The calculation of rate coefficients is a discipline of nonlinear science of importance to much of physics, chemistry, engineering, and biology. Fifty years after Kramers' seminal paper on thermally activated barrier crossing, the authors report, extend, and interpret much of our current understanding relating to theories of noise-activated escape, for which many of the notable contributions are originating from the communities both of physics and of physical chemistry. Theoretical as well as numerical approaches are discussed for single- and many-dimensional metastable systems (including fields) in gases and condensed phases. The role of many-dimensional transition-state theory is contrasted with Kramers' reaction-rate theory for moderate-to-strong friction; the authors emphasize the physical situation and the close connection between unimolecular rate theory and Kramers' work for weakly damped systems. The rate theory accounting for memory friction is presented, together with a unifying theoretical approach which covers the whole regime of weak-to-moderate-to-strong friction on the same basis (turnover theory). The peculiarities of noise-activated escape in a variety of physically different metastable potential configurations is elucidated in terms of the mean-first-passage-time technique. Moreover, the role and the complexity of escape in driven systems exhibiting possibly multiple, metastable stationary nonequilibrium states is identified. At lower temperatures, quantum tunneling effects start to dominate the rate mechanism. The early quantum approaches as well as the latest quantum versions of Kramers' theory are discussed, thereby providing a description of dissipative escape events at all temperatures. In addition, an attempt is made to discuss prominent experimental work as it relates to Kramers' reaction-rate theory and to indicate the most important areas for future research in theory and experiment.

Reaction-rate theory: fifty years after Kramers

Author(s): Vogel, Alfred; Venugopalan, Vasan | Abstract: The mechanisms of pulsed laser ablation of biological tissues were studied. The transiently empty space created between the fiber tip and the tissue surface improved the optical transmission to the target and thus increased the ablation efficiency. It was found that the structure and morphology also affect the energy transport among tissue constituents.

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Mechanisms of pulsed laser ablation of biological tissues.

This article, the fourth in the series, presents kinetic and photochemical data sheets evaluated by the IUPAC Subcommittee on Gas Kinetic Data Evaluation for Atmospheric Chemistry. It covers the gas phase and photochemical reactions of organic halogen species, which were last published in 1997, and were updated on the IUPAC website in 2006/07. The article consists of a summary sheet, containing the recommended kinetic parameters for the evaluated reactions, and four appendices containing the data sheets, which provide information upon which the recommendations are made.

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Evaluated kinetic and photochemical data for atmospheric chemistry: Volume IV – gas phase reactions of organic halogen species

The straightforward and scalable synthesis and patterning of graphene-based nanomaterials remains a technological challenge. Here, the authors use a CO2 infrared laser, under ambient conditions, to directly produce and pattern porous graphene films with three-dimensional networks from commercial polymer films.

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Laser-induced porous graphene films from commercial polymers

A comprehensively tested H2/O2 chemical kinetic mechanism based on the work of Mueller et al. 1 and recently published kinetic and thermodynamic information is presented. The revised mechanism is validated against a wide range of experimental conditions, including those found in shock tubes, flow reactors, and laminar premixed flame. Excellent agreement of the model predictions with the experimental observations demonstrates that the mechanism is comprehensive and has good predictive capabilities for different experimental systems, including new results published subsequent to the work of Mueller et al. 1, particularly high-pressure laminar flame speed and shock tube ignition results. The reaction H + OH + M is found to be primarily significant only to laminar flame speed propagation predictions at high pressure. All experimental hydrogen flame speed observations can be adequately fit using any of the several transport coefficient estimates presently available in the literature for the hydrogen/oxygen system simply by adjusting the rate parameters for this reaction within their present uncertainties. © 2004 Wiley Periodicals, Inc. Int J Chem Kinet 36: 566–575, 2004

https://www.princeton.edu/~combust/research/publications/H2_O2%20Mech%20(Li%20et%20al%20Int%20J%20Chem%20Kin%2036%20566%20575,%202004).pdf

An updated comprehensive kinetic model of hydrogen combustion

The master equation of thermal unimolecular reactions in the fall-off range has been solved for a number of representative molecular systems. Weak collision broadening factors F**W**C(k// /k// infinity ) are derived and represented empirically. Weak collision efficiencies beta //c for the low pressure range are calculated for very high temperatures. Combined with earlier representations (part I) of strong collision broadening factors F**S**C(k//0/k// infinity ), compact empirical expressions for the rate coefficient in the full fall-off range are proposed. These expressions are useful for data representation and modeling of complex reaction system which involve isomerization, dissociation and recombination reactions.

