| 1. |
- Abrahamsson, Erik, 1974, et al.
(författare)
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A new reaction path for the C + NO reaction: dynamics on the 4A'' potential-energy surface.
- 2008
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Ingår i: Physical chemistry chemical physics : PCCP. - : Royal Society of Chemistry (RSC). - 1463-9076 .- 1463-9084. ; 10:30, s. 4400-9
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Tidskriftsartikel (refereegranskat)abstract
- We present a new reaction path without significant barriers for the C + NO reaction, forming ground state N((4)S) and CO. Electronic structure (CASPT2) calculations have been performed for the two lowest (4)A'' states of the CNO system. The lowest of these states shows no significant barriers against reaction in the C + NO or O + CN channels. This surface has been fitted to an analytical function using a many-body expansion. Using this surface, and the previously published (2)A' and (2)A'' surfaces [Andersson et al., Phys. Chem. Chem. Phys., 2000, 2, 613; Andersson et al., Chem. Phys., 2000, 259, 99], we have performed quasiclassical trajectory (QCT) calculations, obtaining rate coefficients for the C((3)P) + NO(X(2)Pi) --> CO(X(1)Sigma(+)) + N((4)S,(2)D) and C((3)P) + NO(X(2)Pi) --> O((3)P) + CN(X(2)Sigma(+)) reactions. We have also simulated the crossed molecular beam experiments of Naulin et al. [Chem. Phys., 1991, 153, 519] The inclusion of the (4)A'' surface in the QCT calculations gives excellent agreement with experiments. This is the first time an adiabatic pathway from C((3)P) + NO(X(2)Pi) to CO(X(1)Sigma(+))+N((4)S) has been reported.
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| 2. |
- Abrahamsson, Erik, 1974, et al.
(författare)
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Classical and quantum dynamics of the O+CN reaction
- 2006
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Ingår i: Chemical Physics. - : Elsevier BV. - 0301-0104. ; 324:2-3, s. 507-514
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Tidskriftsartikel (refereegranskat)abstract
- Electronic structure (CASPT2) calculations have been performed for the (2)Pi and (4)Sigma(-) states of the NCO system. To create the potential energy surfaces, the generalized discrete variable representation (GDVR) method has been used. Wave packet calculations have been performed for the collinear O + CN reaction on both surfaces. These are the first reported quantum dynamics calculations on this reaction. State-to-state reaction probabilities are presented. On the (2)Pi surface, which has an almost 6 eV deep well, we obtain structure in the reaction probabilities at low kinetic energies but at higher energies they are smooth. The (4)Sigma(-) surface is highly exoergic and vibrationally non-adiabatic dynamics is observed. The (4)Sigma(-) surface has an early barrier and as a result we find that translational energy more efficiently promotes the reaction than vibrational energy does. The wave packet results are compared with QCT results. Generally the agreement is good as would be expected but some notable differences are found, particularly for reaction out of the vibrational ground state. (c) 2005 Elsevier B.V. All rights reserved.
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| 3. |
- Abrahamsson, Erik, 1974, et al.
(författare)
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Dynamics of the O + CN Reaction and N + CO Scattering on Two Coupled Surfaces
- 2009
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Ingår i: J. Phys. Chem. A. - : American Chemical Society (ACS). - 1089-5639 .- 1520-5215. ; 113:52, s. 14824-14830
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Tidskriftsartikel (refereegranskat)abstract
- Spin−orbit coupling between the two collinear 2Π and 4Σ− potential energy surfaces for the NCO system are calculated using the RASSI method with CASSCF wave functions as basis set. The GDVR method has been used to interpolate a spin−orbit coupling surface. Wave packet and quasi-classical trajectory surface hopping calculations have been performed and compared for both the O(3P) + CN(X2Σ+) → N(4S) + CO(X1Σ+) reaction and for electronically inelastic scattering in the N + CO channels. The O + CN nonadiabatic reaction probabilities are small. The wavepacket study gives a resonance structure. Also for the N + CO electronically inelastic scattering the wave packet calculations give a distinct resonance structure with peak transition probabilities up to around 10%, which is somewhat lower than the trajectory surface hopping results.
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| 4. |
- Andersson, Patrik U, 1970, et al.
