Bulletin of the American Physical Society
APS March Meeting 2010
Volume 55, Number 2
Monday–Friday, March 15–19, 2010; Portland, Oregon
Session J23: Focus Session: Plyler Prize Session and New Trends in Spectroscopy I |
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Sponsoring Units: DCP Chair: Brooks Pate, University of Virginia Room: C125-C126 |
Tuesday, March 16, 2010 11:15AM - 11:51AM |
J23.00001: Earle K. Plyler Prize for Molecular Spectroscopy Talk: Laser Ablated Metal Atom Reactions to Form Novel Molecules Invited Speaker: A wide variety of laser-ablated metal atom reactions in solid rare gas matrices at cryogenic temperatures to form novel product molecules will be presented. These will include the ion-pair molecule Li$^{+}$O$_{2}^{-}$, the dialane molecule Al$_{2}$H$_{6}$, the thorium methylidene CH$_{2}$=ThH$_{2}$, the thorium borylene FB=ThF$_{2}$, the uranium methylidyne HC$\equiv $UF$_{3}$, the nitride N$\equiv $UF$_{3}$, and other recently prepared uranium bearing molecules. [Preview Abstract] |
Tuesday, March 16, 2010 11:51AM - 12:03PM |
J23.00002: Solving the Puzzle of Tetratomic, 23-Valence-Electron Molecules Marilyn Jacox Because of extensive electron correlation, the ground-state structures, infrared spectra, and chemical bonding properties of a number of tetratomic molecules which possess 23 valence electrons are anomalous. Approaches which have recently been helpful in understanding this phenomenon include \textit{ab initio} calculations and spectroscopic observations of these species trapped in solid neon at temperatures near 4 K. Molecules for which infrared spectra have been obtained and which will be discussed include NNO$_{2}^{-}$, NO$_{3}$, O$_{4}^{+}$, and BF$_{3}^{+}$. [Preview Abstract] |
Tuesday, March 16, 2010 12:03PM - 12:15PM |
J23.00003: Infrared Spectroscopy of Reactive Molecules Isolated in a Solid Parahydrogen Matrix David T. Anderson, Sharon C. Kettwich, Leif O. Paulson Cryogenic solid molecular hydrogen provides a weakly perturbing crystal environment to study the low temperature (2-5 K) infrared spectroscopy of embedded reactive species. Solid hydrogen is considered a quantum solid since the zero-point translational motion of the light hydrogen molecules dominates the physical properties of the crystal. Photodissociation of molecules embedded in the solid hydrogen provide a means of generating molecular species \textit{in situ} that normally would react with H$_{2}$ at room temperature, but at the low temperatures at which hydrogen is a solid these species can be trapped and studied spectroscopically. Recent studies of the photodissociation of ammonia in solid parahydrogen (hydrogen crystals enriched in the para-H$_{2}$ nuclear spin isomer) indicate the imidogen radical (NH) can be isolated and studied using infrared spectroscopy. Other photochemical systems are also being explored and the most recent results and analysis will be presented at the meeting. [Preview Abstract] |
Tuesday, March 16, 2010 12:15PM - 12:27PM |
J23.00004: Collisional $X$- and $A$-State Kinetics of CN using Transient sub-Doppler Hole Burning Michael Hause, Trevor Sears, Gregory Hall We examine the collisional kinetics of the CN radical using transient hole-burning and saturation recovery. Narrow velocity groups of individual hyperfine levels in CN are depleted ($X^{2}\Sigma ^{+})$ and excited ($A^{2}\Pi _{i})$ with a saturation laser, and probed by a counterpropagating, frequency modulated probe beam. Recovery of the unsaturated absorption is recorded following abrupt termination of an electro optically switched pulse of saturation light. Pressure-dependent recovery kinetics are measured for precursors, NCCN and CH$_{3}$COCN, and buffer gases, He, Ar and N$_{2}$. In the case of NCCN, similar recovery kinetics are observed for two-level saturation resonances, where the signal observed is a combination of $X$- and $A$-state kinetics, as well as for three-level crossover resonances, which can be chosen to probe selectively the hole-filling in the $X$ state or the decay of velocity-selected $A$ state radicals. However in the case of CH$_{3}$COCN, the $X$-state kinetics are faster than the $A$-state due to an efficient dipole-dipole rotational energy transfer mechanism. The observed recovery rates are 2-3 times faster than the estimated rotationally inelastic contribution and are a combination of inelastic and velocity-changing elastic collisions. [Preview Abstract] |
Tuesday, March 16, 2010 12:27PM - 12:39PM |
J23.00005: Frequency-stabilized cavity ring-down spectroscopy (FS-CRDS) David Long, Daniel Havey, Mitchio Okumura, Charles Miller, Joseph Hodges FS-CRDS differs from traditional single mode, cw-CRDS by actively stabilizing the optical cavity length through the use of a co-resonant reference beam. This length stabilization in turn stabilizes the cavity's free spectral range (FSR). During a spectral scan, the laser frequency is stepped between successive TEM$_{00}$ modes, resulting in an extremely linear and stable spectral frequency axis. This stability leads to a short term noise equivalent absorption coefficient of 2.5$\times$10$^{-10}$ cm$^{-1}$ Hz$^{-1/2}$. Long-term averaging is also possible, due to the stable frequency axis, allowing for a detection limit of 1.8$\times$10$^{-11}$ cm$^{-1}$, corresponding to a line intensity of 2.5$\times$10$^{-31}$ cm/molec. Using FS-CRDS we have quantitatively measured nine ultraweak electric quadrupole transitions in the O$_{2}$ $\textit{A}$-band having intensities of 3$\times$10$^{-30}$ to 2$\times$10$^{-29}$ cm/molec. Eight of these transitions had not previously been observed. [Preview Abstract] |
