Bulletin of the American Physical Society
APS March Meeting 2010
Volume 55, Number 2
Monday–Friday, March 15–19, 2010; Portland, Oregon
Session Y28: Chemical Dynamics |
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Sponsoring Units: DCP Chair: Wen Li, Wayne State University Room: C124 |
Friday, March 19, 2010 8:00AM - 8:12AM |
Y28.00001: Spinning CO$_{2}$ Molecules into High Rotational States with an Optical Centrifuge Amy Mullin, Liwei Yuan, Allison Robinson, Samuel Teitelbaum We report the first spectroscopic measurements of molecules in an optical centrifuge which generates molecules in very high rotational states. The optical centrifuge combines two ultrafast laser pulses having reverse chirp and circular polarization to generate an intense electric field that undergoes angular acceleration. Molecules with non-uniform polarizability are driven by the field into high rotational states. We have used the optical centrifuge to promote CO$_{2}$ molecules into high rotational states (J$\sim $200) and monitored the effect of the centrifuge on different quantum states using high-resolution transient mid-IR laser absorption. We have observed and characterized the depletion of low angular momentum (J=14) states and the appearance (and subsequent depletion) of middle-J (J=76) states populated by a collisional cascade. Direct detection of CO$_{2}$ molecules in states near J=200 will allow further characterization of the centrifuge. Transient signals are observed only in the presence of both optical centrifuge pulses and for pulses with circular polarization. The ability to control molecular rotation using the optical centrifuge opens a new realm of investigation into the behavior of energized molecules. [Preview Abstract] |
Friday, March 19, 2010 8:12AM - 8:24AM |
Y28.00002: Crossed-beam dc slice imaging studies of Cl($^{2}$P$_{3/2})$ reactions with n-pentane, isopentane, and neopentane Armando Estillore, Laura Visger, Arthur Suits The interaction of chlorine with hydrocarbons has become an important aid in understanding the chemistry in combustion, atmospheric, and marine environments. Here, we present a systematic study of the reactions of ground state Cl($^{2}$P$_{3/2})$ atoms with n-pentane, isopentane, and neopentane. The reactions are studied using crossed molecular beam and dc slice ion imaging techniques. The product alkyl radical $m/z$ = 71 were detected \textit{via} single photon ionization at 157 nm. Center-of-mass translational energy and angular distributions were directly obtained from the images. Product angular distributions showed backward scattering and that most of the available energy ($\sim $50-75{\%}) are partitioned among the products. [Preview Abstract] |
Friday, March 19, 2010 8:24AM - 8:36AM |
Y28.00003: Dynamics of Networked and Metal Cyanides Daniel Weidinger, Douglas Brown, Cassidy Houchins, Jeffrey Owrutsky Time resolved IR spectroscopy was used to characterize the vibrational relaxation dynamics of the CN stretching bands of aqueous molecular metal cyanides and networked metal cyanides in reverse micelles and on surfaces. The vibrational and rotational relaxation dynamics of the CN stretching bands near 2000 cm$^{-1}$ for aqueous molecular cyanides, Au(CN)$_{2}^{-}$, Ag(CN)$_{2}^{-}$, Ni(CN)$_{4}^{2-}$, Pt(CN)$_{4}^{2-}$, Co(CN)$_{6}^{3-}$, Mn(CN)$_{6}^{3-}$, and Ru(CN)$_{6}^{4-}$ have been investigated using ultrafast pump-probe spectroscopy. The spectra and dynamics of Ru(CN)$_{6}^{4-}$ are similar to those previously reported for ferrocyanide. The T$_{1}$ times are significantly longer ($>$30 ps) in the other molecules; Mn(CN)$_{6}^{3-}$ represents an intermediate case with a relaxation time of about 15 ps in water. The spectra and VER dynamics together extend the established theories of metal cyanide bonding and its connection to frequency and intensity. Networked metal cyanides in reverse micelles, including Prussian Blue and analogs with Cu, Ni, Co and Ru were also studied using visible pump-IR probe spectroscopy. Preliminary results suggest that networked metal cyanide VER lifetimes after visible pulse excitation are similar to those from infrared excitation. [Preview Abstract] |
Friday, March 19, 2010 8:36AM - 8:48AM |
Y28.00004: Vibrational spectroscopy and dynamics of the hydrazoic and thiocyanic acids in protic and aprotic solvents Cassidy Houchins, Daniel Weidinger, Jefferey Owrutsky Azide (N$_3^-$) and thiocyanate (NCS$^-$) in solvents are benchmark systems for studying fast vibrational energy relaxation (VER) and solvent-solute interactions. To investigate the effects of solute charge as well as solvent-isotope effects on solute vibrational spectra and dynamics, infrared pump-probe studies have been carried out to determine VER times for CN and antisymmetric NNN stretching bands XNCS and XN3 (X=H, D), in protic and aprotic solvents to compare with the well studied N$_3^-$ and NCS$^-$ anions. The study suggests that protonating the anions may be viewed as a limiting case of hydrogen bonding. Results from extending the measurements to other solvents demonstrate that the trend in VER relaxation times with solvent is similar for HN$_3$ and N$_3^-$, but there is less of a systematic frequency shift for HN$_3$ . [Preview Abstract] |
