Example Internal vibrational excitation

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. 

Internal vibrations and rotations can combine to bring the molecules into orientations necessary to accomplish photoreactions such as /-hydrogen abstraction to form i-BR, the first step after ketone excitation. In the vast majority of examples, excited states with n, n configurations participate in the process shown in Eq. 3b [255], However, there are some notable exceptions [259]. 

Stilbenes and associated molecules provide very good examples of the formation of intermediate unstable isomers which give a chemical route for internal conversion. Upon irradiation, stilbenes undergo a cis-trans isomerization as the predominant reaction. However, under oxidative conditions phenanthrene is also formed.12 It was shown that the phenanthrene came only from c/s-stilbene (13),61 and that an intermediate unstable isomer, nms-dihydrophenanthrene (14), was the precursor of the phenanthrene.62-64 The dihydrophenarithrene was in its ground state, but vibrationally excited, and was formed by a process calculated to be endothermic by 33 10 kcal/mole-1.02 Oxygen or other oxidants converted it to phenanthrene (15), but in the absence of oxidants it was either collisionally stabilized or reverted to m-stilbene. 

In indirect photofragmentation, on the other hand, a potential barrier or some other dynamical force hinders direct fragmentation of the excited complex and the lifetime amounts to at least several internal vibrational periods. The photodissociation of CH3ONO via the 51 state is a representative example. The middle part of Figure 1.11 shows the corresponding PES. Before CH30N0(5i) breaks apart it first performs several vibrations within the shallow well before a sufficient amount of energy is transferred from the N-0 vibrational bond to the O-N dissociation mode, which is necessary to surpass the small barrier. 

Secondary unimolecular reactions in these systems usually result either from production of hot energized species by chemical reaction or from conventional thermal activation. In a few systems, residual excitation from the original photochemical process may be of importance. An interesting and potentially valuable example, due to Srinivasan, is the production of highly vibrationally excited, ground electronic state diene molecules by internal conversion which follows photoexcitation. 

Since the sum of the quantum yields for all the photochemical and radiative processes for most organic compounds is less than unity, it is obvious that internal conversion is a common phenomenon. The reactions that follow internal conversion are best studied in the vapor phase, wherein vibrationally excited molecules have a chance to survive for periods on the order of nanoseconds. The photochemistry of conjugated dienes and trienes offers the best examples of reactions which can be almost exclusively attributed to the internally converted state. It would be desirable to have more obvious means for the identification of such reactions than the phenomenological tests that were used in many of the studies that have been summarized here. One such method is the theoretical approach proposed by Simon. 

As another example consider the internal vibrational energy of a diatomic solute molecule, for example, CO, in a simple atomic solvent (e.g. Ar). This energy can be monitored by spectroscopic methods, and we can follow processes such as thermal (or optical) excitation and relaxation, energy transfer, and energy migration. The observable of interest may be the time evolution of the average... 

Generally in molecular beam studies, both beams have comparable velocities and intersect one another at 90°, and thus the CM velocity vector points at a wide angle intermediate between the two beams. Measurement of the displacement of the laboratory angular distribution of products from the centre-of-mass vector enables an estimate of the velocity of the products to be derived. Reaction products have been velocity analysed (e.g. see refs. 8 and 231) and the results support the view that the product relative translational energy is usually within ca. 1 kcal mole of the reactant relative translational energy. Most of the alkali metal reactions studied to date are exothermic, thus the products must be internally excited. It is believed [8] that, for most reactions, the internal excitation consists mainly of vibrational excitation however, the partition of the vibrational energy between, for example, KI and CH3 is as yet unknown. There are a few exceptions, e.g. the K + HBr reaction where KBr is rotationally excited rather than vibrationally excited [8], and the... 

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