Internal conversion photodissociation

The easiest method for creating many vibrational excitations is to use convenient pulsed visible or near-UV lasers to pump electronic transitions of molecules which undergo fast nonradiative processes such as internal conversion (e.g. porjDhyrin [64, 65] or near-IR dyes [66, 62, 68 and 62]), photoisomerization (e.g. stilbene [12] or photodissociation (e.g. Hgl2 [8]). Creating a specific vibrational excitation D in a controlled way requires more finesse. The easiest method is to use visible or near-UV pulses to resonantly pump a vibronic transition (e.g. 

Acetaldehyde is methyl-substituted formaldehyde, and has a number of similarities with its smaller cousin. In particular, when photodissociated at 308 nm, internal conversion to is rapid, and acetaldehyde can decompose via analogous radical and molecular channels... 

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. 

The photodissociation of aromatic molecules does not always take place at the weakest bond. It has been reported that in a chlorobenzene, substituted with an aliphatic chain which holds a far-away Br atom, dissociation occurs at the aromatic C-Cl bond rather than at the much weaker aliphatic C-Br bond (Figure 4.30). This is not easily understood on the basis of a simple picture of the crossing to a dissociative state, and it is probable that the reaction takes place in the tt-tt Si excited state which is localized on the aromatic system. There are indeed cases in which the dissociation is so fast (< 10-12 s) that it competes efficiently with internal conversion. 1-Chloromethyl-Np provides a clear example of this behaviour, its fluorescence quantum yield being much smaller when excitation populates S2 than when it reaches Figure 4.31 shows a comparison of the fluorescence excitation spectrum and the absorption spectrum of this compound. This is one of the few well-documented examples of an upper excited state reaction of an organic molecule which has a normal pattern of energy levels (e.g. unlike azulene or thioketones). This unusual behaviour is related of course to the extremely fast dissociation, within a single vibration very probably. We must now... 

Fluorescence may be quenched (diminished in intensity or eliminated) due to the deactivation of the lowest excited singlet state of the analyte by interaction with other species in solution. The mechanisms of quenching appear to entail internal conversion, intersystem crossing, electron transfer, and photodissociation as modes of deactivation of the excited analyte-quencher complexes. 

NCCN Photolysis. Cyanogen (NCCN) photodissociation has also been considered a prototypical statistical dissociation reaction. After absorption to the 11,, 1 Au manifolds, the dissociation is thought to proceed via internal conversion to the ground-state surface. Previous studies have measured the rotational state distribution of the CN fragments from photolysis of cyanogen at 193.3 nm [75-77], The threshold photon energy for CN formation was found to be 47,000 200 cm -1 [78]. 

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