(författare)
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Formation of Highly Rovibrationally Excited Ammonia from Dissociative Recombination of NH4
- 2010
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Ingår i: Journal of Physical Chemistry Letters. - : American Chemical Society (ACS). - 1948-7185. ; 1:17, s. 2519-2523
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Tidskriftsartikel (refereegranskat)abstract
- The internal energy distribution of ammonia formed in the dissociative recombination (DR) of NH4+ with electrons has been studied by an imaging technique at the ion storage ring CRYRING. The DR process resulted in the formation of NH3 + H (0.90 ± 0.01), with minor contributions from channels producing NH2 + H2 (0.05 ± 0.01) and NH2 + 2H (0.04 ± 0.02). The formed NH3 molecules were highly internally excited, with a mean rovibrational energy of 3.3 ± 0.4 eV, which corresponds to 70% of the energy released in the neutralization process. The internal energy distribution was semiquantitatively reproduced by ab initio direct dynamics simulations, and the calculations suggested that the NH3 molecules are highly vibrationally excited while rotational excitation is limited. The high internal excitation and the translational energy of NH3 and H will influence their subsequent reactivity, an aspect that should be taken into account when developing detailed models of the interstellar medium and ammonia-containing plasmas.
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| 5. |
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| 6. |
- Andersson, Patrik U, 1970, et al.
(författare)
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Water Condensation on Graphite Studied by Elastic Helium Scattering and Molecular Dynamics Simulations
- 2007
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Ingår i: Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 111:42, s. 15258-15266
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Tidskriftsartikel (refereegranskat)abstract
- Formation of water/ice layers on graphite has been studied in the temperature range from 90 to 180 K by elastic helium scattering, light scattering, and molecular dynamics simulations. Combined helium- and light-scattering experiments show that an ice film that wets the graphite surface is formed at surface temperatures of 100-140 K, whereas three-dimensional ice structures are formed at 140-180 K. Desorption of adsorbed water molecules competes with water incorporation into the ice film, and the ice formation rate is strongly temperature dependent. At 150 K, ice-layer formation takes place at the same time scale as layer reconstruction, and its properties are sensitive to the water deposition rate. The experimental results are compared with kinetics models, and the Johnston-Mehl-Avrami-Kolmogorov model is concluded to well describe the ice-layer formation kinetics in the whole temperature range. Molecular dynamics simulations of water-cluster formation on graphite at 90-180 K show that water molecules and small clusters are highly mobile on the surface, which rapidly results in the nucleation of large and less mobile clusters on the surface. Clusters formed at low temperature tend to have the most molecules in direct contact with the uppermost graphite layer, while multilayer cluster structures are preferred at high temperatures. The results are discussed and compared with earlier studies of water ice formation on solid surfaces.
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| 7. |
- Andersson, Stefan, 1973, et al.
(författare)
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Computational studies of the kinetics of the C+NO and O+CN reactions
- 2003
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Ingår i: Journal of Physical Chemistry A. - : American Chemical Society (ACS). - 1089-5639 .- 1520-5215. ; 107:28, s. 5439-5447
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Tidskriftsartikel (refereegranskat)abstract
- Thermal rate coefficients for the reactions C(3P) + NO(X2Π) → CN(X2Σ+) + O(3P), C(3P) + NO(X2Π) → CO(X1Σ+) + N(2D), and O(3P) + CN(X2Σ+) → CO(X1Σ+) + N(2D) in the temperature range from 5 to 5000 K have been obtained using quasiclassical trajectory calculations. Results are reported for two ab initio potential energy surfaces corresponding to states of 2A‘ and 2A‘ ‘ symmetry. Good agreement between calculated and experimental rate coefficients are obtained for the C + NO reactions for all temperatures, whereas the rate coefficient for the O + CN reaction at room temperature is larger than that found experimentally. The dynamics is considerably different on the two potential energy surfaces with the 2A‘ ‘ giving rate coefficients in better agreement with experiments. The quality of the potential energy surfaces are discussed in the light of new electronic structure calculations including spin−orbit coupling.
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| 8. |
- Barinovs, G, et al.
(författare)
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Propagation of 3D wave packets for nonzero total angular momentum using the split operator method
- 2001
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Ingår i: Journal of Physical Chemistry A. - : American Chemical Society (ACS). - 1089-5639 .- 1520-5215. ; 105:31, s. 7441-7445
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Tidskriftsartikel (refereegranskat)abstract
- Three-dimensional wave packet calculations for total angular momentum quantum number J ≥ 0 have been performed in Jacobi coordinates. To be able to use the split operator propagator together with the fast Fourier transform method, the wave function is transformed and a modified Hamiltonian obtained. The filter diagonalization method has been used to determine a few rovibrational eigenstates of the H2O molecule on the lowest potential energy surface. Good agreement with previous work is obtained.
