order to determine the relative cross section for neutral production. Within the range 0.5eV -- 0.75eV, the spectrum exhibits five sharp peaks in addition to broad threshold behavior. The energies and widths of these resonances will be used to further explain the discrepancies between standing theoretical [1] and experimental [2] values for the electron affinity. \newline [1] S.M. O'Malley and D.R. Beck, \textit{PRA} \textbf{61}, 034501 (2000), \textbf{74}, 042509 (2006); X. Cao and M. Dolg,\textit{ PRA} \textbf{69}, 042508 (2004). \newline [2] V.T. Davis and J.S. Thompson, \textit{PRL} \textbf{88}, 073003 (2002). [Preview Abstract] |
Thursday, June 7, 2007 11:42AM - 12:18PM |
H2.00003: Determination of the Ps-He Momentum-transfer Cross-section Using Time Resolve Doppler Broadening Invited Speaker: This experiment uses Doppler broadening to analyze the thermalization of Positronium (Ps) in Helium gas in order to calculate the momentum-transfer cross-section of Ps. There is wide variation in previous experimental and theoretical results and so this experiment seeks to improve the measurement of this value. Tracking the thermalization of Ps atoms contained within a He gas chamber, Doppler Broadening is used to determine the average energy of Ps with respect to time. From the analysis of this energy as a function of time we can fit the data with a theoretical thermalization model and from this model determine the cross-section of Ps on He. Our work this summer has yielded a value for this cross-section, including work done to determine the corresponding error bars. [Preview Abstract] |
Thursday, June 7, 2007 12:18PM - 12:54PM |
H2.00004: Trapping Single Krypton Atoms for Radioactive Background Measurements Invited Speaker: We report on the construction of an apparatus for the trapping and detection of single metastable krypton atoms, which will be used to measure krypton contamination in other rare gases by Atom Trap Trace Analysis (ATTA). A beam of atoms excited to the $5s[3/2]_2$ ($^3P_2$) metastable state are decelerated in a Zeeman slowing magnet, and loaded into a magneto-optical trap, where their fluorescence is detected using an avalanche photodiode. We estimate that the system will enable us to measure krypton contamination at the $3 \times 10^{-14}$ level in three hours of integration, which is close to the level required for proposed astrophysical detectors using liquid rare gases as a scintillation medium. We will discuss the trap loading and detection efficiency, and possible improvements through the use of an optically excited metastable atom source. [Preview Abstract] |
Thursday, June 7, 2007 12:54PM - 1:30PM |
H2.00005: Isotopic effects in bond rearrangement caused by sudden ionization of ammonia and methane molecules Invited Speaker: The production of H$_{2}^{+}$ and H$_{3}^{+}$ fragments upon dissociation of ammonia and methane molecules involves rearrangements of the molecular bonds. Fast ion impact results in ionizations on time scales of 10 attoseconds, thus freezing the nuclear motion. Our earlier studies of H$_{2}^{+}$ formation upon dissociation of water molecules by fast ions showed a strong isotopic dependence. Recently, we measured isotopic effects in the production rate of H$_{2}^{+}$ and H$_{3}^{+}$ from NH$_{3}$ and CH$_{4}$ ionized by the same ions. Compared to H$_{2}^{+}$, creating H$_{3}^{+}$ requires the rapid cleavage and formation of additional bonds. The process is enhanced, however, by the geometrical similarity of the parent molecules and the triangle-shaped H$_{3}^{+}$ ions. Differences in triangle size result in vibrational excitation and may cause isotopic differences. [Preview Abstract] |
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