Photodissociation electronic state excitation

There are significant differences between tliese two types of reactions as far as how they are treated experimentally and theoretically. Photodissociation typically involves excitation to an excited electronic state, whereas bimolecular reactions often occur on the ground-state potential energy surface for a reaction. In addition, the initial conditions are very different. In bimolecular collisions one has no control over the reactant orbital angular momentum (impact parameter), whereas m photodissociation one can start with cold molecules with total angular momentum 0. Nonetheless, many theoretical constructs and experimental methods can be applied to both types of reactions, and from the point of view of this chapter their similarities are more important than their differences. 

The V (OCO) ion has a structured electronic photodissociation spectrum, which allows us to measure its vibrational spectrum using vibrationally mediated photodissociation (VMP). This technique requires that the absorption spectrum (or, in our case, the photodissociation spectrum) of vibrationally excited molecules differ from that of vibrationally unexcited molecules. The photodissociation spectrum of V (OCO) has an extended progression in the V OCO stretch, indicating that the ground and excited electronic states have different equilibrium V "—OCO bond lengths. Thus, the OCO antisymmetric stretch frequency Vj should be different in the two states, and the... 

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. 

Electronic states and photodissociation dynamics of chloromethyl radical have been studied recently.114-117 Because of the chlorine substitution, there are several low-lying valence excited states (such as l2Ai and 22Bi, which mainly involve the orbitals on the CC1 bond) in addition to the 3s Rydberg state (22Ai), and more dissociation channels are available. 

As mentioned above, the H photofragment yield depends on the SRS vibrational excitation probability, the FC overlap between the vibrational wavefunc-tions in the ground and excited electronic states, the electronic transition dipole moment and on the photodissociation channel. Since in the VMP process the combined SRS-I-UV excitation energies are in the -43,900-44,530 cm i range, the CH3NH2 A state is accessed from all the initially prepared vibrational states. It was therefore concluded [34, 81, 82] that the main player determining the H yield... 

Throughout the chapter, we have presented the methodology of VMP and discussed some specific examples, where controlling the photodissociation products ensuing from an electronically excited state is achievable via preexcitation of specific vibrational states in the initial electronic state. This mode selectivity worked so far, with only two exceptions, for molecules not larger than tetratomic. The main reason for this limit is believed to be IVR however, additional work has to be carried out in larger molecules in attempt to find out what are the limits of size and conditions where nonstatistical behavior is still feasible. 

The work of Fu 3 and co-workers (2,5-7,34) has shown that Cr (CO)5, Mn(CO)5 and Fe(C0)4 return to the lowest electronic states of the initially excited multiplicity within several hundred femtoseconds of the photodissociation of the parent carbonyl. This timescale precludes a spin-orbit induced change from the low-spin manifold and anticipates the accessibility of a conical intersection seam to facilitate radiationless population transfer to the ground state. 

From the three decay times, which each measurement provides, the main point of interest how is the type I quantity T. It characterizes the dynamics of the relevant electronic state of the cluster size under investigation. The data, therefore, allow us to determine directly the photodissociation probabilities 1/r 1 of the observed clusters excited at the energies of the photon irradiation. The corresponding results reflect the stability of the clusters, as graphically presented in Fig. 26. For all measured cluster sizes the fragmentation probabilities at E = 2.00 eV are smaller than those for the other photon energies (figs. 26a, b). For E = 2.00 eV and E = 2.94 eV the curves of the dependence of the photodissociation probability on the cluster size have similar shapes. In Fig. 26b both curves show a particular instability for Kg, which... 

Photodissociation experiments have become one of the most valuable tools in chemical physics for the purpose of understanding how excited electronic states couple to the dissociation continuum. These experiments, and the the-... 

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