Friday, March 19, 2010 8:48AM - 9:00AM |
Y28.00005: On the Unusual Reactivity Patterns in Copper doped Aluminum Cluster Anions Arthur Reber, Patrick Roach, W. Hunter Woodward, A. Welford Castleman Jr., Shiv Khanna We have measured the relative reactivity of Al$_{n}$Cu$^{-}$ clusters (n=11-34) with O$_{2}$. An odd-even alternation is observed that is in accordance with spin-dependant etching, and CuAl$_{22}^{-}$ is observed as a ``magic peak.'' As CuAl$_{22}^{-}$ has 68 valence electrons, it has an open shell within a spherical jellium model and would be expected to be reactive. First principles electronic structure studies indicate that the results from an unusually large splitting of the 2D$^{10}$ sub-shell in the jellium model leading to a HOMO-LIMO gap of 1.24 eV. We show that the splitting results from a geometric distortion of the cluster that may also be understood as a crystal field splitting of the superatomic orbitals. [Preview Abstract] |
Friday, March 19, 2010 9:00AM - 9:12AM |
Y28.00006: Monitoring the Coherent Vibrational Control of Electronic Excitation Transfer Using Ultrafast Pump-Probe Polarization Spectroscopy Jason Biggs, Jeffrey Cina The interplay between nuclear and electronic degrees of freedom in molecular energy-transfer complexes is a subject of current interest. We have proposed a method to use coherent nuclear motion to control the transfer of electronic excitation energy between donor and acceptor moieties in electronically coupled dimers. The underlying electronic and nuclear motion at the level of quantum mechanical amplitudes can be observed using nonlinear wave-packet interferometry(nl-WPI), a form of fluorescence-detected multidimensional electronic spectroscopy. In our control scheme, coherent nuclear motion is induced in the acceptor chromophore prior to direct electronic excitation of the donor. This nuclear motion affects the instantaneous resonance conditions between donor and acceptor moieties and thus affects subsequent energy transfer dynamics. We have developed the framework to simulate four-pulse nl-WPI experiments, and the pump-probe limit thereof, on energy-transfer systems after interaction with a control pulse that induces nuclear motion. We present simulations in the pump-probe limit from model energy-transfer systems subjected to prior impulsive vibrational excitation, and show how pulse polarization can be used to infer electronic dynamics from isotropically oriented dimers. [Preview Abstract] |
Friday, March 19, 2010 9:12AM - 9:24AM |
Y28.00007: Covariant measurements of chaos in chemical reaction dynamics Jason R. Green, R. Stephen Berry, David J. Wales Accurate characterization and measurement of classical chaos are needed to place limits on predictions of atomic motion in few-body and many-body systems. Quantifying chaos requires determination of the many local phase-space directions that indicate the sensitivity of motion to initial conditions. Here we show that coordinate-independent (covariant) directions have the qualitative and quantitative properties needed to provide a detailed understanding of the degree of randomness in reaction dynamics. Furthermore, we show that these directions allow accurate determination of time-scale separation in the exploration of phase space. In particular, they reveal separate time scales in the emerging local ergodicity of a model triatomic Lennard-Jones cluster. Over a range of total energies, there is a phase space region of highly chaotic behavior, characterizing motion in each local potential well, and a region with more regular dynamics, characterized by a longer time scale, corresponding to motion across the saddles. Exploiting covariant directions as a computational tool provides new insight into the dynamics of clusters and floppy molecules, and into quantitative theories of nonequilibrium statistical mechanics. [Preview Abstract] |
Friday, March 19, 2010 9:24AM - 9:36AM |
