Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session D11: Focus Session: Promises and Challenges in Chemical Dynamics III |
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Sponsoring Units: DCP Chair: Laurie Butler, University of Chicago Room: Baltimore Convention Center 303 |
Monday, March 13, 2006 2:30PM - 3:06PM |
D11.00001: Photodissociation of water ice and adsorbed molecules Invited Speaker: The TOF spectra of photofragment hydrogen atoms from the photodissociation of amorphous ice at 193 nm have been measured. The spectra consist of both a fast and a slow components characterized by translational temperatures of 2300 and 120 K, respectively. The photoabsorption of a branched cluster, (H$_{2}$O)$_{6+1}$ that is a (H$_{2}$O)$_{6}$ cyclic cluster attached by a water molecule with the hydrogen bond, is expected to appear at around 200 nm. The source of the hydrogen atoms is attributed to the photodissociation of the ice surface molecules that are attached by the (H$_{2}$O)$_{6}$ water structure with the hydrogen bond. Atmospheric implications are estimated for the photodissociation of the ice particles at 190-230 nm in the region between 80 and 85 km altitude. The photodissociation of ice has also performed at 157 nm. Since the bulk ice absorbs vacuum UV light, hydrogen atoms are produced from both the surface and bulk photodissociation processes. A difference in the UV photodissociation dynamics of chlorine and tricholorofluorocarbon has been observed when those molecules were adsorbed on water ice surfaces. In the photodissociation of Cl$_{2}$ at 300 to 414 nm, the branching ratios of the formation of Cl($^{2}$P$_{1/2})$ with respect to Cl($^{2}$P$_{3/2})$ are different from those reported in the gas phase photodissociation. However, in the photodissociation of CFCl$_{3}$ at 193 nm, the ratio is in good agreement with that reported in the gas phase photodissociation. These results are mostly attributable to the difference in the interaction of the adsorbed molecule with surface water molecules of ice. The kinetic energy distributions of the photofragment chlorine atoms reflect the interaction between the adsorbed molecules and the surface water molecules. The photofragment spectroscopy developed by late Prof. Richard Besohn has been used in this work. [Preview Abstract] |
Monday, March 13, 2006 3:06PM - 3:18PM |
D11.00002: Photodissociation dynamics of ethyl ethynyl ether: A ketenyl radical precursor Maria Krisch, Johanna Miller, Laurie Butler, Hongmei Su, Richard Bersohn, Jinian Shu We investigate the photodissociation dynamics of ethyl ethynyl ether at 193.3 nm with crossed laser-molecular beam photofragment translational spectroscopy and laser-induced fluorescence. We establish ethyl ethynyl ether as the first clean precursor to the ketenyl radical, a key species in combustion reactions. One major bond fission channel was observed for the system, cleavage along the HCCO-C$_{2}$H$_{5}$ bond, leading to ground state C$_{2}$H$_{5}$ (ethyl) radicals and HCCO (ketenyl) radical products in two distinct electronic states. We observed neither cleavage of the other C-O bond nor molecular elimination to form C$_{2}$H$_{4}$ + CH$_{2}$CO (ketene). Ketenyl radicals formed in the higher recoil kinetic energy channel could be either $\tilde {X}(^{2}$A") or \~{A}($^{2}$A') state ketenyl radical. We assign the lower recoil kinetic energy channel to the spin forbidden \~{a}($^{4}$A") state of the ketenyl radical, reached through intersystem crossing. Laser-induced fluorescence from the ketenyl radical peaks after a 20 $\mu $s delay, indicating that it is formed with a significant amount of internal energy and subsequently relaxes to the lowest vibrational level of the ground electronic state, a result consistent with the product assignment. [Preview Abstract] |
Monday, March 13, 2006 3:18PM - 3:30PM |
