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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)
Temperature dependence of collisional energy transfer in highly excited aromatics studied by classical trajectory calculations.
Full text linkThe temperature dependence of the gas-phase collisional relaxation of highly vibrationally excited aromatic molecules has been studied using large scale classical trajectory calculations. The investigations have focused on azulene collisions with different colliders (He, Ar and N-2) as well as pyrazine self-collisions providing the moments of energy transfer (Delta E) and (Delta E-2) in the temperature range 50-1500 K. The interaction well depth epsilon(eff)/k(B) is found to be the key factor controlling the observed T dependence of collisional energy transfer. Systems with a relatively deep interaction well (pyrazine + pyrazine, azulene + Ar, azulene + N-2) show a pronounced negative dependence of - (Delta E) when T 300-400 K) - when the temperature is well above epsilon(eff)/k(B) - all systems behave qualitatively similar, showing only a very weak, slightly negative T dependence, as long as one is still far away from thermal equilibrium
Kinetically controlled selective ionization study on the efficient collisional energy transfer in the deactivation of highly vibrationally excited trans-stilbene
No full textDirect measurements of the gas-phase collisional energy transfer parameters are reported for the deactivation of highly vibrationally excited trans-stilbene molecules, initially prepared with an average energy of about 40 000 cm(-1), in the bath gases argon, CO2, and n-heptane. The method of kinetically controlled selective ionization (KCSI) has been used. Complete experimental collisional transition probability density functions P(E',E) are determined, which are represented by a monoexponential form with a parametric exponent in the argument, P(E',E) proportional to exp[-{(E - E')/(C-0 + C1E)}(Y)] (for downward collisions), well established from earlier KCSI studies. A comparison of the first moments of energy transfer rate constants, k(E,1), or of resulting first moments of energy transfer, (Delta E(E)), for trans-stilbene with those for azulene and toluene clearly shows the considerably more efficient deactivation of trans-stilbene for all bath gases, presumably due to the much greater number of very low-frequency modes of trans-stilbene. However, on a relative scale this gain in deactivation rate of excited trans-stilbene is clearly collider dependent and decreases distinctly with the growing collision efficiency of the larger bath gas molecules
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
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
Multiplex detection of collisional energy transfer using KCSFI
Full text linkA new detection method for obtaining collisional transition probabilities P (E', E) of highly vibrationally excited molecules in the gas phase is presented. The technique employs energy-selective probing of the time-dependent vibrational population distribution by kinetically controlled selective fluorescence (KCSF)". We present experimental results for a test system, the collisional deactivation of toluene by argon, where we use the well-known kinetically controlled selective ionization (KCSI) scheme as a reference for comparison. A newly designed setup is employed that allows simultaneous detection of fluorescence and ionization signals under identical experimental conditions ( kinetically controlled selective fluorescence and ionization = KCSFI"). For the system toluene + argon it is demonstrated that KCSF and KCSI yield identical results. A rate-equation model is presented to understand common features and differences of both approaches. The fluorescence detection scheme shows promise for future investigations on collisional energy transfer. The experimental setup is simpler, because it requires no additional ionization wavelength. This will hopefully give access to the P ( E 0, E) of systems where, e. g., ionization schemes are difficult to implement due to short wavelengths required for the ionization step. A few examples will be outlined briefly
PECT model analysis and predictions of experimental collisional energy transfer probabilities P(E',E) and moments <Delta E> for azulene and biphenylene
Full text linkExperimental collisional energy transfer data from kinetically controlled selective ionization (KCSI) and ultraviolet absorption (UVA) experiments are analyzed in the framework of the partially ergodic collision theory (PECT). Collisions of azulene and biphenylene with different colliders are investigated as case studies. The downward wings of the P(E',E) energy transfer distributions obtained from the PECT model are fitted to the recently introduced "variable-shape"-exponential 3-parameter functional form of P(E,E) obtained from KCSI experiments, P(E',E) &PROP; exp[-{(E - E')/(C-0 + C1E)}(Y)]. The PECT model is able to reproduce the characteristic dependence of the KCSI "shape parameter" Y on the choice of collider, the energy dependent width of the KCSI P(E',E) distributions, described by (α(E) = C-0 + C1E, and the temperature dependence of the UVA data above room temperature. The statistical approach of PECT obviously captures the essence of large molecule energy transfer at chemically significant energies without the need of knowing specific features of the detailed collision dynamics. It therefore shows promise for predicting the shape of P(E,E) in master equation kernels for larger molecules
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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