Statistical model of energy transfer in molecular collisions: De-energization of highly excited toluene

Building transition probabilities for any condition using reduced cumulative energy transfer functions in H2O–H2O collisions

The energy transfer process between highly vibrationally excited H(2)O in thermal equilibrium with a gas bath of H(2)O at different internal energies and temperatures has been studied by classical trajectory calculations. The results were analyzed using a cumulative probability distribution Q(DeltaE) of the amount of energy transferred, obtained by direct count of the number of trajectories that transfer an amount of energy equal to or greater than a certain value DeltaE. Scaling Q(DeltaE) in terms of the mean down and up energies transferred for each group of trajectories results in a unique distribution. This fact and the use of detailed balance constrains were used to propose a methodology that make it possible to build the whole P(E('),E) for any condition by knowing DeltaE and a series of parameters that depend only on the system under study.

Relaxation dynamics of highly vibrationally excited picoline isomers (E(vib) = 38 300 cm(-1)) with CO2: the role of state density in impulsive collisions

Strong collisions of highly vibrationally excited picoline isomers and CO2 (00(0)0) were investigated using high resolution transient IR absorption probing to investigate the role of donor state density. Vibrationally excited 3-picoline and 4-picoline (3-methylpyridine and 4 methylpyridine) with E(vib) = 38300 cm(-1) were prepared by 266 nm excitation followed by rapid internal conversion. Transient IR probe measurements of the nascent rotational and translational energy gain in CO2 (00(0)0) show that large DeltaE collisions for 3- and 4-picoline are similar to those for excited 2-picoline. The probability distributions for the large DeltaE energy transfer of the three isomers have similar dependence on DeltaE. The results are compared with other earlier results demonstrating that the shape of the large DeltaE probability distribution correlates with the DeltaE dependence of the donor vibrational state density. The results are discussed in terms of the GRETCHEN model for collisional relaxation.

Statistical Theory of Collisional Energy Transfer in Molecular Collisions. trans-Stilbene Deactivation by Argon, Carbon Dioxide, and n-Heptane

Recent advances in experimental techniques have made it possible to measure the full conditional probability density P(E, E') of the energy transfer between two colliding molecules in the gas phase, one of which is highly energized and the other in thermal equilibrium at a given temperature. Data have now become available for trans-stilbene deactivation by the three bath gas molecules Ar, CO2, and n-heptane (C7H16). The initial energies of trans-stilbene are set to 10 000, 20 000, 30 000, and 40 000 cm (-1). The results show that exceptionally large amounts of energy are transferred in each collision. By application of our partially ergodic collision theory (PECT), we find that the energy transfer efficiency betaE ranges from a rather normal value of 0.15 for n-heptane at the highest excitation energy to 0.93-nearly in the ergodic collision limit-for the argon bath gas at high excitation energy. Generally, the PECT produces a good fit of the data except for the nearly elastic peak in the case of n-heptane, where PECT produces a rounded and downshifted peak in contrast to a sharply defined elastic maximum of the monoexponential functional fit produced from the original experimental data obtained by kinetically controlled selective ionization in the work of the group of Luther in Göttingen. This problem is analyzed and found to be related partly to the lack of treatment of glancing collisions in the theory with a remaining uncertainty due to the weak dependence of energy transfer efficiency on nearly elastic collisions. A summary of the present state of understanding shows that collisional activation and deactivation of reactant molecules is more efficient and more statistical than has been previously realized.

Kinetically Controlled Selective Ionization Study on the Efficient Collisional Energy Transfer in the Deactivation of Highly Vibrationally Excited trans-Stilbene

Direct 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')/(C0 + C1E)}Y] (for downward collisions), well established from earlier KCSI studies. A comparison of the first moments of energy transfer rate constants, kE,1, or of resulting first moments of energy transfer, <DeltaE(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.

Microcanonical unimolecular rate theory at surfaces. III. Thermal dissociative chemisorption of methane on Pt(111) and detailed balance

A local hot spot model of gas-surface reactivity is used to investigate the state-resolved dynamics of methane dissociative chemisorption on Pt(111) under thermal equilibrium conditions. Three Pt surface oscillators, and the molecular vibrations, rotations, and the translational energy directed along the surface normal are treated as active degrees of freedom in the 16-dimensional microcanonical kinetics. Several energy transfer models for coupling a local hot spot to the surrounding substrate are developed and evaluated within the context of a master equation kinetics approach. Bounds on the thermal dissociative sticking coefficient based on limiting energy transfer models are derived. The three-parameter physisorbed complex microcanonical unimolecular rate theory (PC-MURT) is shown to closely approximate the thermal sticking under any realistic energy transfer model. Assuming an apparent threshold energy for CH(4) dissociative chemisorption of E(0)=0.61 eV on clean Pt(111), the PC-MURT is used to predict angle-resolved yield, translational, vibrational, and rotational distributions for the reactive methane flux at thermal equilibrium at 500 K. By detailed balance, these same distributions should be observed for the methane product from methyl radical hydrogenation at 500 K in the zero coverage limit if the methyl radicals are not subject to side reactions. Given that methyl radical hydrogenation can only be experimentally observed when the CH(3) radicals are kinetically stabilized against decomposition by coadsorbed H, the PC-MURT was used to evaluate E(0) in the high coverage limit. A high coverage value of E(0)=2.3 eV adequately reproduced the experimentally observed methane angular and translational energy distributions from thermal hydrogenation of methyl radicals. Although rigorous application of detailed balance arguments to this reactive system cannot be made because thermal decomposition of the methyl radicals competes with hydrogenation, approximate applicability of detailed balance would argue for a strong coverage dependence of E(0) with H coverage--a dependence not seen for methyl radical hydrogenation on Ru(0001), but not yet experimentally explored on Pt(111).

PECT Model Analysis and Predictions of Experimental Collisional Energy Transfer Probabilities P(E‘,E) and Moments 〈ΔE〉 for Azulene and Biphenylene

Experimental 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) proportional, variant exp[-{(E - E')/(C(0) + C(1)E)}(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 alpha(E) = C(0) + C(1)E, 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.

Crossed beams and theoretical studies of the dynamics of hyperthermal collisions between Ar and ethane

Crossed molecular beams experiments and classical trajectory calculations have been used to study the dynamics of Ar+ethane collisions at hyperthermal collision energies. Experimental time-of-flight and angular distributions of ethane molecules that scatter into the backward hemisphere (with respect to their original direction in the center-of-mass frame) have been collected. Translational energy distributions, derived from the time-of-flight distributions, reveal that a substantial fraction of the collisions transfer abnormally large amounts of energy to internal excitation of ethane. The flux of the scattered ethane molecules increased only slightly from directly backward scattering to sideways scattering. Theoretical calculations show angular and translational energy distributions which are in reasonable agreement with the experimental results. These calculations have been used to examine the microscopic mechanism for large energy transfer collisions ("supercollisions"). Collinear ("head-on") or perpendicular ("side-on") approaches of Ar to the C-C axis of ethane do not promote energy transfer as much as bent approaches, and collisions in which the H atom is "sandwiched" in a bent Ar...H-C configuration lead to the largest energy transfer. The sensitivity of collisional energy transfer to the intramolecular potential energy of ethane has also been examined.