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| 9. |
- Bäck, A, et al.
(författare)
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Comparison of classical and quantum dynamics for collinear cluster scattering
- 2005
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Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 1089-7690 .- 0021-9606. ; 122:14, s. 144711 (1-9)
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Tidskriftsartikel (refereegranskat)abstract
- The collinear dynamics of a cluster of four particles colliding with a fixed particle representing a surface is investigated using a four-dimensional wave packet approach. The properties of the system are chosen to resemble a water cluster interacting with graphite, but a deeper surface-particle potential is also considered causing significant dissociation of the cluster. Having four different product arrangement channels the system is quantum mechanically demanding but still manageable. The dynamical richness makes it a suitable benchmark system for evaluation of classical and quantum/classical schemes. The average energy transferred to the cluster and the three dissociation probabilities are presented as function of the initial state of the cluster. In addition to wave packet data, results obtained using quasiclassical as well as Wigner sampled classical trajectories are presented. The main conclusion is that classical mechanics can describe the dynamics of the system in a very satisfactory way. Including zero-point energy in the classical simulations is particularly important for a good description of dissociation but less important for energy transfer.
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| 10. |
- Bäck, A, et al.
(författare)
-
Investigation of quantum effects on vibrational excitation of CF3Br scattering from graphite
- 2002
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Ingår i: Chemical Physics. - 0301-0104. ; 285:2-3, s. 233-244
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Tidskriftsartikel (refereegranskat)abstract
- Mixed quantum–classical molecular dynamics simulations of vibrational excitation of CF3Br in collisions with graphite have been carried out. The quantum problem is made numerically manageable by a reduced dimensionality treatment of the internal degrees of freedom including only the C–Br stretch and the umbrella motion. These two coordinates are treated quantum mechanically as a 2D wavepacket whereas the translational and rotational degrees of freedom and the surface atoms are treated classically. A 3D wavepacket scheme where the distance to the surface is included among the quantum degrees of freedom is also considered as well as purely classical trajectories. It is found that the total vibrational excitation is quite low. Excitation of the C–Br stretch dominates with only a very small fraction of the excitation energy going into the umbrella motion. The 2D wavepacket calculations and the classical trajectories (initiated without zero point energy) give nearly identical results for the total excitation but the 2D wavepacket model excites the umbrella motion to a lesser extent. The 3D wavepacket method shows a considerable dependence on the initial width of the wavepacket due to the underlying mean-field approximation. All methods show a linear increase in vibrational excitation with increasing surface temperature, in agreement with previous experimental results.
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| 11. |
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| 12. |
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| 13. |
- Kong, Xiangrui, et al.
(författare)
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Dynamics and Kinetics of Methanol-Graphite Interactions at Low Surface Coverage
- 2019
-
Ingår i: Chemphyschem. - : Wiley. - 1439-4235 .- 1439-7641. ; 20:17, s. 2171-2178
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Tidskriftsartikel (refereegranskat)abstract
- The processes of molecular clustering, condensation, nucleation, and growth of bulk materials on surfaces, represent a spectrum of vapor-surface interactions that are important to a range of physical phenomena. Here, we describe studies of the initial stages of methanol condensation on graphite, which is a simple model system where gas-surface interactions can be described in detail using combined experimental and theoretical methods. Experimental molecular beam methods and computational molecular dynamics simulations are used to investigate collision dynamics and surface accommodation of methanol molecules and clusters at temperatures from 160 K to 240 K. Both single molecules and methanol clusters efficiently trap on graphite, and even in rarified systems methanol-methanol interactions quickly become important. A kinetic model is developed to simulate the observed behavior, including the residence time of trapped molecules and the quantified Arrhenius kinetics. Trapped molecules are concluded to rapidly diffuse on the surface to find and/or form clusters already at surface coverages below 10(-6) monolayers. Conversely, clusters that undergo surface collisions fragment and subsequently lose more loosely bound molecules. Thus, the surface mediates molecular collisions in a manner that minimizes the importance of initial cluster size, but highlights a strong sensitivity to surface diffusion and intermolecular interactions between the hydrogen bonded molecules.
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| 14. |
- Kong, Xiangrui, et al.
