Polyatomic molecules highly-excited vibrational

Abstract. The development of modern spectroscopic techniques and efficient computational methods have allowed a detailed investigation of highly excited vibrational states of small polyatomic molecules. As excitation energy increases, molecular motion becomes chaotic and nonlinear techniques can be applied to their analysis. The corresponding spectra get also complicated, but some interesting low resolution features can be understood simply in terms of classical periodic motions. In this chapter we describe some techniques to systematically construct quantum wave functions localized on specific periodic orbits, and analyze their main characteristics. 

The principal reaction discussed above forms oxygen molecules in high vibrational levels of the ground state. This is chemi-excitation but is not chemiluminescence vibration-rotation transitions of homonuclear molecules are forbidden. For such cases electronic absorption spectroscopy is the required technique. For reactions in which a heteronuclear diatomic (or a polyatomic) molecule is excited these transitions are allowed. They are overtones of the molecular transitions that occur in the near infrared. These excited products emit spontaneously. The reactions are chemiluminescent, their emission spectra may be obtained and analyzed in order to deduce the detailed course of the reaction. 

It is quite natural to carry out the resonance photoionization of molecules via their electronically excited states. But this is not the only possibility. Polyatomic molecules can also be excited via their highly excited vibrational states (Bagratashvili et al. 1983). The former approach has found the widest application (Lubman 1990), thanks to the availability of high-power U V lasers, whereas the latter is not so widespread, as powerful tunable IR lasers are scarce. However, both these methods deserve a brief consideration. 

To achieve highly selective and sensitive detection of molecules, it was suggested that one should use laser resonance excitation of molecular vibrations, followed by photoionization of the vibrationaUy excited molecules, that is, vibrationally mediated photoionization (Ambartzumian and Letokhov 1972). As a matter of fact, it proved very difficult to realize this idea because the shift of the VUV absorption bands of the molecules caused by their vibrational excitation was too small. However, there exists another possibility for polyatomic molecules, namely multiphoton (MP) excitation of high-lying vibrational states by high-power IR laser pulses timed to resonance with the pertinent vibrational transitions (Chapter 11). The highly excited vibrational states can be photoionized by another UV or VUV laser pulse. This molecular-photoionization experiment was performed in accordance with the scheme... 

Hippier H, Troe J and Wendelken H J 1983 Collisional deactivation of vibrationally highly excited polyatomic molecules. II. Direct observations for excited toluene J. Chem. Phys. 78 6709... 

McCoy A B and Siebert E L 1996 Canonical Van VIeck pertubation theory and its applications to studies of highly vibrationally excited states of polyatomic molecules Dynemics of Moiecuies end Chemicei Reections ed R E Wyatt and J Z H Zhang (New York Dekker) p 151... 

In addition to the previously mentioned disadvantages, all of these methods have another drawback in the large molecule photofragment velocity measurements. For example, in the studies of UV photon photodissociation of polyatomic molecules, like alkene and aromatic molecules, molecules excited by the UV photons quickly become highly vibrationally excited in the ground electronic state through fast internal conversion, and dissociation occurs in the ground electronic state. 

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. 

A number of studies of unimolecular reactions induced by single-photon vibrational overtone excitation have appeared this year. The basic motivation behind such experiments has been the possibility of observing mode-selective photochemistry resulting from the apparently highly localized nature of overtones of X-H stretching vibrations (X = C, O, etc.) in polyatomic molecules. A series of studies by Jasinski, Frisoli, and Moore examined the isomeriza-... 

E.E.Nikitin, Vibrational relaxation of highly-excited polyatomic molecules, Doklady Akad.Nauk SSSR, 239, 380 (1978)... 

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