D11.00003: Multiphoton Photodissociation of Several Halocarbons. Bob Quandt The 2 x 193 nm photodissociations of CHCl$_{3 }$, CFCl$_{3}$, CF$_{3}$CCl$_{3}$, CCl$_{4}$, CH$_{3}$-CCl$_{3}$ and CH$_{3}$CH$_{2}$-CCl$_{3 }$have been examined using dispersed fluorescence. It was found that the initial photodissociation of CHCl$_{3}$ forms large amounts of CH(A$^{2}\Delta )$ while the photodissociation of CCl$_{4}$ forms lesser but still significant amounts of CX(A$^{2}\Delta )$ The photodissociations of CH$_{3}$-CCl$_{3}$ and CH$_{3}$CH$_{2}$-CCl$_{3 }$ produce CH$_{3}$-C and CH$_{3}$CH$_{2}$-C presumably in the A$^{2}\Delta $ state. The exact photoproducts of the fluorinated species are currently unknown. Fluorescence rise time measurements show that the CH(A$^{2}\Delta )$ and CCl(A$^{2}\Delta )$ photoproducts quickly react to form C$_{2 }$(d$^{3}\Pi _{g})$.~However, formation of C$_{2 }$(d$^{3}\Pi _{g})$ is attenuated when the primary photoproducts are CF$_{3}$C, CH$_{3}$-C and CH$_{3}$CH$_{2}$-C and disappears completely when it is CF. In addition, the atomic and molecular halogen photoproduct channels were investigated using ab initio calculations. Intrinsic Reaction Coordinate calculations were performed at the MP2 level of theory using the LANL2DZ basis set in order to characterize the dissociation pathways for all of species investigation.. The results of the calculations show the presence of three transition states and an ion-pair isomer intermediate for all molecules. The broken symmetry structure of the transition states for the formation of molecular bromine is in agreement with the first step of the addition mechanism proposed by Cain and co-workers for CX$_{2 }$+Y$_{2}$ reactions. ~ ~ ~ ~ [Preview Abstract] |
Monday, March 13, 2006 3:30PM - 3:42PM |
D11.00004: Thick and thin slices of photoproducts: correlated state distributions in ketene dissociation Gregory Hall, Anatoly Komissarov, Mike Minitti, Arthur Suits We have revisited the correlated product distribution of CH$_{2}$ + CO in the photodissociation of ketene at an energy 2350 cm$^{-1}$ above the barrierless singlet dissociation threshold. Using time-sliced ion imaging, we find the speed distribution of state-selected CO fragments to be quite different from previous measurements. For each CO rotational state observed, the deviations of the coincident CH$_{2}$ distribution from a statistical Phase Space Theory can be accurately described with a single parameter in the spirit of a linear surprisal. The distributions are consistent with calculations of exit channel rotational dynamics starting at a variational transition state. [Preview Abstract] |
Monday, March 13, 2006 3:42PM - 3:54PM |
D11.00005: State-resolved reactive scattering by slice imaging: A new view of the Cl+ethane reaction Arthur Suits, Wen Li, Cunshun Huang The reactions of chlorine atoms with alkanes have been the subject of intense scrutiny as model systems for the detailed investigation of polyatomic reaction dynamics. Interest in these systems has accelerated in recent years as new experimental methods have provided the means of gaining insight into the reaction with quantum state specificity. In this presentation, we will show state-resolved crossed beam scattering results for the reaction Cl + C$_{2}$H$_{6} \quad \to $ HCl + C$_{2}$H$_{5}$, obtained using DC slice imaging. The HCl (v=0, J) images, recorded at a range of well-defined collision energies, show strongly coupled angular and translational energy distributions revealing features of the reaction not seen in previous studies. The overall distribution is mainly forward-sideways scattered with respect to the Cl beam, with a translational energy distribution peaking near the collision energy. However, there is a substantial backscattered contribution that is very different. It shows a sharp peak near the energetic limit, but extending to much lower energy, implying substantial internal excitation in the ethyl radical co-product. These results provide new insight into the reaction, and they are considered in terms of alternative models of the dynamics. This work represents the first genuine crossed-beam study in which a product other than the methyl radical was detected with quantum state specificity, showing the promise of the approach generally for high resolution state-resolved reactive scattering. [Preview Abstract] |
Monday, March 13, 2006 3:54PM - 4:06PM |
D11.00006: Propargyl Radical: ab initio Anharmonic Modes and the Polarized Infrared Absorption Spectra of Matrix-Isolated HCCCH2 Barney Ellison, Xu Zhang, Evan Jochnowitz, Mark Nimlos, John Stanton, Mychel Varner Propargyl Radical: ab initio Anharmonic Modes and the Polarized Infrared Absorption Spectra of Matrix-Isolated HCCCH2 The propargyl radical has twelve fundamental vibrational modes, Gamma(vib)(HCCCH2) = 5a1 + 3b1 + 4b2, and nine have been detected in a cryogenic matrix. Ab initio coupled-cluster anharmonic force field calculations were used to help guide the assignments. The experimental HCCCH2 matrix frequencies (cm-1) and polarizations are: a1 modes - 3308.5 +/- 0.5, 3028.3 +/- 0.6, 1935.4 +/- 0.4, 1440.4 +/- 0.5, 1061.6 +/- 0.8; b1 modes - 686.6 +/- 0.4, 483.6 +/- 0.5; b2 modes - 1016.7 +/- 0.4, 620 +/- 2. We recommend a complete set of gas-phase vibrational frequencies for the propargyl radical, HCCCH2 X 2B1. [Preview Abstract] |
