Translational energy effects

The absence of ion translational energy effects on the C8 distribution is also indicated by the lack of an applied field effect. We conclude that vibrational de-excitation is occurring and that argon is much more effective for vibrational de-excitaton than is neon. Such a large difference in vibrational deactivation efficiency between neon and argon is somewhat surprising. However, deactivation of ions may be more sensitive to polarizability effects than is deactivation of neutral molecules. 

A bimoleciilar reaction can be regarded as a reactive collision with a reaction cross section a that depends on the relative translational energy of the reactant molecules A and B (masses and m ). The specific rate constant k(E ) can thus fonnally be written in tenns of an effective reaction cross section o, multiplied by the relative centre of mass velocity... 

In principle, the reaction cross section not only depends on the relative translational energy, but also on individual reactant and product quantum states. Its sole dependence on E in the simplified effective expression (equation (A3.4.82)) already implies unspecified averages over reactant states and sums over product states. For practical purposes it is therefore appropriate to consider simplified models for tire energy dependence of the effective reaction cross section. They often fonn the basis for the interpretation of the temperature dependence of thennal cross sections. Figure A3.4.5 illustrates several cross section models. 

U ] Rettner C T and Stein H 1987 Effect of the translational energy on the chemisorption of N2 on Fe(111) activated dissociation via a precursor state Phys. Rev. Lett. 59 2768... 

Effect of Translational Energy on Ion-Molecule Reaction Rates... 

There are several methods for studying the effects of translational energy on ion-molecule reactions. The first class of experiments uses... 

Gustafsson and Lindholm (19) have shown the effects of translational energy on charge transfer reactions with H2, N2, and CO. They observe that endothermic reaction cross-sections increase with increasing kinetic... 

We also observe other effects of translational energy in methane which are similar to those found in hydrogen. There is an abstraction reaction for parent ions which also produces a hydronated methane. In studying the reaction... 

Hydrocarbon Reactions. Some interesting features of reactions involving olefins and saturated hydrocarbons are shown by investigating the effects of translational energy. The reactions ... 

A significant recent experimental advance is the introduction of tandem mass spectrometers for studying ion-molecule reactions. Examining various isotope effects as a function of translational energy can provide detailed information about reaction mechanisms. Tandem experiments can also observe many of the possible reaction channels for a given collision complex. Such information provides valuable clues to the chemical and physical nature of the intermediates in ion-neutral interactions. 

In order to see the effect of the rotational excitation of the parent H2O molecules on the OH vibrational state distribution, the experimental TOF spectrum of the H atom from photodissociation of a room temperature vapor H2O sample has also been measured with longer flight distance y 78 cm). By integrating each individual peak in the translational energy spectrum, the OH product vibrational distribution from H2O photodissociation at room temperature can be obtained. 

Fig. 14 are the simulated distributions including the different parent rotational levels. An interesting observation from these distributions is that the shape of the multiplet peak corresponding to each 011 (/I) rotational level for the perpendicular polarization is not necessarily the same as that for the parallel polarization see for example the peak labelled v = 0, N = 22. From the simulations, relative populations are determined for the OH (A) product in the low translational energy region from H2O in different rotational levels for both polarizations. The anisotropy parameters for the OH product from different parent rotational levels are determined. Experimental results indicate that the ft parameters for the 011 (/I) product from the three parent H2O levels Ooo, loi, I11, are quite different from each other. Most notably, for the 011 (/I, v 0, N = 22) product the ft parameter from the foi H2O level is positive while the ft parameters from the Ooo and In levels are negative, indicating that the parent molecule rotation has a remarkable effect on the product anisotropy distributions of the OH(A) product. The state-to-state cross-sections have also been determined, which also are different for dissociation from different rotational levels of H2O. 

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