High-energy excimer

Fig. 12. Trends of relative intensity (I) of different components of fluorescence spectra of copolymers of N-vinylcarbazole (NVQ with (—)-menthyl vinyl ether vs. NVC content — — monomer —H— high energy excimer —A—low energy excimer...

Phillips and Schug (24) have suggested that the 390 nm emission, observed when PET is excited with high energy electrons, might be from a triplet state or an excimer. Since the triplet states of both PET and DMT are lower in energy (MSO nm), it is unlikely that the emission is from a triplet state. In addition, excimer formation and emission should not effect the absorption-excitation processes therefore, it is unlikely that the 390 nm emission is from an excimer. 

Triplet photoaddition of simple non-cyclic monoolefins is unknown. The sensitized dimerization of ethyl vinyl ether gives exclusively head-to-head adducts, Eq. 21, and probably should not be classed as an example of simple acyclic olefin. Usually the triplets have high energies and are severly twisted. 55> Some cyclic rigid molecules, Eq. 20, that do dimerize 63> do not incorporate substituents that allow regioselectivity to be determined. Butadiene gives principally head-to-head dimerization, Eq. 19, concordant with the PMO prediction, and so does indene, Eq. 22. The anti dimer that is formed would not be expected from a singlet excimer reaction. 

Using PMDR methods, the broad excimer emission formed at defect sites in the lattice of hexachlorobenzene crystal at 1.6°K (by doping it with high-energy triplet molecules at 1.6°K) is shown (38) to have zf transitions. This provides strong evidence that it is a triplet excimer. For molecules with atoms having nuclear spin, transitions between hyperfine levels of different zf levels are also observed. 

Polyvinylcarbazole exhibits excimer emission from the carbazole unit. By use of picosecond pulse radiolysis, Tagawa et al. (1979) showed that excitation produces two excimers, the normal sandwich excimer which fluoresces at 420 nm and another which fluoresces at 375 nm. The latter high energy species decays to the lower energy species very quickly and therefore good time resolution is necessary in order to see both. 

The main reactions taking place when chloromethylated polystyrene (CMS) and chloromethylated poly(diphenylsiloxane) (SNR) are irradiated with high energy electrons or deep UV (KrF excimer laser, 248 nm) radiation have been studied. The results are discussed in terms of short-lived reactive species generated using pulse radiolysis and laser (248 nm) photolysis techniques. 

A pulsed dye laser pumped with flash lamps is used in most cases, as here the selection of different dyes, frequency doubling and Raman shifting, which is possible as a result of the high energies available, allows the whole spectral range down to 200 nm to be covered. However, diode lasers, particularly for multistep excitation, and dye lasers pumped with an excimer or a nitrogen laser can also be applied. 

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