Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session V18: Chemical Dynamics & Molecular Spectroscopy |
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Sponsoring Units: DCP Chair: Elliot Bernstein, Colorado State University Room: Colorado Convention Center 103 |
Thursday, March 8, 2007 11:15AM - 11:27AM |
V18.00001: Ultrafast 2D IR vibrational echo chemical exchange spectroscopy Junrong Zheng, Michael Fayer Ultrafast 2D IR vibrational echo chemical exchange spectroscopy, akin the 2D NMR methods, is applied to the study of dynamics of weakly hydrogen bonded solute-solvent complexes in liquid solutions. The strengths of the solute-solvent hydrogen bonds are adjusted by modifying the chemical structures of the solutes and solvents. For the eight samples studied, the formation enthalpies vary from -0.6 kcal/mol to -2.5 kcal/mol, and the dissociation time constants vary from 3 ps to 32 ps. The dissociation rates of the hydrogen bonds are found to be strongly correlated with their formation enthalpies. The correlation can be described with an equation similar to the Arrhenius equation. As another example of chemical exchange spectroscopy, the rate of carbon-carbon single bond rotational isomerization of an ethane derivative in room temperature liquid solution is measured. Based on the experimental results and density functional theory calculations, the time constant for the ethane internal rotational isomerization under the same conditions is about 12 ps. [Preview Abstract] |
Thursday, March 8, 2007 11:27AM - 11:39AM |
V18.00002: Dynamic Nuclear Polarization (DNP) Using Nitroxide Radicals Brandon Armstrong, Evan McCarney, Songi Han The theory of the Overhauser Effect in liquids is well established, but measured DNP enhancements from nitroxide radicals depart significantly from prediction. To achieve large signal enhancements, milli-molar concentrations of radicals are needed, a regime where Heisenberg exchange of the unpaired electron is significant. Therefore, the three electron transitions resulting from hyperfine interactions with the $^{14}$N nucleus cannot be treated as independent in a DNP experiment. Furthermore, the relaxation rate of $^{14}$N can be easily on the same order of magnitude or even greater than the relaxation rate of the unpaired electron, contributing to the mixing of the hyperfine states, even in the absence of Heisenberg exchange. We present a quantitative study and a new model of $^{1}$H DNP enhancement of water by varying radical concentrations and solvent viscosities of natural abundance $^{14}$N versus $^{15}$N isotope enriched 4-Oxo-TEMPO free radical at 0.35 Tesla. [Preview Abstract] |
Thursday, March 8, 2007 11:39AM - 11:51AM |
V18.00003: Comparison of Electronically Excited Photodissociation between Nitramine Energetic Materials and Model Systems Yuanqing Guo, Margo Greenfield, Atanu Bhattacharya, Elliot Bernstein Nitramine energetic materials (RDX, HMX and CL20) have broad applications as explosives and fuels. Model systems (1,4-dinitropiperazine, nitropiperidine, nitropyrrolidine and DMNA) have similar molecular structures, but they are unable to be used as fuels and explosives. To elucidate the difference between them, both nanosecond and femtosecond mass resolved excitation spectroscopy have been employed to investigate the mechanisms and dynamics of the electronically excited photodissociation of these materials. NO is a dominant dissociation product. Based upon the experimental observation and calculations of potential energy surfaces for these systems, we suggest that energetic materials dissociate from their ground electronic states after relaxing from the first excited states, and that the model systems dissociate from their excited state. In both cases a nitro-nitrite isomerization is part of the reaction mechanism. Parent ions of DMNA and nitropyrrolidine are observed in fs experiments. All the other molecules generate NO as a product even in fs time regime. [Preview Abstract] |
Thursday, March 8, 2007 11:51AM - 12:03PM |
