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  • 1
    Source: ACS Legacy Archives
    Topics: Chemistry and Pharmacology , Physics
    Type of Medium: Electronic Resource
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  • 2
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 97 (1992), S. 1809-1817 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Collisional deactivation of highly vibrationally excited gas phase toluene-d8 and benzene-d6 pumped at 248 nm, has been investigated by monitoring the time resolved infrared fluorescence from the C–D stretch modes near 4.3 μm. For toluene-d8, energy transfer data were obtained for about 20 collider gases, including unexcited toluene-d8; for benzene-d6, only a few colliders were investigated. For both systems the data were analyzed by an inversion technique that converts the fluorescence decay to the bulk average energy, from which is calculated the average energy transferred per collision, 〈〈ΔE〉〉inv. Data obtained earlier for benzene-d0 were reanalyzed and the revised results are reported. Results for both normal and deuterated excited species show 〈〈ΔE〉〉inv to be nearly directly proportional to the vibrational energy 〈〈E〉〉inv of the excited molecule from 5 000 to 25 000 cm−1. However, for pure toluene-d8, benzene-d6, and a few other collider gases at high energies, the slope of the 〈〈ΔE〉〉inv vs 〈〈E〉〉inv curve is reduced and even becomes negative at sufficiently high energies. The results obtained for normal and deuterium-containing species are discussed in terms of possible quantum effects and mechanisms for energy transfer. In particular, it is considered likely that V–T/R energy transfer dominates over V–V, and the lowest frequency vibrational modes are the conduits for the energy transfer, in agreement with results for small molecules. Attention is called to a fundamental difference between classical and quantum statistics and how this difference may adversely affect classical trajectory simulations of large molecules.
    Type of Medium: Electronic Resource
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  • 3
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The collisional loss of vibrational energy from gas-phase toluene, pumped by a pulsed KrF laser operating at 248 nm, has been observed by monitoring the time-resolved infrared fluorescence from the C–H stretch modes near 3.3 μm. The fragmentation quantum yield of toluene pumped at 248 nm was determined experimentally to be ∼6%. Energy-transfer data were obtained for 20 collider gases, including unexcited toluene, and analyzed by an improved inversion technique that converts the fluorescence intensity to the bulk average energy, from which is extracted 〈〈ΔE〉〉, the bulk average amount of energy transferred per collision. Comparisons are presented of these results with similar studies of benzene and azulene, and with the time-resolved ultraviolet absorption study of toluene carried out by Hippler et al. [J. Chem. Phys. 78, 6709 (1983)]. The present results show 〈〈ΔE〉〉 to be nearly directly proportional to the vibrational energy of the excited toluene from 5000 to 25 000 cm−1. For many of the colliders at higher energies, the energy dependence of 〈〈ΔE〉〉 is somewhat reduced. A simple method is described for obtaining good estimates of 〈ΔE〉d (the energy transferred per collision in deactivating collisions) by carrying out an appropriate least-squares analysis of the 〈〈ΔE〉〉 data. The values of 〈ΔE〉d are then used in master-equation calculations to investigate possible contributions from "supercollisions'' (in which surprisingly large amounts of energy are transferred) in the deactivation of toluene.
    Type of Medium: Electronic Resource
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  • 4
    ISSN: 1573-0662
    Keywords: ozone episode ; air pollution ; biomass burning
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Geosciences
    Notes: Abstract Results of field measurements carried out from June 15 to December 31, 1995, in Córdoba city (Argentina) are presented. During this field campaign, surface ozone mixing ratios were generally around 30–35 ppb (afternoon peak). However, during the first week of September, days with excessive ozone values close to 100 ppb were found. These elevated ozone concentrations appeared together with high values of NOx, CO, PM10, and an unusual meteorological situation for this time of the year. These results made this episode an interesting one to be studied in more detail. In this work, we used chemical and meteorological data to trace the region from where the assumed precursors were emitted and we identified possible source characteristics.
    Type of Medium: Electronic Resource
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  • 5
    ISSN: 0538-8066
    Keywords: Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Models that simulate atmospheric photochemistry require the use of a stiff ordinary differential equations (ODEs) solver. Since the simulation of the chemical transformations taking place in the system takes up to 80 percent of the CPU time, the numerical solver must be computationally fast. Also, the residual error from the solver must be small. Because most accurate solvers are relatively slow, modelers continue to search for timely, yet accurate integration methods. Over the past years an extensive number of articles have been dedicated to this subject. One of the highly debated questions is whether one should construct specialized algorithms or instead use general methods for stiff ODEs. In the present article we use the second alternative. We apply three linearly (semi-)implicit methods from the classical stiff ODE literature which we modified to implement the sparse routines to solve the system of equations describing a complex kinetic mechanism. © 1998 John Wiley & Sons, Inc. Int J Chem Kinet 30: 349-358, 1998
    Additional Material: 4 Ill.