Theory of Thermal Unimolecular Reactions in the Fall‐off Range. II. Weak Collision Rate Constants

Quasiclassical trajectory calculations of the energy transfer of highly vibrationally excited benzene and hexafluorobenzene (HFB) molecules colliding with helium, argon and xenon have been performed. Deactivation is found to be more efficient for HFB in accord with experiment. This effect is due to the greater number of low frequency vibrational modes in HFB. A correlation between the energy transfer parameters and the properties of the intramolecular potential is found. For benzene and HFB, average energies transferred per collision in the given energy range increase with energy. Besides weak collisions, more efficient ‘‘supercollisions’’ are also observed for all substrate–bath gas pairs. The histograms for vibrational energy transfer can be fitted by biexponential transition probabilities. Rotational energy transfer reveals similar trends for benzene and HFB. Cooling of rotationally hot ensembles is very efficient for both molecules. During the deactivation, the initially thermal rotational distribution heats up more strongly for argon or xenon as a collider, than for helium, leading to a quasi‐steady‐state in rotational energy after only a few collisions.

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Trajectory simulations of collisional energy transfer in highly excited benzene and hexafluorobenzene

A versatile model for ultraviolet (UV) laser ablation of polymers is presented, which is very successfully applied to the calculation of a variety of different properties of this process, including the influence of plume attenuation dynamics. The polymer is described as a system of chromophores with two possible electronic states. The model is based on the combination of photothermal decomposition and photodissociative bond breaking in the electronically excited state. Laser induced chemical modifications are incorporated via different absorption coefficients for the initial and for the modified polymer after absorption of UV light. Dynamic attenuation of the expanding ablation plume and heat conduction are taken into account. The results of the theoretical calculations are compared with the results of three different series of experiments performed with polyimide (PI) and polymethylmethacrylate at the excimer laser wavelength 248 nm and with PI also at 308 nm: (1) Measurement of the ablation rate as a function of fluence for four different pulse durations between 20 and 250 ns; (2) Measurements of the ablation rate as a function of fluence for five different laser irradiation spot radii between 10 and 150 μm, and (3) Time resolved measurement of the dynamic plume attenuation at the ablating laser wavelength as a function of fluence for four different pulse durations between 20 and 250 ns. The model leads to a prediction of etch rates, ablation thresholds, plume attenuation, and surface temperatures during the ablation process, which is in good agreement with the experimental results. The observed increase of the ablation rate with increasing pulse length and with decreasing laser spot size can be explained by the model as a consequence of laser induced modified absorption in combination with the dynamic shielding of the expanding plume.

Ultraviolet laser ablation of polymers: spot size, pulse duration and plume attenuation effects explained.

The thermal decomposition of H2O2 was studied behind reflected shock waves using absorption spectroscopy at 215 nm with light from a laser source and at 215, 230, and 290 nm with light from a UV lamp. Due to the improved detection sensitivity for H2O2 and HO2, rate constants for the reactions H2O2
(+Ar) → 2HO (+Ar)
(1), HO2 + HO2 → H2O2 + O2
(3) and HO + HO2 → H2O + O2
(4) could be derived with much better precision than possible in earlier work. Rate constant minima for reactions (3) and (4) near 800 and 1100 K, respectively, were reconfirmed. Falloff curves of reaction (1) near to the low pressure
limit were measured in experiments at about 1, 4, and 15 bar. Limiting low pressure rate constants for reaction (1) of about k1,0 = [Ar]1016.36±0.23exp[−21 962(±608) K/T] cm3 mol−1 s−1 were obtained over the temperature range 950–1250 K. The deviations from the low pressure limit at relatively low pressures may provide experimental evidence for non-exponential lifetime distributions of dissociative H2O2 such as recently observed in classical trajectory calculations on the ab initio potential of H2O2.

Shock wave study of the unimolecular dissociation of H2O2 in its falloff range and of its secondary reactions

Complete 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 (<0.1%). For larger colliders the second term disappears co...

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

Klaus Luther

Papers

Theory of Thermal Unimolecular Reactions in the Fall‐off Range. II. Weak Collision Rate Constants

Trajectory simulations of collisional energy transfer in highly excited benzene and hexafluorobenzene

Ultraviolet laser ablation of polymers: spot size, pulse duration and plume attenuation effects explained.

Shock wave study of the unimolecular dissociation of H2O2 in its falloff range and of its secondary reactions

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