Monday, March 13, 2006 4:06PM - 4:18PM |
D11.00007: H-atom elimination of $n$-propyl and \textit{iso}-propyl radicals: a photodissociation study Jingsong Zhang, Weidong Zhou, Yan Yuan The H-atom elimination channels in the UV photodissociation of jet-cooled $n$-propyl and \textit{iso}-propyl radicals are studied in the region of 237 nm using the high-$n$ Rydberg-atom time-of-flight technique. Upon excitation to the 3$p$ state by the UV photolysis radiation, $n$-propyl radical and \textit{iso}-propyl radical dissociate into the H atom and propene products. The product center-of-mass translational energy release of both $n$-propyl and \textit{iso}-propyl radicals have bimodal distributions. The H-atom product angular distribution in $n$-propyl is anisotropic (with \textit{$\beta $} $\sim $ 0.5), and that in \textit{iso}-propyl is isotropic. The overall average translational energy release is $\langle $E$_{T}\rangle $ $\sim $ 0.27E$_{avail}$ for $n$-propyl and $\langle $E$_{T}\rangle \quad \sim $ 0.21E$_{avail}$ for \textit{iso}-propyl. The bimodal translational energy distributions indicate two dissociation pathways: (i) a unimolecular dissociation pathway from the ground-state propyl after internal conversion from the 3$p$ state, and (ii) a repulsive pathway directly connected with the excited state of the propyl radical. Isotope labeling has also been carried out. The possible photodissociation mechanisms will be discussed. [Preview Abstract] |
Monday, March 13, 2006 4:18PM - 4:30PM |
D11.00008: Photoinitiated decomposition of hydorxyalkyl radicals: Rydberg-valence interactions Hanna Reisler, Jie Wei, Boris Karpichev, Laura Edwards The photoinitiated dynamics of hydroxyalkyl radicals is investigated on the ground and excited surfaces. Overtone excitation of four quanta of OH stretch vibration of the hydroxymethyl radical gives rise to O-H bond fission via tunneling, without prior isomerization to the methoxy radical. Excitation of hydroxyethyl radicals to the lowest $s$ and $p$ Rydberg excited states accesses dissociative states lying as low as 2.5 eV above the ground state. These states undergo conical intersections with each other and the ground state, which are much more efficient than comparable interactions in the hydroxymethyl radical, and display increased opportunities for accessing crossing seams. Both O-H and C-H bond fission channels are observed. Dissociation mechanisms involving direct and indirect bond fission, isomerization and nonadiabatic interactions will be discussed. Research supported by DOE. [Preview Abstract] |
Monday, March 13, 2006 4:30PM - 4:42PM |
D11.00009: Determining the Products and Mechanism of the C$_{2}$Cl$_{3}$+O$_{2}$ Reaction by Time-Resolved FTIR Emission Spectroscopy Hongmei Su, Tiancheng Xiang, Shaolei Zhao, Congyun Shi The reaction products and mechanism of the C$_{2}$Cl$_{3}$ radical with O$_{2}$ have been studied by time-resolved FTIR emission spectroscopy. The chlorinated vinyl radical, C$_{2}$Cl$_{3}$, was generated by 248 nm photolysis of a C$_{2}$Cl$_{4}$ precursor. Vibrationally excited CO, CO$_{2}$ and ClCO products were observed. The time evolution of the emission intensity shows that CO$_{2}$ and ClCO are primary reaction products. In contrast, CO emission does not decay until \textit{ms} time scale indicating that CO products are due to secondary reactions, most likely, the further dissociation of ClCO. Two primary reaction channels are identified: C$_{2}$Cl$_{3}$ + O$_{2} \quad \diamondsuit $ CO$_{2}$ + CCl$_{3}$ $\diamondsuit $ ClCO + COCl$_{2}$ When reacting with O$_{2}$, the observed products and channels of the chlorinated vinyl radical, C$_{2}$Cl$_{3}$, turned out to be very similar to its counterpart, vinyl radical C$_{2}$H$_{3}$. Morokuma's DFT study$^{[1]}$ of the reaction C$_{2}$H$_{3}$+O$_{2}$ shows that the main reaction path proceeds through a three-membered CCO ring adduct. The question is, does C$_{2}$Cl$_{3}$ go through the same reaction path as C$_{2}$H$_{3}$? Does the substitution of H atom by heavier Cl atom make any difference in regard to the reaction mechanism? Further investigation combining \textit{ab initio }calculation is under way. \newline \newline [1] A.M.Mabel, E.W.G.Diau, M.C.Lin, and K.Morokuma, J.Am.Chem.Soc. 118, 9759 (1996) [Preview Abstract] |
