162,115 research outputs found
On the accuracy of collisional energy transfer parameters for reaction kinetics applications: detailed evaluation of data from direct experiments
No full textIt is shown that the spread among the various "direct'' experimental data in the literature, so unsatisfactory for their application in chemical kinetics, can be removed consistently. Underlying agreement within very small uncertainties is demonstrated for the case of the much studied collisional relaxation of highly vibrationally excited azulene. Benchmark experimental data for the collisional energy transfer of highly vibrationally excited azulene obtained by the method of "kinetically controlled selective ionization (KCSI)'' (U. Hold, T. Lenzer, K. Luther and A. C. Symonds, J. Chem. Phys., 2003, 119, 11192) are used for a detailed comparison with earlier measurements employing time-resolved ultraviolet absorption (UVA) and infrared fluorescence (IRF). The experimental UVA and IRF traces are simulated by convolution of the transient vibrational distributions g(E) during relaxation obtained from KCSI measurements with the respective calibration curves of the UVA and IRF experiments. The differences between such simulations and the experimental curves are traced back to non-negligible contributions of azulene self-collisions in the UVA and IRF data. Astonishing quantitative agreement is reached when azulene/bath gas mixing ratios of the corresponding UVA/IRF experiments are fully accounted for in the KCSI simulations. The influence of self-collisions is thus quantitatively assessed as an important source of error in addition to the well-known problem of calibration curve uncertainties in UVA and IRF detection as discussed earlier (T. Lenzer, K. Luther, K. Reihs and A.C. Symonds, J. Chem. Phys., 2000, 112, 4090)
Collisional energy transfer probabilities ofhighly excited molecules from KCSI. III. Azulene: P(E',E) and moments of energy transfer for energies up to 40000 cm-1 via self-calibrating experiments
No full textComplete experimental transition probability density functions P(E-',E) have been determined for collisions between highly vibrationally excited azulene and several bath gases over a wide energy range. This was achieved by applying 2-color "kinetically controlled selective ionization (KCSI)" [U. Hold, T. Lenzer, K. Luther, K. Reihs, and A. C. Symonds, J. Chem. Phys. 112, 4076 (2000)]. The results are "self-calibrating," i.e., independent of any empirical calibration curve, as usually needed in traditional energy transfer experiments like time-resolved ultraviolet absorption or infrared fluorescence. The complete data set can be described by our recently introduced monoexponential 3-parameter P(E-',E) form with a parametric exponent Y in the argument, P(E-',E)proportional toexp[-{(E-E-')/(C-0+C-1.E)}(Y)]. For small colliders (helium, argon, xenon, N-2, and CO2) the P(E-',E) show increased amplitudes in the wings compared to a monoexponential form (Y1) than that provided by a monoexponential. Approximate simulations show that the wings of P(E-',E) at amplitudes and (1/2). The energy transfer parameters presented in this study have benchmark character in certainty and accuracy, e.g., with only 2%-5% uncertainty for our data below 25 000 cm(-1). Deviations of previously reported first moment data from ultraviolet absorption and infrared fluorescence measurements can be traced back to either the influence of azulene self-collisions or well-known uncertainties in calibration curves. (C) 2003 American Institute of Physics
Collisional energy transfer of highly vibrationally excited toluene and pyrazine: Transition probabilities and relaxation pathways from KCSI experiments and trajectory calculations.
No full textNew experimental results for the collisional energy transfer of highly vibrationally excited toluene and pyrazine employing the method of "kinetically controlled selective ionization (KCSI)" are presented. By means of a master equation approach we determine complete and detailed collisional transition probabilities P(E',E) for energies up to 50 000 cm(-1). The same monoexponential representation P(E',E) proportional to exp[ - ((E - E')/alpha (1)(E))(Y)] (for E' less than or equal to E) with a parametric exponent Y in the argument and linearly energy dependent alpha (1)(E) = C-0 + C1E successfully used in our earlier investigation [T. Lenzer, K. Luther, K. Reihs and A. C. Symonds, J. Chem. Phys., 2000, 112, 4090] can reproduce the toluene and pyrazine results for the whole range of bath gases studied. The parameters Y, C-0 and C-1 of P(E',E) show a smooth increase with the size of the collider. An approximately linear energy dependence of the first moment of energy transfer [DeltaE] is observed for all bath gases. Literature data from infrared fluorescence (IRF) experiments in general show significantly smaller - [DeltaE] values outside the uncertainty limits of the KCSI results. It is shown that this can mainly be traced back to the critical dependence of the IRF data on small uncertainties in the calibration curve. Some of the trends with respect to the energy transfer efficiencies of different colliders observed in the KCSI experiments are easily rationalized on the basis of accompanying trajectory calculations on the deactivation of highly vibrationally excited pyrazine by n-propane and CO2. The negligible influence of the V-V relaxation channel in the pyrazine + CO2 system observed in earlier IR diode laser studies is confirmed
[Report to Chief J. E. Curry, by an unknown author #1]
No full textReport to Chief J. E. Curry, by an unknown author. The report contains a list of officers who gave depositions to the United States Attorney
[Report to Chief J. E. Curry, by an unknown author #2]
No full textReport to Chief J. E. Curry, by an unknown author. The report contains a list of officers who gave depositions to the United States Attorney
On-line in-situ characterization of CO2 RESS processes for benzoic acid, cholesterol and aspirin
