Polyatomic molecules electronic continua

There can be a difference between the dissociation of polyatomic molecules and delayed ionization in the nature of the initial excitation. In ZEKE spectroscopy the state that is optically accessed (typically via an intermediate resonantly excited state) is a high Rydberg state, that is a state where most of the available energy is electronic excitation. Such a state is typically directly coupled to the continuum and can promptly ionize, unlike the typical preparation process in a unimolecular dissociation where the state initially accessed does not have much of its energy already along the reaction coordinate. It is quite possible however to observe delayed ionization in molecules that have acquired their energy by other means so that the difference, while certainly important is not one of principle. 

Figure 3.25 Outline of the absorption spectrum of a rigid polyatomic molecule. The bands corresponding to electronic transitions are broad as they include vibrational and rotational transitions and they coalesce to form an absorption continuum...

Photodissociation of small polyatomic molecules is an ideal field for investigating molecular dynamics at a high level of precision. The last decade has seen an explosion of many new experimental methods which permit the study of bond fission on the basis of single quantum states. Experiments with three lasers — one to prepare the parent molecule in a particular vibrational-rotational state in the electronic ground state, one to excite the molecule into the continuum, and finally a third laser to probe the products — are quite usual today. State-specific chemistry finally has become reality. The understanding of such highly resolved measurements demands theoretical descriptions which go far beyond simple models. 

The recent progress of computational quantum chemistry has made it possible to get realistic descriptions of vibrational frequencies for polyatomic molecules in solution. The first attempt in this direction was made by Rivail el al. [1] by exploiting a semiempirical QM molecular model coupled with a continuum description of the medium to compute vibrational frequency shifts for molecular solutes. An extension to ab initio QM methods, including the treatment of electron correlation effects and electrical and mechanical anharmonicities, was then proposed [2 1] in the framework of the Polarizable Continuum Model (PCM). 

From the point of view of chemical reaction dynamics, the most interesting case is that of unbound excited states or excited states coupled to a dissociative continuum that is, photodissociation dynamics. The dissociative electronically excited states of polyatomic molecules can exhibit very complex dynamics, usually involving nonadiabatic processes. The TRPES and TRCIS may be used to study the complex dissociation dynamics of neutral polyatomic molecules, and below we will give two examples of dissociative molecular systems that have been studied by these approaches, NO2 and (NO)2. 

---