Monday, March 13, 2006 4:42PM - 4:54PM |
D11.00010: Experimental and Theoretical Investigation of the Electronic Spectroscopy of H$_{2}$CN Paul J. Dagdigian, Alexey Teslja, Michael Banck, Wolfgang Eisfeld The electronic spectrum of H$_{2}$CN, recorded through cavity ring-down spectroscopy, is reported. The radical was prepared by 193 nm photolysis of monomeric formaldoxime vapor. Two diffuse features are observed in the 34800-35800 cm$^{-1}$ spectral range, along with the $A - X$ (1,0) band of the OH co-fragment. These are assigned as vibronic transitions to the ground and 2$b_{1}$ (umbrella mode) levels of the second excited, $^{2}A_{1}$ state from the ground $^{2}B_{2}$ state. Experimental and simulated rotational profiles of these bands agree extremely well with each other for an assumed type-B electric-dipole allowed $^{2}A_{1} \quad \leftarrow \quad ^{2}B_{2}$ transition appropriate to this transition. A theoretical investigation of the dissociation pathways for electronically excited H$_{2}$CN is also presented. The upper states of the observed bands cannot dissociated directly, but higher $b_{1}$ levels are above the excited-state dissociation limit. [Preview Abstract] |
Monday, March 13, 2006 4:54PM - 5:06PM |
D11.00011: Ab initio analysis of the BCN, NBC and CNB fragments formation Robert Jara, Vladimir Makarov, Gerardo Morell, Brad Weiner The Laser Ablation technique applied to the synthesis of nano-structured materials produce unique compositions, with correspondingly unique physical and chemical properties. In order to do smart utilization of this method, fundamental understanding of the precursor formation and incorporation mechanisms for the growth of different materials is crucial. Particularly important is to know what fragments are formed during the ablation process, in order to understand its dynamic evolution and eventual incorporation in the synthesized material. In connection to the synthesis of Boron Carbonitride, we have focused our attention on the dynamic evolution of the primary reactions upon BCN ablation using the \textit{ab initio} method (Gaussian-98 software pack). The results (including the formation energy, direct and inverse rate constants, etc.) were estimated by considering the elementary reactions as bimolecular reactions: reactions between atoms, and atoms with diatomic molecules. The temperature dependence of the dissociation rate constant of three-atomic fragments by different channels was also analyzed using the RRKM theory. It was found that BCN and NBC are the most thermodynamically stable fragments. This theoretical analysis helps understand the strong tendency for BCN to grow stoichiometrically under a variety of experimental conditions. [Preview Abstract] |
Monday, March 13, 2006 5:06PM - 5:18PM |
D11.00012: Variable (\textit{T}$_{g}$, \textit{T}$_{s}$) Measurements of Alkane Dissociative Sticking Coefficients Leticia Valadez, Kristy DeWitt, Heather Abbott, Kurt Kolasinski, Ian Harrision Dissociative sticking coefficients $S(T_{g}, \quad T_{s})$ for CH$_{4}$ and C$_{2}$H$_{6}$ on Pt(111) have been measured as a function of gas temperature ($T_{g})$ and surface temperature ($T_{s})$ using an effusive molecular beam. Microcanonical unimolecular rate theory (MURT) was employed to extract transition state characteristics [e.g., $E_{0}$(CH$_{4})$ = 52.5$\pm $3.5 kJ/mol$^{-1}$ and $E_{0}$(C$_{2}$H$_{6})$ = 26.5$\pm $3 kJ/mol$^{-1}$]. MURT allows our $S(T_{g}, \quad T_{s})$ values to be directly compared to other supersonic molecular beam and thermal equilibrium sticking measurements. The $S(T_{g}, \quad T_{s})$ depend strongly on $T_{s}$, however, only for CH$_{4}$ is a strong $T_{g}$ dependence observed. The fairly weak $T_{g}$ dependence for C$_{2}$H$_{6}$ suggests that vibrational mode specific behavior and/or molecular rotations play stronger roles in the dissociative chemisorption of C$_{2}$H$_{6}$ than they do for CH$_{4}$. Interestingly, thermal $S(T_{g}=T_{s})$ predictions based on MURT modeling of our CH$_{4}$/Pt(111) data are three orders of magnitude \textit{higher} than recent thermal equilibrium measurements on supported Pt nanocrystallite catalysts [J. M. Wei, E. Iglesia, J. Phys. Chem. B \textbf{108}, 4094 (2004)]. [Preview Abstract] |
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