(författare)
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Environmental molecular beam studies of ice surface processes
- 2011
-
Ingår i: Physics and Chemistry of Ice 2010 (The 12th International Conference on the Physics and Chemistry of Ice, Sapporo, Japan, September 5-10, 2010, Eds. Y. Furukawa, G. Sazaki, T. Uchida, N. Watanabe, Hokkaido University Press, Sapporo, Japan, 2011). - 9784832903616 ; , s. 79-88
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Konferensbidrag (refereegranskat)abstract
- The development and application of a new molecular beam setup for studies of gas-ice interactions at pressures in the 0.001 mbar range is described. Direct simulation Monte Carlo calculations have been applied to optimize the experimental design with respect to gas transmission through the high pressure zone outside the ice surface and pressure along the surface plane. In a first application of the new setup elastic helium scattering and light scattering have been used to study the formation of water, methanol and water-methanol ice on a graphite surface in the temperature range from 180 to 215 K. Water does not wet the graphite surface under these conditions, in agreement with earlier results obtained below 180 K, while methanol readily forms a thin layer on the surface. Methanol enhances water ice nucleation and influences the properties of the ice formed, but does not have a major impact on ice growth or evaporation once a thick water-containing ice layer has formed. The potential for further development of the molecular beam technique are discussed.
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| 15. |
- Kong, Xiangrui, et al.
(författare)
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Water Accommodation and Desorption Kinetics on Ice
- 2014
-
Ingår i: Journal of Physical Chemistry A. - : American Chemical Society (ACS). - 1089-5639 .- 1520-5215. ; 118:22, s. 3973-3979
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Tidskriftsartikel (refereegranskat)abstract
- The interaction of water vapor with ice remains incompletely understood despite its importance in environmental processes. A particular concern is the probability for water accommodation on the ice surface, for which results from earlier studies vary by more than 2 orders of magnitude. Here, we apply an environmental molecular beam method to directly determine water accommodation and desorption kinetics on ice. Short D2O gas pulses collide with H2O ice between 170 and 200 K, and a fraction of the adsorbed molecules desorbs within tens of milliseconds by first order kinetics. The bulk accommodation coefficient decreases nonlinearly with increasing temperature and reaches 0.41 +/- 0.18 at 200 K. The kinetics are well described by a model wherein water molecules adsorb in a surface state from which they either desorb or become incorporated into the bulk ice structure. The weakly bound surface state affects water accommodation on the ice surface with important implications for atmospheric cloud processes.
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| 16. |
- Krems, RV, et al.
(författare)
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Resonance and reversibility of vibrational relaxation of HF in high temperature Ar bath gas
- 2002
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Ingår i: Journal of Chemical Physics. - 1089-7690 .- 0021-9606. ; 117:1, s. 166-171
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Tidskriftsartikel (refereegranskat)abstract
- The Boltzmann averaged rate constants for total vibrational relaxation of HF(v = 1) in collisions with Ar are computed in the range of temperatures between 100 and 1500 K. The computed rate constants overestimate the experimental measurements at high temperatures by a large factor. It is concluded that the deviation between theory and experiment cannot be explained by inaccuracy of the PES or dynamical approximations made. It is shown that increasing initial rotational energy enhances a resonant character of the vibrational energy transfer to a great extent. An assumption is made that total vibrational relaxation of HF(v = 1) at high temperatures is determined by competition between vibrational relaxation to a resonant level (v = 0,jres), vibrational excitation from the resonant level, and purely rotational relaxation of HF(v = 0,jres). It is demonstrated that at high temperatures the latter process can be significantly slower than vibrationally inelastic transitions and rotational relaxation of HF(v = 0,jres) may in fact be a rate-limiting stage of vibrational relaxation.
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| 17. |
- Markovic, Nikola, 1961, et al.
