Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session V37: Chemical Dynamics and Molecular Spectroscopy |
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Sponsoring Units: DCP Chair: Alan Aspuru-Guzik, Harvard University Room: 409 |
Thursday, March 19, 2009 8:00AM - 8:12AM |
V37.00001: Laser-excitation of molecular systems within stochastic time-dependent current-density-functional theory Heiko Appel, Massimiliano Di Ventra In this talk we investigate the excited electron dynamics of molecular systems due to laser excitation. The system dynamics is described within the recently proposed stochastic time-dependent current-density-functional theory [1,2]. \\[2mm] $[1]$ Massimiliano Di Ventra and Roberto D'Agosta, Phys. Rev. Lett. \textbf{98}, 226403 (2007).\\ $[2]$ Roberto D'Agosta and Massimiliano Di Ventra, Phys. Rev. B \textbf{78}, 165105 (2008). [Preview Abstract] |
Thursday, March 19, 2009 8:12AM - 8:24AM |
V37.00002: Nonlinear Response Functions in Model Dissipative Anharmonic Systems Mohammad Sahrapour, Nancy Makri We report the results of simulations of third order response functions ($R^{(3)}(\tau _3 ,0,\tau _1 )=\mbox{Tr}\left\{ {\hat {\alpha }(\tau _3 )[\hat {\alpha },[\hat {\alpha },[\hat {\alpha }(\tau _1 ),\rho _0 ]]]} \right\}$ where $\hat {\alpha }$ is the polarizability) for harmonic, Morse, and anharmonic model systems in a linearly dissipative environment. These simulations are carried out via the iterative path integral methodology developed earlier in our group which delivers efficient, numerically exact long time quantum dynamics. We find that even minor anharmonicity in the potential qualitatively changes the response function; rotating the pattern seen by 45$^{o}$ in the $\tau _1 -\tau _3 $ plane. We also observe that modulations in the $\tau _3 $ direction increase in frequency as we go to a more anharmonic potential. As the temperature is increased, these modulations also appear in the $\tau _1 $ direction. It is also found that asymmetry in the potential, at least at temperatures considered here, does not have a significant effect. Finally, in all three systems we notice that decay in the $\tau _3 $ direction is faster than in the $\tau _1 $ direction. The observed sensitivity of the response function to anharmonicities in the potential can be exploited to construct more accurate molecular potentials once the appropriate non-linear spectroscopic experiments have been performed. [Preview Abstract] |
Thursday, March 19, 2009 8:24AM - 8:36AM |
V37.00003: Hamiltonian Monodromy: Unexpected behavior of quasi-linear molecules, atoms in traps and of hydrogen in crossed fields J.B. Delos, C. Schleif, D. Sadovskii, G. Dhont, B. Zhilinskii A system exhibits monodromy if we take the system around a closed loop in its parameter space, and we find that the system does not come back to its original state. Many systems have this property, including quasi-linear molecules, atoms in a trap or a hydrogen atom in crossed fields. Using classical perturbation theory, Sadovskii and Cushman predicted the presence of monodromy in perpendicular fields. It shows up as a defect in the lattice of quantum states. When the fields are tilted from perpendicular, these lattice defects undergo a series of bifurcations. Atoms in a trap can display a newly discovered dynamical manifestation of monodromy. This phenomenon will also occur with oriented dipolar molecules in fields or with quasilinear molecules. (Supported by NSF and Region Nord--Pas-de-Calais) [Preview Abstract] |
Thursday, March 19, 2009 8:36AM - 8:48AM |
V37.00004: On the internal photorelaxation mechanism of DNA Eric Bittner We propose a model for the photo-deactivation mechanism for DNA based upon accurate quantum chemical and molecular dynamical evaluations of model Watson/Crick nucleoside pairs and stacked pairs. Our results corroborate recent ultrafast experimental studies on DNA oligonucleotides and suggest that following photo-excitation to a local $\pi-\pi^*$ state, the excitation is rapidly delocalized over several (3-4) bases on an ultrafast time-scale. However, this delocalized state is unstable with respect to the motions of the protons involved in hydrogen-bonding between Watson/Crick pairs and rapidly re-localizes to a charge-transfer state on a longer time-scale ranging from 10 to 100 ps. This state, too, is unstable and relaxes via a conical intersection with the ground state near the geometry of the enol- and imino-tautomeric form. We suggest that this internal deactivation mechanism is responsible for the intrinsic photostability of DNA. [Preview Abstract] |
