Laser high-concentration

The relative ease with which lasers can produce high concentrations of excited states can be important in initiating multi-molecular photochemistry. It is trivial to produce 0.1 M or greater photon "concentration" in a 1 y volume over a 10 ns period of time. Subsequent multimolecular reactions of excited states or labile photofragments are limited principally by the unimolecular lifetimes involved. 

Triplet-triplet annihilation (TTA), i.e., deactivation of triplet molecules as a result of their interaction, is one of the main pathways of triplet state decay in solutions, crystals and the gas-phase. TTA may become the determining pathway of triplet state deactivation under conditions of high concentrations of triplet-state molecules resulting in particular from powerful laser photoexcitation. 

The stratosphere is often referred to as the ozone layer, because of the relatively high concentrations produced by photochemical reactions in this region of the atmosphere. Ozone, derived from the Greek word meaning to smell, was first discovered by Schonbein in 1839. It has a rather pungent smell, which is sometimes noticeable around copy machines and laser printers that use discharge processes. 

The mechanism of the cycloaddition appears to be concerted for various reagents however, for several cases, radical cation cycloaddition-cycloreversions have a stepwise component. For example, CIDNP effects observed during the PET induced dimerization of spiro[2.4]heptadiene (97) identify a dimer radical cation with spin density only on two carbons of the dienophile fragment this intermediate must be a doubly linked radical cation ( 99 + 282,283 pulsed laser experiment at high concentrations of 97 supports a second dimer radical cation at high... 

On the other hand, the formation of a dimeric product in the laser photolysis of 30 in the absence of quenchers can be qualitatively interpreted by taking into account the spin distribution in DPC. Thus, the reaction of DPC with DPC according to Scheme 9.34 will lead to the observed products. Such reactions would be favored in the presence of an intense excimer laser, which creates a high concentration of DPC. " °... 

The study of the short-lived excited states is limited by the low concentrations in which they are created on excitation with normal light sources. The use of high intensity sources such as flash lamps with suitable flashing rates (Section 10.3) and laser sources (Section 10.2) have helped in the study of the triplet states of molecules. The triplet-triplet absorption, triplet lifetimes, intersystem crossing rates from triplet to ground singlet State, etc. can be effectively measured because of high concentrations of the... 

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