(författare)
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A linearized path integral description of the collision process between a water molecule and a graphite surface
- 2008
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Ingår i: JOURNAL OF PHYSICAL CHEMISTRY A. - : American Chemical Society (ACS). - 1089-5639 .- 1520-5215. ; 112:8, s. 1701-1711
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Tidskriftsartikel (refereegranskat)abstract
- Quantum effects in the scattering and desorption process of a water molecule from a graphite surface are investigated using the linearized path integral model. The graphite surface is quantized rigorously using the fully quantum many-body Wigner transform of the surface Boltzmann operator, while the water molecule is treated as rigid. Classical dynamics with these quantized initial conditions show that quantizing the surface at 100 and 300 K results in markedly different results, compared to a fully classical analysis. The trapping probability (defined as the probability of multiple encounters with the surface) is not sensitive to the choice of dynamical treatment, but the residence time on the surface is much shorter in the quantum case. At 300 K the transiently trapped molecules desorb from the surface with a rate constant which is 60−70% larger than the corresponding classical value. Lowering the surface temperature to 100 K decreases the quantum rate constant by approximately a factor of 3 while all trapped molecules stick to the surface in the classical case. The stability of the quantum initial state for the highly anisotropic graphite crystal is discussed in detail as well as the dynamical consequences of energy redistribution during the scattering process. The graphite surface application demonstrates that the Boltzmann operator Wigner transform for a system with 900 degrees of freedom can be obtained by the so-called gradient implementation [Poulsen et al. J. Chem. Theory Comput. 2006, 2, 1482] of the underlying Feynman−Kleinert effective frequency theory, an implementation that only requires a force and potential routine for the system at hand, and hence is applicable to any molecule−surface collision problem.
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| 18. |
- Markovic, Nikola, 1961, et al.
(författare)
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Mixed quantum-classical scattering dynamics of CF3Br
- 2004
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Ingår i: Journal of Physical Chemistry A. - : American Chemical Society (ACS). - 1089-5639 .- 1520-5215. ; 108:41, s. 8765-8771
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Tidskriftsartikel (refereegranskat)abstract
- The vibrational excitation of CF3Br scattering from graphite has been studied using mixed quantum−classical methods. A previously investigated 2D model for the intramolecular degrees of freedom [Bäck, A.; Marković, N. Chem. Phys. 2002, 285, 233] has been extended to 3D including all vibrations of a1 symmetry, improving the dynamical description of the umbrella mode. We investigate the details of the excitation process for a few selected initial conditions as well as the general effect of surface temperature for ensembles of randomly sampled trajectories. Quantum results are obtained from 3D wave packet propagations and calculations based on the time-dependent Gauss−Hermite discrete variable representation method. When the quantum data are compared with classical results it is confirmed that quantization of the internal degrees of freedom does indeed have a very small effect for the present system. Considering vibrational excitation from the ground state, almost perfect agreement between quantum and classical calculations is found, provided that the classical trajectories are initialized without vibrational energy.
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| 19. |
- Papagiannakopoulos, Panos, et al.
(författare)
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Surface Transformations and Water Uptake on Liquid and Solid Butanol near the Melting Temperature
- 2013
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Ingår i: Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 117:13, s. 6678-6685
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Tidskriftsartikel (refereegranskat)abstract
- Water interactions with organic surfaces are of central importance in biological systems and many Earth system processes. Here we describe experimental studies of water collisions and uptake kinetics on liquid and solid butanol from 160 to 200 K. Hyperthermal D2O molecules (0.32 eV) undergo efficient trapping on both solid and liquid butanol, and only a minor fraction scatters inelastically after an 80% loss of kinetic energy to surface modes. Trapped molecules either desorb within a few ms, or are taken up by the butanol phase during longer times. The water uptake and surface residence time increase with temperature above 180 K indicating melting of the butanol surface 4.5 K below the bulk melting temperature. Water uptake changes gradually across the melting point and trapped molecules are rapidly lost by diffusion into the liquid above 190 K. This indicates that liquid butanol maintains a surface phase with limited water permeability up to 5.5 K above the melting point. These surface observations are indicative of an incremental change from solid to liquid butanol over a range of 10 K straddling the bulk melting temperature, in contrast to the behavior of bulk butanol and previously studied materials.
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| 20. |
- Rääf, Christopher, et al.