V18.00004: UV excited electronic state decomposition of energetic materials and model systems using fs laser spectroscopy Margo Greenfield, Yuanqing Guo, Atanu Bhattacharya, Elliot Bernstein Time resolved (fs) photodissociation experiments have been performed in efforts to elucidate the dynamics controlling the excited electronic state decomposition of the energetic materials RDX and HMX and their associated model systems (dimethylnitramine, nitropyrrolidine, nitropiperidine, and dinitropiperazine). The initial decomposition product of the energetic materials and model systems is the NO molecule. Femtosecond pump-probe techniques have been employed to measure the photodissociation dynamics of these systems via the initial NO product at three wavelengths (226 nm, 228 nm, 230 nm). The NO molecule has a non-resonant two-photon absorption at 228 nm and 230 nm and single photon resonant absorption for the A$^{2}\Sigma \leftarrow $X$^{2}\Pi $(0,0) transition. Both pump-probe transients at non-resonant absorption and resonant absorption wavelengths indicate the dynamics of the energetic material's decomposition from the excited electronic state is faster than the time duration of our laser pulse (180 fs) and notably different from some of the model systems. [Preview Abstract] |
Thursday, March 8, 2007 12:03PM - 12:15PM |
V18.00005: Photoinduced ring-opening mechanism in several model diarylethenes Petra Sauer, Roland E. Allen Recent experimental results suggest that derivatives of diarylethenes may be viable candidates for switches in molecular devices. In these molecules, the ring-opening and ring-closing reactions are induced with laser pulses of different frequencies. We have shown that the ring-opening mechanism in the simplest of all diarylethenes, stilbene, occurs via a HOMO-LUMO avoided crossing and subsequent depopulation of the excited state with minimal involvement of other orbitals. We now show that the same photoinduced ring-opening process for both oxygen ($C_{10}H_{8}O_{2}$) and sulfur ($C_{10}H_{8}S_{2}$) containing diaryethenes involves higher order excited states. We will show simulation results in which the initial laser pulse excitation induces a transition from HOMO to LUMO. Due to both nuclear motion and symmetry changes, this laser pulse also excites some percentage of the electronic population out of orbitals lower in energy than the HOMO and into orbitals higher in energy than the LUMO. In order for the ring-opening event to occur, the electronic population in the higher excited states is first transferred, via a series of avoided crossings, into the LUMO. A subsequent avoided crossing between the HOMO and LUMO then allows ring-opening. [Preview Abstract] |
Thursday, March 8, 2007 12:15PM - 12:27PM |
V18.00006: Cooling and Trapping of NH radicals L. Paul Parazzoli, Carlos Romero, Daniel Lobser, Heather Lewandowski In the past decade, cooling and trapping of atoms has allowed physicists to probe the nature of quantum mechanics on a macroscopic scale. Molecules, having a more complex structure, are considerably more difficult to cool. However, it is their complex structure, including rovibrational states and permanent dipole moments, which make them so interesting. We cool metastable NH ($^{1}\Delta )$ radicals using supersonic expansion coupled with Stark deceleration. The NH radicals are created by photolysis of HN$_{3}$ during supersonic expansion. The supersonic expansion produces a cold beam of radicals, which is loaded into a Stark decelerator. The Stark decelerator uses time varying inhomogeneous electric fields to decelerate the NH molecules. The resulting molecular sample has a temperature of 10 -100 mK. Further cooling will be explored using interactions with ultracold Rubidium atoms. [Preview Abstract] |
Thursday, March 8, 2007 12:27PM - 12:39PM |
V18.00007: First Principles Simulations of THz Spectra of Acephate: Insight Into the Phonon Signatures. Yiming Zhang, Xihong Peng, Yunqing Chen, Saroj Nayak, X.-C. Zhang Acephate is an insecticide that kills insects by disrupting nervous system functions. THz spectroscopy offers a unique tool for detecting trace amount of these materials. Using a combination of solid state first principles simulations and gas phase quantum mechanical modeling we have studied phonon spectra of acephate compound. This talk will present a detailed vibrational spectra analysis over a wide range of frequency and our computational data will be compared with available experimental results. [Preview Abstract] |
Thursday, March 8, 2007 12:39PM - 12:51PM |
