Trapped Electrons in Rigid Polar solution. A Study of Recombination Luminescence

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Abstract
The afterglow (A) and thermoluminescence (TL) of tryptophan in ethyleneglycol/water‐glass irradiated with 250 nm UV‐light at 77°K have been studied. Both A and TL are attributed to recombination between tryptophan ions and trapped electrons produced by ionization of tryptophan by triplet absorption. The excitation energy of the molecule formed by the recombination is between 3.0 and 4.0 eV. The intensity I of A decays according to I = const t−1.09. The glow curve has two peaks: TL1 at 90°K and TL2 at 135°K. TL2 is associated with a glass transition of the solvent and occurs in the same temperature region as the optical absorption from the trapped electron disappears.The effect of electron scavengers on the thermoluminescence shows that the glow curve may be interpreted in terms of the distance be the trapped electron and the positive ion. A and TL1 is associated with electrons that are trapped fairly close to their mother ions whose recombination is guided by the Coulomb force. The electrons associated with TL2 are trapped at larger distances but probably so large as the mean trapping distance. As judged from scavenging experiments a lower limit of the mean trapping distance is 20 Å. On their way back to the mother ions the electrons do not behave like “thermal” but rather like solvated ones with respect to reactivity with H. The yield of trapped electrons increases and the electrons also seem to be trapped closer to their mother ions when the irradiation temperature increases. This may be caused by increased mobility of the molecular dipoles or OH‐groups and/or by the higher thermal energy of the electrons prior to trapping.