(författare)
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Introduction of a method to calculate cumulative age- And gender-specific lifetime attributable risk (LAR) of cancer in populations after a large-scale nuclear power plant accident
- 2020
-
Ingår i: PLoS ONE. - : Public Library of Science (PLoS). - 1932-6203. ; 15:2
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Tidskriftsartikel (refereegranskat)abstract
- The effect of age and gender in risk estimates related to long-term residence in areas contaminated by nuclear power plant fallout was evaluated by applying the lifetime attributable risk (LAR) concept to an existing exposure model that was previously used for cumulative effective dose estimates. In this study, we investigated the influence of age distribution on the number of cancer cases by applying five different age distributions from nuclear power-producing countries (India, Japan, South Korea, and the United States), and Egypt because of intentions to develop nuclear power. The model was also used to estimate the effective dose and gender-specific LAR as a function of time after fallout for the offspring of the population living in 137Cs fallout areas. The principal findings of this study are that the LAR of cancer incidence (excluding non-fatal skin cancers) over 70 y is about 4.5 times higher for newborn females (5.4% per MBq m-2 of initial 137Cs ground deposition) than the corresponding values for 30 y old women (1.2% per MBq m-2 137Cs deposition). The cumulative LAR for newborn males is more than 3 times higher (3.2% versus 1.0% per MBq m-2 137Cs deposition). The model predicts a generally higher LAR for women until 50 y of age, after which the gender difference converges. Furthermore, the detriment for newborns in the fallout areas initially decreases rapidly (about threefold during the first decade) and then decreases gradually with an approximate half-time of 10-12 y after the first decade. The age distribution of the exposed cohort has a decisive impact on the average risk estimates, and in our model, these are up to about 65% higher in countries with high birth rates compared to low birth rates. This trend implies larger average lifetime attributable risks in countries with a highly proportional younger population. In conclusion, the large dispersion (up to a factor of 4 between newborns and 30 y olds) in the lifetime detriment per unit ground deposition of 137Cs over gender and age in connection with accidental nuclear releases justifies the effort in developing risk models that account for the higher radiation sensitivity in younger populations.
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| 21. |
- Thomson, Erik S, et al.
(författare)
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Collision Dynamics and Solvation of Water Molecules in a Liquid Methanol Film
- 2011
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Ingår i: Journal of Physical Chemistry Letters. - : American Chemical Society (ACS). - 1948-7185. ; 2:17, s. 2174-2178
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Tidskriftsartikel (refereegranskat)abstract
- Environmental molecular beam experiments are used to examine water interactions with liquid methanol films at temperatures from 170 to 190 K. We find that water molecules with 0.32 eV incident kinetic energy are efficiently trapped by the liquid methanol. The scattering process is characterized by an efficient loss of energy to surface modes with a minor component of the incident beam that is inelastically scattered. Thermal desorption of water molecules has a well characterized Arrhenius form with an activation energy of 0.47 ± 0.11 eV and pre-exponential factor of 4.6 × 10^(15±3) s^(–1). We also observe a temperature-dependent incorporation of incident water into the methanol layer. The implication for fundamental studies and environmental applications is that even an alcohol as simple as methanol can exhibit complex and temperature-dependent surfactant behavior.
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| 22. |
- Thomson, Erik S, et al.
(författare)
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Collision dynamics and uptake of water on alcohol-covered ice
- 2013
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Ingår i: Atmospheric Chemistry and Physics. - : Copernicus GmbH. - 1680-7316 .- 1680-7324. ; 13:4, s. 2223-2233
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Tidskriftsartikel (refereegranskat)abstract
- Molecular scattering experiments are used to investigate water interactions with methanol and n-butanol covered ice between 155 K and 200 K. The inelastically scattered and desorbed products of an incident molecular beam are measured and analyzed to illuminate molecular scale processes. The residence time and uptake coefficients of water impinging on alcohol-covered ice are calculated. The surfactant molecules are observed to affect water transport to and from the ice surface in a manner that is related to the number of carbon atoms they contain. Butanol films on ice are observed to reduce water uptake by 20 %, whereas methanol monolayers pose no significant barrier to water transport. Water colliding with methanol covered ice rapidly permeates the alcohol layer, but on butanol water molecules have mean surface lifetimes of less than or similar to 0.6 ms, enabling some molecules to thermally desorb before reaching the water ice underlying the butanol. These observations are put into the context of cloud and atmospheric scale processes, where such surfactant layers may affect a range of aerosol processes, and thus have implications for cloud evolution, the global water cycle, and long term climate.
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| 23. |
- Tomsic, A, et al.
(författare)
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Collision dynamics of large water clusters on graphite
- 2003
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Ingår i: Journal of Chemical Physics. - 1089-7690 .- 0021-9606. ; 119:9, s. 4916-4922
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Tidskriftsartikel (refereegranskat)abstract
- The emission of neutral cluster fragments during collisions of large water clusters with graphite surfaces has been investigated using molecular beam techniques. Water clusters with an average size of up to 1.4·104 molecules per cluster collide with the surface with a velocity of 1380 ms–1. Angular distributions for emitted large fragments are shifted towards the tangential direction and become increasingly narrow with increasing fragment size. The kinetic energy in the surface normal direction is efficiently transferred to internal degrees of freedom and to surface modes, while the momentum parallel to the surface plane is less affected by the surface interaction. Both a direct scattering channel and an emission channel mediated by cluster evaporation are concluded to be of importance for the collision outcome. The results for the evaporation-mediated emission channel agree well with previous experimental investigations and with recent molecular dynamics simulations, and the observations regarding the direct scattering channel qualitatively agree with the dynamics observed for macroscopic particles colliding with surfaces.
