Yuan Lee

Dynamics of Molecular Dissociation


Abstract

Internal conversions of benzene and its derivatives are known to take place in subpicosecond time scale, when the excitation photon wavelengths are shorter than 2500Å. However, because of the sharing of internal energy among a large number of vibrational degrees of freedom in the hot molecules, the dissociation of a hot molecule is a rather slow process. For example, for a benzene molecule with the internal energy of 146 kcal/mole, which is far exceeding the C-H bond dissociation energy of 110 kcal/mole, the average dissociation lifetime is as long as 10 μsec.

One of the very interesting aspects of benzene and its derivatives is the fact that below the bond dissociation energy, there are many structural isomers and the activation energy for isomerization also lies below the bond dissociation energy. Consequently, isomerization processes are expected to take place before a molecule finds its way to dissociate, and these isomerization processes tend to complicate the dissociation of aromatic hydrocarbons and certainly affect the dynamics of dissociation processes.

In order to understand the complex isomerization processes of vibrationally excited molecules prior to dissociations, we have constructed a new molecular beam apparatus in which identification and velocity distribution of dissociation products from many different channels can be carried out simultaneously. This apparatus employs time-delayed photoionization by a vacuum UV laser. The laser is directed perpendicular to the propagation direction of the molecular beam and cuts across the center of mass of the dissociated molecules to produce a line of ionized products. The displacement of products from the center of mass reveal the information on the recoil velocities of products, and the masses of dissociation products were measured after ions are accelerated to a constant momentum by applying short electric pulses, and using a cylindrical electrostatic energy analyzer.

Using deuterium labeling of methyl group in toluene, it was found that excessive scrambling of H atom on methyl group and H atom on the benzene ring took place prior to dissociation. From the products yields and the extent of isotope scrambling, as well as from the rate of dissociation and isomerization, it was found that the direct dissociation of CH3 is competing with the isomerization of hot toluene into cyclohepta triene. Extensive scrambling of deuterium and hydrogen occurs along the ring once the hot cyclohepta triene is formed.


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