No full textRapid expansions of supercritical solutions (RESS) of benzoic acid, cholesterol and aspirin in supercritical CO2 have been used to investigate the influence of a systematic variation of the pre-expansion temperature and pressure, the distance from the RESS nozzle and the amount of added co-solvent on properties like the average particle diameter D-av and the width of the particle size distribution sigma. The properties of the CO2 expansion have been characterized by a 1-dimensional flow-field model using the Span - Wagner equation of state. Particle detection was performed on-line and in-situ using laser-based three wavelength extinction measurements (3-WEM). For benzoic acid we found a decrease in Dav with increasing pre-expansion pressure, and an increase in Dav with increasing pre-expansion temperature. This is probably due to a lower mass flow rate, which is associated with a lower pre-expansion pressure or higher pre-expansion temperature. This in turn results in a longer residence time in the expansion region and thus a longer particle growth time. Furthermore, a decrease in pre-expansion pressure or an increase in pre-expansion temperature is associated with a decrease in saturation, corresponding to an increase in the critical particle radius and a decrease in the nucleation rate. The size of the benzoic acid particles ranged from about 100 to 500 nm. In addition, we found no obvious correlation between Dav and the distance from the RESS nozzle for benzoic acid and aspirin particles. The particle size was roughly 350 nm and 160 nm for these two solutes, respectively. Obviously, the particle growth processes have already ceased not too far away from the Mach disc. In addition, for cholesterol expansions in CO2 there was no correlation between the amount and type of co-solvent added. Particle sizes of similar to 100 nm were obtained for methanol, ethanol and isopropanol co-solvents. This is most likely due the low solubility of cholesterol in supercritical CO2, compared with molecules such as benzoic acid, which results in a change of D-av which is too small to be detected using 3-WEM
Gas-phase collisional relaxation of the CH<sub>2</sub>I radical after UV photolysis of CH<sub>2</sub>I<sub>2</sub>
No full textTransient UV absorption spectra and kinetics of the CH2I radical in the gas phase have been investigated at 313 K. Following laser photolysis of 1-3 mbar CH2I2 at 308 nm, transient spectra in the wavelength range 330-390 nm were measured at delay times between 60 ns and a few microseconds. The change of the absorption spectra at early times was attributed to vibrational cooling of highly excited CH2I radicals by collisional energy transfer to CH2I2 molecules. From transient absorption decays measured at specific wavelengths, time-dependent concentrations of vibrationally "hot" and "cold" CH2I and CH2I2 were extracted by kinetic modeling. In addition, the transient absorption spectrum of CH2I radicals between 330 and 400 nm was reconstructed from the simulated concentration-time profiles. The evolution of the absorption spectra of CH2I radicals and CH2I2 due to collisional energy transfer was simulated in the framework of a modified Sulzer-Wieland model. Additional master equation simulations for the collisional deactivation of CH2I by CH2I2 yield values in reasonable agreement with earlier direct studies on the collisional relaxation of other systems. In addition, the simulations show that the shape of the vibrational population distribution of the hot CH2I radicals has no influence on the measured UV absorption signals. The implications of our results with respect to spectral assignments in recent ultrafast spectrokinetic studies of the photolysis of CH2I2 in dense fluids are discussed
Murder on the mountain: author talk with Peter J. Wosh
No full textAuthor talk by Peter J. Wosh on May 5th, 2022, on his book, "Murder on the Mountain: crime, passion, and punishment in gilded age New Jersey.
Collisional energy transfer probabilities of highly excited molecules from kinetically controlled selective ionization (KCSI). II. The collisional relaxation of toluene: P(E′,E) and moments of energy transfer for energies up to 50 000 cm-1.
No full textComplete and detailed experimental transition probability density functions P(E',E) have been determined for the first time for collisions between a large, highly vibrationally excited molecule, toluene, and several bath gases. This was achieved by applying the method of kinetically controlled selective ionization (KCSI) (Paper I [J. Chem. Phys. 112, 4076 (2000), preceding article]). An optimum P(E-',E) representation is recommended (monoexponential with a parametric exponent in the argument) which uses only three parameters and features a smooth behavior of all parameters for the entire set of bath gases. In helium, argon, and CO2 the P(E-',E) show relatively increased amplitudes in the wings-large energy gaps \E'-E\-which can also be represented by a biexponential form. The fractional contribution of the second exponent in these biexponentials, which is directly related to the fraction of the so-called "supercollisions," is found to be very small ( and of the square root of . The energy transfer parameters presented in this study form a new benchmark class in certainty and accuracy, e.g., with only 2%-7% uncertainty for our data below 25 000 cm(-1). They should also form a reliable testground for future trajectory calculations and theories describing collisional energy transfer of polyatomic molecules. (C) 2000 American Institute of Physics. [S0021-9606(00)01604-4]
Transient lens spectroscopy of ultrafast internal conversion processes in citranaxanthin.
No full textThe ultrafast internal conversion (IQ dynamics of the apocarotenoid citranaxanthin have been studied for the first time by means of two-color transient lens (TL) pump-probe spectroscopy. After excitation into the high-energy edge of the S-2 band by a pump pulse at 400 nm, the subsequent intramolecular processes were probed at 800 nm. Experiments were performed in a variety of solvents at room temperature. Upper limits for the S-2 lifetime tau(2) on the order of 100-200 fs are estimated. The S-1 lifetime tau(1) varies only slightly between solvents (10-12 ps), and the only clear decrease is observed for methanol (8.5 ps). The findings are consistent with earlier results from transient absorption studies of other apocarotenoids and carotenoid ketones and transient lens experiments Of C-40 carbonyl carotenoids. Possible reasons for the observed weak solvent dependence of tau(1) for citranaxanthin are discussed
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