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| 24. |
- Tomsic, A, et al.
(författare)
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Molecular-dynamics simulations of cluster-surface collisions: Emission of large fragments
- 2001
-
Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 1089-7690 .- 0021-9606. ; 115:22, s. 10509-10517
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Tidskriftsartikel (refereegranskat)abstract
- Large-scale classical molecular-dynamics simulations of (H2O)n (n = 1032,4094) collisions with graphite have been carried out. The clusters have an initial internal temperature of 180 K and collide with an incident velocity in the normal direction between 200 and 1000 m/s. The 1032-clusters are trapped on the surface and completely disintegrate by evaporation. The 4094-clusters are found to partly survive the surface impact provided that the surface is sufficiently hot. These clusters are trapped on the surface for up to 50 ps before leaving the surface under strong evaporation of small fragments. The time spent on the surface is too short for full equilibration to occur, which limits the fragmentation of the clusters. The size of the emitted fragment is roughly 30% of the incident cluster size. The cluster emission mechanism is found to be very sensitive to the rate of the surface-induced heating and thus to the surface temperature. The incident cluster velocity is less critical for the outcome of the collision process but influences the time spent on the surface. The trends seen in the simulations agree well with recent experimental data for collisions of large water clusters with graphite [Chem. Phys. Lett. 329, 200 (2000)].
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| 25. |
- Tomsic, A, et al.
(författare)
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Molecular dynamics simulations of cluster-surface collisions: Mechanisms for monomer emission
- 2001
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Ingår i: Physical Chemistry Chemical Physics. - 1463-9084 .- 1463-9076. ; 3:17, s. 3667-3671
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Tidskriftsartikel (refereegranskat)abstract
- Molecular dynamics simulations of (H2O)4094-clusters impacting with a velocity of 470 ms–1 in the normal direction on a graphite surface kept at 1400 K were performed. The aim was to clarify the behavior of water molecules and other small fragments emitted during the collision event. The results agree well with previous experimental studies and with the results of a thermokinetic model for evaporation of small fragments during cluster scattering. About 80% of the evaporating water molecules come in close contact with the hot surface, and their translational degrees of freedom are partly accommodated to the temperature of the surface, especially in the direction normal to the surface plane, leading to a high translational temperature in this direction. The results stress the importance of energy transfer from the hot surface to the cluster and explain the high translational temperature determined from experimental angular distributions.
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| 26. |
- Tomsic, A, et al.
(författare)
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Scattering of Ice particles from a graphite surface: A molecular dynamics simulation study
- 2003
-
Ingår i: Journal of Physical Chemistry B. - : American Chemical Society (ACS). - 1520-5207 .- 1520-6106. ; 107:38, s. 10576-10582
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Tidskriftsartikel (refereegranskat)abstract
- Large-scale classical trajectory calculations of (H2O)n, n ≤ 25 159, colliding with a graphite surface have been carried out in order to relate the phenomenon of direct scattering to the initial conditions of the collision. The collisions were performed at normal incidence with the incident velocity ranging from 50 to 2000 ms-1 and at surface temperatures between 300 and 1400 K. Upon impact, the cluster is deformed elastically (reversibly) and plastically (irreversibly), and if the elastically stored energy is larger than the binding energy between the cluster and the surface, the cluster scatters directly from the surface. The partitioning between elastic and plastic deformation is governed by the initial conditions (cluster temperature, incident velocity, incident cluster size, and surface temperature). At low incident velocities the scattering probability is controlled by adhesion and at high incident velocities by plastic deformation, and the direct scattering is thus confined to a narrow range of incident velocities. The results are in qualitative agreement with recent experimental studies of water clusters scattering from graphite.
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| 27. |
- Öjekull, Jenny, 1973, et al.