V18.00008: Hydrophobic dependence of molecules at the liquid/solid interface as studied with infrared-visible sum frequency generation spectroscopy Bryan Hsu, Veronique Lachat, Gabor Somorjai, Mohsen Yeganeh We report IR-vis SFG spectra obtained at the interface of liquids with hydrophobic and hydrophilic solid surfaces. The hydrophilic and hydrophobic surfaces used were sapphire and a dense methyl-terminated sapphire surface from chemically bonded octadecyltrichlorosilane (OTS), respectively. Orientation calculations of SSP (IR, vis, and SF polarizations) and SPS polarizations of acetonitrile on the OTS and sapphire showed tilt angles of approximately 90 and 40 degrees, respectively. The CD3 symmetric stretch of methanol (d4) at OTS and sapphire showed a blue-shift for the latter but no shift for the former when compared to FTIR of the bulk liquid. This may be due to changing H-bonding characteristics with methanol orienting its hydroxyl end towards sapphire and away from OTS. PPP spectra of n-heptane (d16) and n-hexadecane (d34) showed weaker intensity CD3 antisymmetric stretches on sapphire compared to OTS, with a larger decrease for n-hexadecane. This can be interpreted as the hydrocarbons curling away from sapphire more so than OTS, especially with n-hexadecane. [Preview Abstract] |
Thursday, March 8, 2007 12:51PM - 1:03PM |
V18.00009: Observation of the $\widetilde{A}-\widetilde{X}$ Electronic Transition of the Jet-Cooled Methyl Peroxy Radical by High Resolution CRDS Patrick Dupr\'{e}, Shenghai Wu, Patrick Rupper, Terry Miller Reactive intermediates are of crucial importance both for combustion and atmospheric chemistry. By using our new home made Fourier Transform limited (10--30~MHz) Ti:Sa laser source we have probed the vibrationless level of the first electronic state (in the near-IR range) of both ${\rm CH}_{3}{\rm OO}$ and ${\rm CD}_{3}{\rm OO}$ radical species. The radicals are formed inside a ${\rm Ne/He/O}_{2}/{\rm CH}_{3}{\rm I}$ plasma created by a DC or a RF electrical discharge. The supersonic jet expansion necessary for the rotational cooling ($\sim20\,{\rm K}$) is obtained by a pulsed slit nozzle ($\sim50\times0.5\,{\rm mm}^{2}$). The near-IR radiation, obtained by Stimulated Raman Scattering (SRS) is injected inside a high finesse cavity. A sensitivity of the order of $\sim20\times10^{-9}\,/{\rm pass}/\sqrt{{\rm Hz}}$ is currently obtained. Spectrum with a resolution $\sim350\,{\rm MHz}$ for ${\rm CD}_{3}{\rm OO}$ clearly shows rotational and spin-rotation structure with effects of the internal methyl group rotation possibly evolved. [Preview Abstract] |
Thursday, March 8, 2007 1:03PM - 1:15PM |
V18.00010: Modeling the transient vibrational dynamics of photofragments Steve Young, Sara Mason, Hai-Lung Dai, Andrew Rappe Knowledge of radical spectroscopy and the structure of radicals is important in many scientific areas, such as atmospheric systems, combustion reactions, biological processes, and more. Because many radicals are transient, unstable, and generally produced in small quantities, they are often difficult to characterize spectroscopically. In this talk, we will present our synthesis of theoretical and experimental data to understand the behavior of radical photofragments. The first part of the talk outlines our approach to understanding vibrationally hot but electronically cold radical dynamics, with direct molecular dynamics and performing electronic structure calculations using DFT within the GAMESS package. We will then summarize our recent development and application of time-resolved FTIR emission spectroscopy for the study of photofragments. Finally, we will present a joint theoretical and experimental investigation of the dynamics of the vinyl radical, including characterization of the complex interaction of rotation, alpha-proton motion, and anharmonic effects, and discuss their influence on the IR spectrum. [Preview Abstract] |
Thursday, March 8, 2007 1:15PM - 1:27PM |
V18.00011: Raman spectra of benzene derivatives adsorbed on metal substrates D.A. Alexson, S.C. Badescu, O.J. Glembocki, S.M. Prokes, R.W. Rendell We study the molecular orientations of several benzene derivatives on large Ag and Au clusters via first-principles calculations. We find the lowest-energy structures, several local minima and the diffusion barriers for benzene, nitrobenzene, 2,4-dinitrotoluene (DNT) and 1,4-benzenedimethanethiol (BDMT). The theoretical calculations are compared to experimental measurements of SERS for 2,4-DNT and 1,4-BDMT on Ag and Au coated dielectric nanowires. [Preview Abstract] |