Y28.00008: Anion photoelectron spectroscopy of ZnOH, combined experimental and theoretical study Ivan Iordanov, K.D.D. Gunaratne, Jorge Sofo, A.W. Castleman Jr. Information about ZnOH anion has been obtained for the first time using TOF mass spectrometry combined with the Velocity Map Imaging Apparatus. Vertical detachment energies (VDE), nature of the HOMO and LUMO, photoelectron angular distributions of the studied species are reported. Combined with our experimental study, we report on first-principles calculations of ZnOH using density functional theory (DFT) with the Perdew-Burke-Ernzerhof (PBE) functional and also calculations using couple-cluster singles, doubles and triples (CCSD(T)) method. The calculations are used to determine the ground state geometry, vibrational modes and detachment energies of the neutral and anion ZnOH cluster. These results are used to explain the characteristics of the experimental ZnOH spectrum. [Preview Abstract] |
Friday, March 19, 2010 9:36AM - 9:48AM |
Y28.00009: Understanding performance and corrosion behavior of photo-electrode in terms of energetics of water-derived radicals on Ga-V (V=N,P,As) and GaP:N (110) surfaces: First-principles study Woon Ih Choi, John Turner, Yong-Hyun Kim, Kwiseon Kim Holes supplied from sunlight will detach the hydrogen atoms of H$_{2}$O as protons, leaving energetic O, H, or OH radicals. Therefore energetics of water-derived radicals on photo-electrode surface is important factor which determine its performance. Based on first-principles electronic structure and total energy calculations, we have studied reactions of -H, -O and -OH on the (110) surface of photo-cathode Ga-V and GaP:N materials, where V is N, P, and As. Zero-point energy and chemical potential of H$_{2}$ and O$_{2}$ gases are considered after static calculations. We have found that atomic oxygen on the GaN surface prefers being detached as O$_{2}$ to forming Ga-O. On the other hand, GaP and GaAs surfaces can have a strong Ga-O bond, hindering formation of O$_{2}$ gas and thus promoting surface corrosion. On GaP and GaAs surfaces hydrogen easily evolves as H$_{2}$ gas but on GaN not. Doped nitrogen in GaP to improve corrosion resistivity are tend to be clustered specially on the outermost surface region. These surface nitrogen atoms are expected to protect the surface and at the same time reduce hydrogen evolution rate. [Preview Abstract] |
Friday, March 19, 2010 9:48AM - 10:00AM |
Y28.00010: Beyond vibrational dissipation and decoherence. A mixed quantum/semiclassical theory for small-molecule and host-lattice dynamics in low-temperature media. Craig Chapman, Xiaolu Cheng, Jeffrey Cina Bath-mediated dynamics play an important role in condensed phase processes. Coupling of a small-molecule chromophore vibration to a cryogenic host can lead to coherent motion of the lattice atoms as in addition to giving rise to vibrational decoherence and vibrational energy relaxation, which can in turn affect ultrafast spectroscopic signals. We present a mixed quantum/semiclassical theory to simulate the dynamics of these systems that makes use of several key features of cryogenic chromophore-host complexes. In particular we exploit their well defined structure, and weak coupling aspects by framing a theory that treats the large-amplitude vibrational motion and the small-amplitude bath dynamics with differing degrees of rigor. The system is treated fully quantum mechanically while the bath is treated as a collection of multidimensional Gaussian wave packets. Calculated linear absorption spectra and linear wave packet interferometry signals for a simple test case are presented. A nonlinear version of wave packet interferometry has been shown to be an effective method of reconstructing molecular wave packets using a variety of pulse durations and interpulse delays, and may have applications in low-temperature environments as well. [Preview Abstract] |
Friday, March 19, 2010 10:00AM - 10:12AM |
Y28.00011: Coulomb explosion in femtosecond laser-cluster interactions Christian Chenard-Lemire, Laurent J. Lewis, Michel Meunier The products of laser ablation of thin film or bulk materials and the laser fragmentation of clusters depend strongly on the actual process that leads to the ejection of matter. For fs pulses, a few thermal and non-thermal processes have been identified. In this study, we assess the importance of the cold Coulomb Explosion (CE) process in various situations. While it is understood that CE can become important at high fluences for large-gap dielectrics, it remains unclear whether or not it can occur in semiconductors and metals. In order to clarify this issue, we have used computer simulations to model the interaction of fs laser pulses with cluster of C (diamond), c-Si and Al. We use a novel approach (electron Force Field) that allows the electronic and ionic excitations for relatively large scale (a few thousand atoms) systems to be simulated at the quantum level. Preliminary results for C clusters show that CE is very important even at fluences near threshold; small c-Si clusters also show evidence of CE. [Preview Abstract] |
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