Thursday, March 19, 2009 8:48AM - 9:00AM |
V37.00005: Tunneling Splitting in the Rotationally Resolved Electronic Spectrum of 1,3-Benzodioxole Jessica A. Thomas, Leonardo Alvarez-Valtierra, David W. Pratt Gas phase rotationally resolved electronic spectra were collected for the origin and several vibronic transitions of 1,3- benzodioxole. For each band, an autocorrelation program identified the presence of two overlapping spectra which were each fit using a least-squares algorithm to determine the rotational constants. Interpretation of these constants and how they change from one band to the next gives information about the physical causes of this splitting, which will be discussed. [Preview Abstract] |
Thursday, March 19, 2009 9:00AM - 9:12AM |
V37.00006: Vacuum Ultraviolet Absorption of Supercritical Water David Bartels, Ireneusz Janik, Timothy Marin The first continuum $\mathop X\limits^\sim \to \mathop A\limits^\sim $ peak in the gas phase absorption spectrum of H$_{2}$O (maximum at 7.4 eV) , is ascribed to promotion of a nonbonding valence electron to a dissociative excited state which is an admixture of antibonding sigma and 3s Rydberg orbitals. Due to the large spatial extent of this orbital the$\mathop X\limits^\sim \to \mathop A\limits^\sim $ transition is strongly perturbed by the local environment. In liquid water, this peak is notably broadened and shifted to the blue, with an absorption maximum of 8.3 eV at room temperature. An obvious question is how this transition changes as a function of the water density in the supercritical regime as the system transitions from liquid to gas. As density decreases, most water molecules will be located at an interface between large clusters and voids. We will describe an experiment to measure the VUV absorption of the $\mathop X\limits^\sim \to \mathop A\limits^\sim $ transition in supercritical water (T$>$374 \r{ }C, P$>$220 bar ) where the density can be tuned continuously. Experimentally this requires over six orders of magnitude detector dynamic range in the vacuum UV and a short path (ca. 1 micron) high temperature/pressure cell with sapphire windows. This has not been accomplished in any previous experiment. Results will be discussed in the light of recent ab initio calculations of the spectrum in large and small clusters. [Preview Abstract] |
Thursday, March 19, 2009 9:12AM - 9:24AM |
V37.00007: Vibrational Coupling Pathways in the $\nu _{3}$ CH Stretch Fundamental Region of Methanol as Revealed by Coherence-Detected FTMW-IR Spectroscopy Sylvestre Twagirayezu, David S. Perry, Justin L. Neill, Matt T. Muckle, Brooks H. Pate Rotational state-selection on the methanol E species transitions 2$_{0}\leftarrow $3$_{-1}$ and 2$_{1}\leftarrow $3$_{0}$ is used to record the infrared (IR) spectra of the connected rotational levels of jet-cooled methanol, CH$_{3}$OH and CH$_{3}$OD. The observed spectra of CH$_{3}$OH contain twelve interacting vibrational bands in the interval 2755-2855 cm$^{-1}$, whereas a direct state count gives 14 vibrations in this interval grouped into 6 tiers by coupling order. The isotope dependence of the spectra confirms that the dominant coupling pathway is a third order coupling of the CH stretch to a combination of the COH bend and an HCH bend. [Preview Abstract] |
Thursday, March 19, 2009 9:24AM - 9:36AM |
V37.00008: Ab Initio Torsion-Wag Surface for the Ethyl Radical Ram S. Bhatta, David S. Perry The torsion-wag potential of the ethyl radical has a 6-fold barrier to internal rotation and the minimum energy path involves deviations of the CH$_{2}$ wag angle of 6 to 11 degrees on either side of planar. Partially optimized 2-dimensional surfaces were calculated at the B3LYP, MP2, and CCSD(T) levels with 6-311++G(d,p) and 6-311++G(3df, 2p) basis sets and they were fit to a function containing a polynomial in the wag angle $\tau $ and trigonometric functions of the torsional angle $\alpha $. Comparison is made with the corresponding surfaces for CH$_{3}$NH$_{2}$ and CH$_{3}$OH$_{2}^{+}$. Unlike CH$_{3}$CH$_{2}^{\bullet }$, both have a substantial barrier to inversion. The dominant torsion-wag coupling term in all three cases has the form $\tau $cos3$\alpha $. [Preview Abstract] |
Thursday, March 19, 2009 9:36AM - 9:48AM |
