On the Mechanisms of X‐Ray‐Induced Excitation of Aromatic Molecules in Polar Solution

Abstract

A8STRACTStudies of the X-ray-and UV-induced luminescence of tryptophan and tyrosine in solution between 77°K and 300 0K are reviewed. The fluorescence and phosphorescence spectra observed during X-irradiation are identical to those induced by UV light. bur the phosphorescence to fluorescence ratios obtained with X-rays are much larger than those found wi th UV light. The experiments seem to show that charge recombination is the dominating mode of the X-ray-induced excitation at low temperatures, giving rise to roughly 8,5010 of the X-ray-induced luminescence at 77 0K. The fraction of the X-ray-induced excitation which results from charge recombination decreases with increasing temperature and becomes negligible above 220 oK. This causes the yield of X-ray-induced excitation of the radiative levels of tryptophan to decrease from G = ,5 excited molecules i 100 eV absorbed in solute. at 77"K, to G = 0.4 at room temperature. The mechanisms for quenching of charge recombination are discussed.

Ionization Processes in Polar Solution of Tryptophan at 77° K

“…Hence, according to this treatment, the probability that a photo-ejected electron will undergo spontaneous recombination before trapping is PH '" 0.4. The value of I'R for X-rays can be estimated in two ways: According to a work of Steen [18] the probability of spontaneous recombination after X-ray induced ionization decreases with a factor of 1.2 when the irradiation temperature increases from 77 to 123°K. In the same temperature region the probability of trapping PT increases by a factor of 1.25.…”

“…Finally the value of PH may be estimated from the G-values. The G-value of spontaneous recombination at 77°K is~= 4.3 [18]. Assume that the yield of trapped electrons arising from tryptophan is the same as that of trapped electrons in pure ED/H 20 [10], 1.e., G e = 2.0.…”