Polyatomic molecules overview

In this chapter we shall first outline the basic concepts of the various mechanisms for energy redistribution, followed by a very brief overview of collisional intennoleciilar energy transfer in chemical reaction systems. The main part of this chapter deals with true intramolecular energy transfer in polyatomic molecules, which is a topic of particular current importance. Stress is placed on basic ideas and concepts. It is not the aim of this chapter to review in detail the vast literature on this topic we refer to some of the key reviews and books [U, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, and 32] and the literature cited therein. These cover a variety of aspects of tire topic and fiirther, more detailed references will be given tliroiighoiit this review. We should mention here the energy transfer processes, which are of fiindamental importance but are beyond the scope of this review, such as electronic energy transfer by mechanisms of the Forster type [33, 34] and related processes. 

In this section, we present an overview of the photoabsorption cross section (o ) and the photoionization quantum yields (rh) for normal alkanes, C H2 +2 ( = 1 ), as a function of the incident photon energy in the vacuum ultraviolet range, and of the number of carbon atoms in the alkane molecule, because normal alkanes are typical polyatomic molecules of chemical interest. In Fig. 5, the vertical ionization potentials of the valence electrons, which interact with the vacuum ultraviolet photons, in each of these alkane molecules are indicated to show how the outer- and inner-valence orbitals associated with carbon 2p and 2s orbitals, respectively, locate in energy [7]. 

The terms mode-selective and bond-selective dissociation refer to the control of the dissociation products in VMP. The terms are usually used as synonyms although, strictly speaking, the former should refer to selective preexcitation of a vibrational mode and the latter to the resulting selective bond cleavage. Control of the dissociation products in VMP has been extensively reviewed [28-31] and our discussion will focus on molecules studied (or continued to be smdied) after the latest comprehensive review was published [31], An exception will be a short overview on the VMP of water isotopologues since it was the extensive theoretical and experimental investigations of these molecules, in particular H2O and HOD, that opened a new era of detailed smdies of state-to-state photodissociation out of specific rovibrationally excited states of polyatomic molecules. 

In this chapter we shall first outline the basic concepts of the various mechanisms for energy redistribution, followed by a very brief overview of collisional intermolecular energy transfer in chemical reaction systems. The main part of this chapter deals with true intramolecular energy transfer in polyatomic molecules, which is a topic of particular current importance. Stress is placed on basic ideas and concepts. It is not the aim of this chapter to review in detail the vast literature on this topic we refer to some of the key reviews and books [JT,... 

Very recently, Shustorovich and coworkers have extended the conceptual framework of this initial BOC-MP model to prove that the quadratic exponential potential (QEP), expressed in terms of a unity bond index (UBI) after normalization, provides a general, accurate description of any two-center, quasi-spherical interaction. As a result, the formalism has been renamed the UBI-QEP method [19-22]. This recent work includes a newly developed formalism to calculate the heat of adsorption of polyatomic molecules, such as ethylene and acetylene, without bond energy partitioning [19,22]. The reader is invited to study these latter papers to learn the most recent and accurate applications of this approach, which will be referred to as the BOC-MP/UBI-QEP method in this chapter. In this section, only an overview of the initial BOC-MP method will be provided, and greater detail is provided in these publications of Shustorovich [16-18]. 

A brief synopsis of MBPT in section II is followed by a discussion in section III of the present applications to the ground state 0( P) + H2O nonreactive potential energy surface. Section IV describes the fitting procedure, followed in section V by an overview of the classical dynamics method as applied to collisional excitation of polyatomic molecules. Section VI presents the results of this study. 

In the following sections on the interaction of radiation with gas molecules we begin with an overview of the physical principles of radiative transitions in molecules in Sections 3.1 and 3.2, proceed to discussions of the properties of diatomic and polyatomic molecules in Sections 3.3 and 3.4, and, finally, examine line strengths in Section 3.5 and line shapes in Section 3.6. Interactions of radiation with solid and liquid surfaces, as well as cloud particles, are the subject of Sections 3.7 and 3.8. For further information on molecular spectroscopy we refer the reader to text books, such as Pauling Wilson (1935), Herzberg (1939,1945,1950), Townes Schawlow (1955), or Steinfeld (1974). The book by Murcray Goldman (1981) is... 

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