V37.00009: Terahertz Investigations of Extraordinarily Efficient Conduction in a Redox Active Ionic Liquid. Verner Thorsmolle, Jan Brauer, Guido Rothenberger, Daibin Kuang, Shaik Zakeeruddin, Michael Gr\"atzel, Jacques Moser Iodine added to iodide-based ionic liquids leads to extraordinarily efficient charge transport, vastly exceeding expectancy for such viscous systems. Using terahertz time-domain spectroscopy, in conjunction with dc conductivity and viscosity measurements we unravel the conductivity pathways in 1-methyl-3-propylimidazolium iodide melts. Applying low temperatures, we demonstrate for the first time conduction entirely due to a Grotthus bond-exchange mechanism at iodine concentrations higher than 3.9 M. The terahertz and transport results are reconciled in a model providing a quantitative description of the conduction by physical diffusion and the Grotthus bond-exchange process. These novel results are of great importance for the fundamental understanding of conduction in molten salts and for applications where ionic liquids are used as charge-transporting media such as in batteries and dye-sensitized solar cells. [Preview Abstract] |
Thursday, March 19, 2009 9:48AM - 10:00AM |
V37.00010: Investigation of the Order-Disorder Transition in the Hybrid Inorganic-Organic System [(CH$_{3})_{2}$NH$_{2}$]Zn(HCOO)$_{3}$ by means of $^{1}$H NMR T. Besara, P. Jain, A.P. Reyes, P.L. Kuhns, N.S. Dalal, H.W. Kroto, A.K. Cheetham [(CH$_{3})_{2}$NH$_{2}$]Zn(HCOO)$_{3}$, a hybrid ABX$_{3}$ perovskite, with A=(CH$_{3})_{2}$NH$_{2}$, B=Zn and X=HCOO, undergoes a paraelectric-antiferroelectric transition around 156 K. Synchrotron studies indicate that hydrogen bonding between the H-atoms in the NH$_{2}$ group and O-atoms from the formate group is involved. The dimethylamine cation is disordered with nitrogen existing in three different positions, but not known whether statically or dynamically. We have investigated it by means of spin-lattice relaxation time, T$_{1}$, using proton NMR. We find that the cation is dynamically disordered and that the transition involves its slowing down. Evidence is seen for tunneling of the CH$_{3}$ groups, and for the compound becoming a glass, with the cation displaying several metastable equilibrium geometries (T$_{1}$ trajectories). [Preview Abstract] |
Thursday, March 19, 2009 10:00AM - 10:12AM |
V37.00011: High Resolution Cavity Ringdown Spectroscopy of Jet-Cooled Reactive Intermediates Gabriel Just, Patrick Rupper, Linsen Pei, Terry Miller Alkyl peroxy radicals long have been well known to be key intermediates in atmospheric chemistry as well as in low temperature combustion. For the last several years, our group has generated a data set for these radicals using room temperature cavity ringdown spectroscopy. We have recently extended our investigations of the peroxy radicals to obtain a high resolution data set of spectra under jet cooled conditions using a quasi-Fourier-transform-limited laser source and a supersonic slit jet discharge expansion.Over the last few years, we have developed our capability to obtain narrow-bandwidth, near infrared (NIR) radiation for performing high resolution cavity ringdown spectroscopy using the post-amplification of a Ti:Sa ring laser. The NIR light can be generated by either stimulated Raman shifting or by difference frequency mixing by combining the second harmonic of a Nd:YAG laser with our post- amplified light in a BBO crystal.Using this apparatus, we have observed isomer and conformer specific spectra of the following species : methyl peroxy, CH$_3$O$_2$, ethyl peroxy, C$_2$H$_5$O$_2$, propyl peroxy, C$_3$H$_7$O$_2$, and phenyl peroxy, C$_6$H$_5$O$_2$ . These spectra show rotationally resolved structure with a temperature of $\sim$15 K as well as other structure attributable to spin-rotation interactions, tunneling splittings etc. [Preview Abstract] |
Thursday, March 19, 2009 10:12AM - 10:24AM |
