Luminescence Kinetics Following Pulse Irradiation. II. DNA

The spectra and decay kinetics of the luminescence of calf-thymus DNA following irradiation by 4.3-MeV electrons are reported over a temperature range from 93° to 400°K. The emission is followed from 2 μsec to 1 sec after the start of a 1.6-μsec electron pulse. All the emission spectra observed can be accounted for by the presence of three components with emission peaks at 350, 410, and 470 nm. Emission at 93°K exhibits only 350- and 470-nm components which can be clearly resolved. Apart from the decay of the 350-nm component at 93°K, which follows a reciprocal time–power law, none of the emission decay curves at any temperature can be fitted to a simple mathematical function. It is shown that much if not all the phosphorescence in the temperature range studied results from the trapping of excitation energy. Above 200°K the depths of the traps responsible are distributed about a mean of 0.36 eV and this conclusion is confirmed by thermoluminescent studies. In addition to the decay above, at 93°K the emission at times less than 50 μsec after irradiation depends on the previous dose history of the sample and a possible explanation is presented.