Dissociation into atoms and radicals

The distinction between a truly continuous absorption spectrum and a banded absorption spectrum for diatomic molecules may be made by instruments of relatively low resolving power. Even though individual rotational lines are not resolved, a discrete spectrum will have sharp band heads and the appearance will in no way resemble the appearance of a continuum. 

As we have seen in discussing the behavior of diatomic molecules, there is often great difficulty in identifying a predissociation spectrum since the appearance may at one extreme be essentially that of a truly discrete spectrum and at the other that of a true continuum. The facts of photochemistry may be of great use to the spectroscopist in distinguishing between predissociation and truly discrete spectra. 

For many polyatomic molecules the problem is far more difficult. A few of the simple polyatomic molecules such as formaldehyde, nitrogen dioxide, carbon dioxide, and ammonia have spectra which can be adequately resolved, and treated in rigorous fashion by the theoreticians. For most of the others the treatment is much less satisfactory. 

A linear polyatomic molecule may be treated by the same relatively simple rules as are used for diatomic molecules provided it is not necessary to treat those vibrations which tend to make the molecule nonlinear. Since the symmetry is changed when such vibrations are excited in the upper state, transitions to such levels will, in general, be weak. 

The two linear molecules of most concern to the photochemist are carbon dioxide37 47 and nitrous oxide37. Carbon dioxide absorbs only in the relatively far ultraviolet. The primary dissociation is almost certainly 

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During this study, we have found that laser intensity is one of the important factors that control laser surface chemistry. At a small laser intensity, molecules adsorbed on solid surfaces dissociate into atoms and radicals. Some of these atoms or radicals react with atoms of the solid substrates. At a large laser intensity, atoms are photoablated from the solid surfaces to react with the molecules adsorbed or in the gas phase. Hence, we describe in this paragraph a) the dynamical study of UV laser photodissociation of halogen or metal-containing molecules on solid surfaces, b) reactions of atoms generated in the photodissociation of an adsorbate with solid surfaces, and c) reactions of molecules in the gas phase with the photoelectrons or metal atoms generated on intense laser irradiation of solid surfaces. 

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