V37.00012: Rotationally Resolved $\tilde{A}^2$A$_1$ - $\tilde{X}^2$E Electronic Spectra of Symmetric Methoxy Radicals: CH$_3$O and CD$_3$O Ming-Wei Chen, Dmitry Melnik, Jinjun Liu, Terry A. Miller Methoxy radical has attracted spectroscopic interest for more than twenty years. Microwave measurements of CH$_3$O and CD$_3$O with precision on the kHz scale have determined the $\tilde{X}^2$E parameters. Jet-cooled laser induced fluorescence (LIF) spectra have also been observed by our group with high-resolution ($\Delta\nu\sim$250 MHz) and high-accuracy ($\Delta\sigma\sim$50 MHz), for the 3$^2_0$ and 6$^1_0$ bands of the $\tilde{A}^2$A$_1$ - $\tilde{X}^2$E$_{3/2}$ electronic transition. Since the ground state component E$_{1/2}$ is $\sim$60 cm$^{-1}$ energetically higher than the E$_{3/2}$ spin component, the $\tilde{X}^2$E$_{1/2}$ state is not thermally populated in a jet-cooled environment. However, our complementary stimulated emission pumping (SEP) experiment with the same resolution and accuracy as the LIF work directly interrogates the $\tilde{X}^2$E$_{1/2}$ level of CH$_3$O and CD$_3$O by depleting the fluorescence from the $\tilde{A}^2$A$_1$ 3$^2$ levels. The global analysis of the microwave, LIF, and SEP data breaks correlations in the microwave data and provides better determinations for the $\tilde{X}$ and $\tilde{A}$ states' parameters. Comparison of the values for CH$_3$O, $^{13}$CH$_3$O, and CD$_3$O allows us to separate first-order from second-order electronic and vibrational contributions based upon the isotopic dependencies of the effective ground state parameters. [Preview Abstract] |
Thursday, March 19, 2009 10:24AM - 10:36AM |
V37.00013: The Photoprotective Properties of Adenine: Time-resolved Photoelectron Spectroscopy at different excitation wavelengths Susanne Ullrich, Nick L. Evans, William M. Potter, Amanda N. Brouillette The UV photostability of biomolecules is determined by their excited state electronic relaxation mechanisms. To be effective, these mechanisms must operate on ultrafast timescales in order to dominate over competing photochemical processes that potentially lead to destruction of the biomolecule. Femtosecond time-resolved photoelectron spectroscopy (TRPES) provides unique capabilities for studying photoinduced processes in small polyatomic molecules. Changes in the PES, observed as the delay between the pump and probe pulses is scanned, can be associated with electronic configurational changes during the relaxation process. Analysis based on ionization correlations allows us to extract the electronic character of the excited states in addition to their lifetimes. Details of the experimental setup and technique will be presented in this talk as well as our initial results on the deactivation pathways in the DNA base adenine following excitation by wavelengths between 245-266 nm. [Preview Abstract] |
Thursday, March 19, 2009 10:36AM - 10:48AM |
V37.00014: Observation of the $\tilde{A}$ -- $\tilde{X}$ Electronic Transitions of Cyclopentyl and Cyclohexyl Peroxy Radicals Via Cavity Ringdown Spectroscopy Rabi Chhantyal Pun, Phillip Thomas, Terry Miller Organic peroxy radicals are important intermediates in combustion chemistry. These molecules, formed from the addition of oxygen to alkyl radicals, are pivotal species in many atmospheric processes. We have previously targeted the $\tilde{A}$-$\tilde{X}$ transitions of straight and branched aliphatic peroxy radicals. This research has now been extended towards cyclic systems starting with cyclopentyl peroxy (C$_5$H$_9$O$_2$) and cyclohexyl peroxy (C$_6$H$_{11}$O$_2$), which are predicted by $ab initio$ and DFT calculations to have four and two low-lying conformers respectively. Both have conformers which differ on the orientation of the O-O bond with respect to the cyclic carbon skeleton namely, $cis$- and $gauche$-, with C$_6$H$_{11}$O$_2$ also having conformers with axial and equatorial placement on the ring. We observe strong bands for both peroxies in the near-IR which are favorably assigned as the origin and the O-O stretch in agreement with the calculations. We have also obtained the spectrum of C$_6$D$_{11}$O$_2$ which facilitates the assignments of the weaker vibrational structure in C$_6$H$_{11}$O$_2$. [Preview Abstract] |
Thursday, March 19, 2009 10:48AM - 11:00AM |
V37.00015: Dynamic light scattering in an aqueous solution of 3-methylpyridine Dimitry Ivanov, Anna Trubetskaya, Andrei Kostko, Mikhail Anisimov, Jan Sengers We report a set of dynamic light scattering experiments in an aqueous solution of 3-methylpyridine. The dynamic correlation functions appear to exhibit two modes: one associated with a normal diffusion process and another one with network relaxation. The observed correlations seem to be associated with long-living nonequilibrium structures. To obtain further insight into this phenomenon we have made systematic studies of the nature of the observed dynamics as a function of time and concentration. [Preview Abstract] |
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