(författare)
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Dissociative recombination of H+(H2O)3 and D+(D2O)3 water cluster ions with electrons: Cross sections and branching ratios
- 2007
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Ingår i: The Journal of chemical physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 127, s. 194301-194309
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Tidskriftsartikel (refereegranskat)abstract
- Dissociative recombination (DR) of the water cluster ions H+(H2O)3 and D+(D2O)3 with electrons has been studied at the heavy-ion storage ring CRYRING (Manne Siegbahn Laboratory, Stockholm University). For the first time, absolute DR cross sections have been measured for H+(H2O)3 in the energy range of 0.001–0.8 eV, and relative cross sections have been measured for D+(D2O)3 in the energy range of 0.001–1.0 eV. The DR cross sections for H+(H2O)3 are larger than previously observed for H+(H2O)n (n=1,2), which is in agreement with the previously observed trend indicating that the DR rate coefficient increases with size of the water cluster ion. Branching ratios have been determined for the dominating product channels. Dissociative recombination of H+(H2O)3 mainly results in the formation of 3H2O+H (probability of 0.95±0.05) and with a possible minor channel resulting in 2H2O+OH+H2 (0.05±0.05). The dominating channels for DR of D+(D2O)3 are 3D2O+D (0.88±0.03) and 2D2O+OD+D2 (0.09±0.02). The branching ratios are comparable to earlier DR results for H+(H2O)2 and D+(D2O)2, which gave 2X2O+X (X=H,D) with a probability of over 0.9.
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| 28. |
- Öjekull, Jenny, 1973, et al.
(författare)
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Dissociative recombination of NH4+ and ND4+ ions : Storage ring experiments and ab initio molecular dynamics
- 2004
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Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 120:16, s. 7391-7399
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Tidskriftsartikel (refereegranskat)abstract
- The dissociative recombination (DR) process of NH4+ and ND4+ molecular ions with free electrons has been studied at the heavy-ion storage ring CRYRING (Manne Siegbahn Laboratory, Stockholm University). The absolute cross sections for DR of NH4+ and ND4+ in the collision energy range 0.001-1 eV are reported, and thermal rate coefficients for the temperature interval from 10 to 2000 K are calculated from the experimental data. The absolute cross section for NH4+ agrees well with earlier work and is about a factor of 2 larger than the cross section for ND4+. The dissociative recombination of NH4+ is dominated by the product channels NH3+H (0.85+/-0.04) and NH2+2H (0.13+/-0.01), while the DR of ND4+ mainly results in ND3+D (0.94+/-0.03). Ab initio direct dynamics simulations, based on the assumption that the dissociation dynamics is governed by the neutral ground-state potential energy surface, suggest that the primary product formed in the DR process is NH3+H. The ejection of the H atom is direct and leaves the NH3 molecule highly vibrationally excited. A fraction of the excited ammonia molecules may subsequently undergo secondary fragmentation forming NH2+H. It is concluded that the model results are consistent with gross features of the experimental results, including the sensitivity of the branching ratio for the three-body channel NH2+2H to isotopic exchange.
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| 29. |
- Öjekull, Jenny, 1973, et al.
(författare)
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Dissociative recombination of water cluster ions with free electrons : Cross sections and branching ratios
- 2008
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Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 128:4, s. 44311-
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Tidskriftsartikel (refereegranskat)abstract
- Dissociative recombination (DR) of water cluster ions H+(H2O)(n) (n=4-6) with ree electrons has been studied at the heavy-ion storage ring CRYRING (Manne Siegbahn Laboratory, Stockholm University). For the first time, branching ratios have been determined for the dominating product channels and absolute DR cross sections have been measured in the energy range from 0.001 to 0.7 eV. Dissociative recombination is concluded to result in extensive fragmentation for all three cluster ions, and a maximum number of heavy oxygen-containing fragments is produced with a probability close to unity. The branching ratio results agree with earlier DR studies of smaller water cluster ions where the channel nH(2)O+H has been observed to dominate and where energy transfer to internal degrees of freedom has been concluded to be highly efficient. The absolute DR cross sections for H+(H2O)(n) (n=4-6) decrease monotonically with increasing energy with an energy dependence close to E-1 in the lower part of the energy range and a faster falloff at higher energies, in agreement with the behavior of other studied heavy ions. The cross section data have been used to calculate DR rate coefficients in the temperature range of 10-2000 K. The results from storage ring experiments with water cluster ions are concluded to partly confirm the earlier results from afterglow experiments. The DR rate coefficients for H+(H2O)(n) (n=1-6) are in general somewhat lower than reported from afterglow experiments. The rate coefficient tends to increase with increasing cluster size, but not in the monotonic way that has been reported from afterglow experiments. The needs for further experimental studies and for theoretical models that can be used to predict the DR rate of polyatomic ions are discussed.
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