Thursday, March 8, 2007 1:27PM - 1:39PM |
V18.00012: ABSTRACT WITHDRAWN |
Thursday, March 8, 2007 1:39PM - 1:51PM |
V18.00013: Single photon ionization of hydrogen bonded clusters with a desk-top size soft x-ray laser: (HCOOH)$_{n }$ and (HCOOH)$_{m}$(H$_{2}$O)$_{n}$ Scott Heinbuch, Feng Dong, Jorge Rocca, Elliot Bernstein Pure neutral (HCOOH)$_{n}$ clusters and mixed (HCOOH)$_{m}$(H$_{2}$O)$_{n}$ clusters are investigated employing time of flight mass spectroscopy and single photon ionization at 26.5 eV (from a soft x-ray laser). The distribution of pure (HCOOH)$_{n }$clusters is dependant on experimental conditions. At certain conditions a magic number is found at n = 5. During the ionization process, neutral clusters suffer little fragmentation because almost all excess energy above the vertical ionization energy is taken away by the photoelectron. Metastable dissociation rate constants of (HCOOH)$_{n}^{+}$ are measured in the range of (0.1--0.8)x10$^{4}$ s$^{-1}$ for cluster sizes of 4$<$n$<$9. The rate constants display an alternating behavior between monomer and dimer loss that can be attributed to the structure of the cluster. When small amounts of water are added to the formic acid, the predominant signals in the mass spectrum are still (HCOOH)$_{n}^{+}$ cluster ions. Also observed are the mixed cluster series (HCOOH)$_{m}$(H$_{2}$O)$_{n }$for m=1-8 and n=0-4. A magic number in the cluster series n=1 at m=5 is observed. The mechanisms and dynamics of the formation of these neutral and ionic clusters are discussed. [Preview Abstract] |
Thursday, March 8, 2007 1:51PM - 2:03PM |
V18.00014: Formation and Distribution of Neutral Transition Metal Oxide Clusters: Single Photon Ionization at 26.5 eV Feng Dong, Scott Heinbuch, Jorge Rocca, Elliot Bernstein A single photon of an EUV laser (26.5 eV) has enough energy to ionize any metal oxide cluster generated in a molecular beam. Neutral vanadium, niobium, and tantalum oxide clusters are studied by single photon ionization employing a 26.5 eV EUV laser. During the ionization process, metal oxide clusters are virtually free of fragmentation. The most stable neutral metal oxide clusters under saturated oxygen conditions can be represented as (MO$_{2})_{0,1}$(M$_{2}$O$_{5})_{y}$ (M=V, Nb, Ta). Both O-rich and O-deficient clusters can be observed. Oxygen-rich metal oxide clusters with high ionization energy are detected by 26.5 eV, but not by 10.5 eV, ionization. For O-rich clusters M$_{x}$O$_{y}$H$_{z}$ species are also observed for the first time. Given these experimental capabilities, neutral cluster reactions and reactivity can be studied. We will present preliminary results of these studies. [Preview Abstract] |
Thursday, March 8, 2007 2:03PM - 2:15PM |
V18.00015: UV Single Photon Dissociation of Furazan Based Energetic Materials: DAAF Atanu Bhattacharya, Yuanqing Guo, Margo Greenfield, Elliot Bernstein The new series of furazan-based energetic materials is characterized by low sensitivity to impact and friction. They have broad application as fuels and propellants; however, extra nitro functional groups attached to the furazan ring (e.g. 4,4$^{\prime} $-dinitro-3,3$^{\prime} $-azoxyfurazan) adversely impact the thermal stability of these energetic materials. In order to evaluate the effect of nitro functional groups on furazan-based energetic materials the decomposition of 4,4$^{\prime} $-diamino-3,3$^{\prime} $-azoxyfurazan (DAAF), from excited electronic states, has been investigated by UV excitation (8 ns duration) and time of flight mass spectroscopy. The NO molecule is observed as an initial product. Three vibronic transitions of NO are characterized. Simulation of the NO [A $^{2}\Sigma $ (v$^{\prime} $=0)$\leftarrow $X $^{2}\Pi $ (v$\prime $=1)] transition and fitting to the intensity ratios among NO vibronic transition yields rotational and vibrational temperatures of 30 K and 1265 K, respectively. Compared with NO gas spectra, under comparable condition, the NO from decomposition of DAAF is vibrationally hot and rotationally cold. [Preview Abstract] |
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