    Type of Medium: Electronic Resource
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  • 6
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 95 (1991), S. 8108-8119 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The time-resolved infrared fluorescence technique has been used to study V–V and V–T/R energy transfer to carbon dioxide from highly excited benzene, benzene-d6, toluene, and toluene-d8. The highly vibrationally excited aromatics in the electronic ground state are obtained by radiationless transitions after pumping with a KrF laser at 248 nm to the S1 excited electronic level. The V–V energy transfer from the excited parent to the asymmetric stretch mode of CO2 was measured by observing the characteristic emission of CO@B|2 near 4.3 μm. From these measurements, the probability per collision of formation of CO*2 was determined as a function of the internal energy in the excited aromatic. In all cases investigated, this probability is ≤0.1% at the initial excitation energy of 40 000 cm−1 and it is approximately directly proportional to the vibrational energy of the excited aromatic. The total concentration of CO@B|2 produced as a result of the many collisions needed to totally deactivate the excited aromatic amounted to 〉5% of the initial concentration of the excited aromatic and the quantitative values obtained are in excellent agreement with other work.A simple dipole–dipole interaction model is shown to explain the observed magnitude of V–V energy transfer and it is used to predict the amount of energy transferred to the bending mode of CO2. A key feature of this model is that the states of the highly vibrationally excited polyatomic are assumed to be broadened by rapid intramolecular vibrational redistribution of energy. In addition to the V–V energy-transfer measurements, the average energy lost per collision by the excited aromatic was determined as a function of the vibrational energy of the aromatic, and the rate constants were determined for CO*2 deactivation by the nondeuterated species. For the deuterated species, the results implicated a contribution from resonant V–V transfer between the C–D stretch modes and the asymmetric stretch mode of CO2. The overall results for the CO2 collider gas indicate that V–V energy transfer contributes a relatively small portion of the total energy transfer, and that portion can be described with the dipole–dipole interactions model.
    Type of Medium: Electronic Resource
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  • 7
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 92 (1990), S. 4793-4804 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The time dependent thermal lensing technique has been used to measure the vibrational relaxation of NO2 (initially excited at 21 631 cm−1) by Ar, Kr, and Xe. The energy transfer analysis was carried out in terms of 〈〈ΔE〉〉, the bulk average energy transferred per collision. This quantity was found to have a very strong dependence on vibrational energy, with a marked increase at energies greater than about 10 000 cm−1, where several electronic excited states (2B2, 2B1, and 2A2) mix with the ground state (2A1). This effect may be due to large amplitude vibrational motions associated with the coupled electronic states. Even at low energies, deactivation is faster than in other triatomic systems, probably because NO2 is an open shell molecule and electronic curve crossings provide efficient pathways for vibrational deactivation. The V–T rate constant for deactivation of NO2(010) by argon is estimated to be (5.1±1.0)×10−14 cm3 s−1. Results obtained for NO@B|2–NO2 collisions gave 〈〈ΔE〉〉 values in good agreement with literature results from fluorescence quenching experiments, indicating that V–T may be more important than V–V energy transfer in the quenching process.
    Type of Medium: Electronic Resource
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  • 8
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 2239-2253 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: High-order spectroscopic data for the reactant are used exclusively to determine both the sum of open reactive channels and the density of states, which are used in a statistical theory to predict dissociation rate constants. Practical methods are introduced for calculating sums of reactive channels and densities of states, when couplings among all degrees of freedom are included. An empirical method is described for reconciling spectroscopic parameters with known dissociation energies (also determined spectroscopically). The predicted k(E,J)'s and thermal k∞(T) for NO2 dissociation are in good agreement with experimental data, especially when the effects of electronically excited states are included. The predicted low pressure thermal rate constants are generally in fair agreement with experiment, although a slightly different temperature dependence is calculated; this discrepancy is probably due to the absence of unknown higher order spectroscopic terms and to the crude corrections made for excited electronic states. When high order spectroscopic (or theoretical) data are available and when the effects due to excited electronic states are considered, this theory is useful for predicting, fitting, and interpreting unimolecular rate data.
    Type of Medium: Electronic Resource
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  • 9
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 108 (1998), S. 2383-2394 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The time dependent thermal lensing (TDTL) technique has been used to study collisional energy transfer from highly excited CS2 in baths of Xe, Kr, and Ar, and from highly excited SO2 in Kr and Ar. Bath gas pressures ranged from about 50 to about 600 Torr. The data were analyzed by simulating the observed TDTL signals with a unified hydrodynamic TDTL theory. The results are expressed in terms of 〈ΔE〉, the bulk average energy transferred per collision as a function of 〈E〉, the mean energy content. The results show that 〈ΔE〉 increases dramatically at 〈E〉(approximate)17 500–23 500 cm−1 for CS2 deactivation, and at 〈E〉(approximate)18 000–22 500 cm−1 for SO2 deactivation. This enhancement of energy transfer, which was observed previously in NO2 and CS2 deactivation, has been linked to the presence of nearby excited electronic states. Furthermore, at lower energy, our results reveal an unusual systematic dependence of 〈ΔE〉 on bath pressure; energy transfer per collision is significantly more efficient at lower collision frequency. These results and data from the literature can be explained with a phenomenological model which includes collisional vibrational relaxation within each of two sets of vibronic levels, and collision-induced intersystem crossing (CIISC), which exhibits mixed order kinetics. © 1998 American Institute of Physics.
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  • 10
    Publication Date: 2019-08-15
    Print ISSN: 0167-6369
    Electronic ISSN: 1573-2959
    Topics: Energy, Environment Protection, Nuclear Power Engineering
    Published by Springer
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