Tuesday, March 16, 2010 12:39PM - 1:15PM |
J23.00006: Multi-frequency THz Heterodyne Spectroscopy using Electro-Optic Sampling Invited Speaker: Multi-frequency heterodyne spectroscopy, developed by two groups (Schiller as well as van der Weide, Keilmann and co-workers) uses one optical femtosecond frequency comb (FFC) to probe a sample. A second FFC with a slightly detuned spacing is used as a multi frequency local oscillator to uniquely map the broadband optical spectroscopic information to the RF domain where it can be easily analyzed. Researchers at NIST (Coddington et al) have realized the full potential of this technique by tightly locking the detuned combs together using optical locking techniques. It is of considerable interest to extend such capabilities to access the so-called molecular vibrational ``fingerprint'' range of approximately $10$ to $100$ THz ($300$ to $3000$ cm$^{-1}$). A transfer of the direct heterodyne detection approach used in the optical regime down to this frequency range is fraught with difficulties including significantly lower power of the probe THz frequency comb. In addition, a low noise detector with a relatively fast RF response ($>100$ MHz at a minimum) is required. An alternative, indirect detection technique for detecting THz signals is electro-optic sampling (EOS). It has employed for time domain THz spectroscopic applications for a number of years with a demonstrated spectral detection ranging from 0.5 THz range to over 100 THz. Through careful analysis of the EOS we show how electro-optic sampling of THz frequency comb by a detuned optical FFC followed by direct optical detection of the optical sampling beam enables conversion of the THz spectroscopic data directly to the RF domain. In particular, we show there is a one-to-one correspondence between a detected RF heterodyne beat and THz comb element. Numerical simulations predict excellent signal to noise ratio of the RF beats (20 dB) with modest acquisition times (10 $\mu$s). We will also summarize our progress toward experimental realization of such a system. [Preview Abstract] |
Tuesday, March 16, 2010 1:15PM - 1:27PM |
J23.00007: Cavity Enhanced Absorption Spectroscopy using a Prism Cavity and Supercontinuum Source Kevin K. Lehmann, Paul S. Johnston The multiplex advantage of current cavity enhanced spectrometers is limited by the limited high reflectivity bandwidth of the dielectric mirrors used to construct the high finesse cavity. We report on our development of a spectrometer that uses Brewster's angle retroreflectors that is excited with supercontinuum radiation generated by a 1.06 $\mu$m pumped photonic crystal fiber, which covers the 500-1800 nm spectral range. Recent progress will be discussed including modeling of the prism cavity losses, alternative prism materials for use in the UV and mid-IR, and a new higher power source pumped by a mode-locked laser. [Preview Abstract] |
Tuesday, March 16, 2010 1:27PM - 1:39PM |
J23.00008: Large amplitude local-bending eigenstates in the SEP spectrum of acetylene Adam Steeves, Josh Baraban, Robert Field The bending dynamics of acetylene undergo a normal-to-local transition in the vicinity of twelve quanta of excitation. We report the observation, by stimulated emission pumping, of the first few eigenstates exhibiting local-mode behavior. Assignments are made on the basis of complementary information in spectra recorded from intermediate levels with {\it gerade} and {\it ungerade} vibrational symmetry. The highest energy state currently assigned ($N_{\mathrm{bend}}=16$) is characterized by a large ($> 75^{\circ}$) distortion from the linear geometry at its classical turning point. Large geometrical distortions will lead to unique electronic, collisional, and chemical properties for these eigenstates. [Preview Abstract] |
Tuesday, March 16, 2010 1:39PM - 1:51PM |
J23.00009: Molecular Structure and Dynamics Probed by Rydberg Electrons Fedor Rudakov, Peter Weber Probing molecular structure as a chemical reaction unfolds has been a long standing goal in chemical physics. Most spectroscopic and diffraction techniques work well when the molecule is cold and thus vibrational motion is minimized. Yet, in order to initiate a chemical reaction, a large amount of energy has to be pumped into the molecule. Therefore, most well-established techniques are generally inapplicable to studying ultrafast molecular transformations except for highly favorable cases. In our research we demonstrated that Rydberg electrons are very sensitive to the molecular structure. Photoionization of the molecule out of Rydberg states reveals a purely electronic spectrum not encumbered by vibrational motion. Thus, the technique is largely insensitive to the vibrations of the molecule. As an example of using Rydberg electrons as a probe for molecular structure we report on the isomerizations of cyclohexadiene and quadricyclane on an ultrafast time scale. [Preview Abstract] |
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