Velocity distribution 592 molecular reaction

Conventional molecular beam reactive scattering studies have excelled in the determination of the angular and velocity distributions of reaction products, but direct information on the internal state distributions has been sparse. One of the most important of the non-beam methods for learning about the partitioning of reaction energy into the internal degrees of freedom of the products has been infra-red chemiluminescence studies. Unfortunately, this technique has hitherto been limited to hydride compounds, principally hydrogen halides. We present an alternative technique based on electronic fluorescence spectroscopy. 

Highly exothermic chemical reactions can give ions as products. Such reactions are called chemi-ionization reactions. When one or more of the reactants are formed by pulsed photolysis, and the ion product(s) are time-resolved with a mass spectrometer, this comprises a FPTRMS experiment. The ions can be extracted by a biased pair of electrodes, one of which has an aperture through which ions pass to enter a quadrupole mass spectrometer. The time resolution of this experiment is superior to FPTRMS experiments between neutrals, since the ion velocities are set by the electrode bias voltage, and the thermal molecular velocity distribution limitation is no longer a factor. With modest electrode bias voltages, the detection time can be as short as 2 /is. 

Fig. 15. Centre-of-mass contour plots of the HF angular and velocity distribution from the reaction H + F2 performed under crossed-molecular beam conditions at two relative collisional energies. (Reproduced from ref. 510 by permission of the authors and D. Rideal Publishing Company.)...

Miller and Kusch (3 ) determined the molecular composition of KI vapor by measurement of the velocity distribution of the molecules in the beam produced as the vapor effused through a small slit in a source. The analysis was based on an assumption that the velocity distribution within the oven is Maxwellian and that the vapor effuses through the ideal slit of kinetic theory. The velocity distributions of potassium and thallium atomic beams were found to be in excellent agreement with the theoretical distributions so the determination of the molecular composition of KI beams was tried. Using the derived equilibrium constants, we calculate the enthalpy change of the dissociation reaction by the 2nd and 3rd law methods. The results are presented in the following table. 

The standard theories of chemical kinetics are equilibrium theories in which a Maxwell-Boltzmann energy (or momentum or internal coordinate) distribution of reactants is postulated to persist during a reaction. In the collision theory, mainly due to Hinshelwood,7 the number of energetic, reaction producing collisions is calculated under the assumption that the molecular velocity distribution always remains Maxwellian. In the absolute... 

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