Rate from molecular beam studies

Increased attention has been focused on vibrational, rotational, and translational nonequilibria in reacting systems as well. To account for these nonequilibrium effects, it is becoming increasingly traditional to express specific reaction-rate constants in terms of sums or integrals of reaction cross-sections over states or energy levels of the reactants involved [3], [11]. This approach helps to relate the microscopic and macroscopic aspects of rate processes and facilitates the use of fundamental experimental information, such as that obtained from molecular-beam studies [57], in calculation of macroscopic rate constants. Proceeding from measurements at the molecular level to obtain the rate constant defined in equation (4) remains a large and ambitious task. 

Kineticists have been excited by the prospect of obtaining detailed information on the dynamics of chemical reactions at molecular level from molecular beam studies. The experimental difficulties of the technique are formidable and the results have been limited mainly to the reactions of alkali metal atoms. Molecular beam studies of transfer reactions are omitted here, where the emphasis is on bulk kinetics and the measurements of rate coefficients and Arrhenius parameters (activation energies and A-factors). 

In light of previous experimental and theoretical work on the F f H2 reaction, it can be seen why an experisient of this complexity is necessary in order to observe dynamic resonances in this reaction. The energetics for this reaction and its isotopic variants are displayed in Figure 1. Chemical laser (11) and infrared chemiluminescence (12) studies have shown that the HF product vibrational distribution is hi ly inverted, with most of the population in v=2 and v°°3. A previous crossed molecular beam study of the F + D2 reaction showed predominantly back-scattered DF product (13). These observations were combined with the temperature dependence of the rate constants from an early kinetics experiment (14) in the derivation of the semiempirical Muckerman 5 (M5) potential energy surface (15) using classical trajectory methods. Although an ab initio surface has been calculated (16), H5 has been the most widely used surface for the F H2 reaction over the last several years. 

The rates of proton reactions have been shown to span at least 15 orders of magnitude. A wealth of kinetic data has produced a good qualitative description of the proton-transfer process in the form of reaction mechanisms. For reactions in solution there is a dearth of experimental data on the intimate details of proton-transfer reactions. Detailed theories of proton-transfer reactions are still developing. However, these are often based on information obtained from gas-phase reactions (i.e., molecular beam studies), which are free of solvent participation. The transport of hydrogen ions through water and ice have been very important in understanding the structure and properties of these materials. " The structure of the solvated proton in water as well as in other solvents has been one of keen experimental and theoretical interest."... 

The effect of oxidizing atmospheres on the reduction of NO over rhodium surfaces has been investigated by kinetic and IR characterization studies with NO + CO + 02 mixtures on Rh(lll) [63], Similar kinetics was observed in the absence of oxygen in the gas phase, and the same adsorbed species were detected on the surface as well. This result contrasts with that from the molecular beam work [44], where 02 inhibits the reaction, perhaps because of the different relative adsorption probabilities of the three gas-phase species in the two types of experiments. On the other hand, it was also determined that the consumption of 02 is rate limited by the NO + CO adsorption-desorption... 

The two-route mechanism (1) was qualitatively substantiated by Winter-bottom [45], A system of steps corresponding to this mechanism was first given by Kuchaev and Nikitushina [46] who also studied a steady-state kinetic model. Rate constants for mechanism (1) were reported by Cassuto et al. [48, 49, 65,107,108]. All except k3 were determined using the molecular beam method. The value for k3 was obtained from the solution of the inverse problem. It is these constants that will be applied by us here. 

Surface reaction rate data were determined in independent studies in which the diffusion constraint was removed by molecular beam techniques. Predicted values for the overall reaction rate, computed by coupling this data with diffusion rates from boundary layer theory, are in excellent agreement with experimental values for ribbons and wires. 

A few studies on the photophysics of rotationally resolved states of formaldehyde have been recently published. A. C. Luntz [J. Chem. Phys., j>9, 3436 (1978)] reported the lifetimes of the K -sublevels of the 4 SVL of I CO in a molecular beam. K. Shibuya and E. K. C. Lee [J. Chem. Phys., 69, 5558 (1978)] reported the fluorescence quantum yields from various rotational levels (J1, K ) of the 41 SVL of H2CO. J. C. Weisshaar and C. 3. Moore [J. Chem. Phys., 7J), 5135 (1979)] reported the collisionless decay rates of single rotational levels of the 4 ... 

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