Excitation, dissipative, and emissive mechanisms in biochemicals.

Triplet states, because of their long lifetimes and reactive properties, are of fundamental concern to radiation biologists. Our interest in the role of these states in radiation damage was stimulated by the observation that the ratios of fluorescence to phosphorescence (F/P) excited by x irradiation of the aromatic amino acids and trypsin are less, by a factor of about 10 to 100, than those observed for ultraviolet (uv) excitation of the lowest-lying excited states (1). This implies that x irradiation greatly enhances the relative population of excited triplet states or that fluorescence or phosphorescence quantum efficiencies are changed. Three mechanisms were proposed to account for this enhanced triplet population: (1) intersystem crossing, from singlet to triplet manifold during the energetic cascade from high-lying excited states to the lowest-energy, emitting states; (2) the interaction and breakup of collective excitations; or (3) direct spin exchange between incident, slow electrons and orbital electrons in the absorbing molecules. Possible methods to investigate these mechanisms are, respectively, (1) irradiation with subionizing vacuum uv (4 to 10 eV); (2) irradiation with high-energy vacuum uv (-25 eV); and (3) irradiation with slow electrons (<100 eV). Preliminary evidence bearing on the first and third possibilities has come recently from studies in our two laboratories and is summarized below. Experimental details are given in companion reports (2, 3). Following the initial absorption of energy, a series of relaxation and emissive events will occur which ultimately leave the absorbing system (which now may be altered chemically) in its ground state. The individual processes may be very fast ('10-15 second for light emission) or very slow (100 seconds for decay of triplet