Thermoluminescence of solutions in squalane after γ-irradiation

Abstract

The isothermal luminescence and thermoluminescence of squalane and its solutions has been studied following irradiation with γ-rays at 77 K. The main solute used was biphenyl which gives excimer fluorescence under these conditions as well as monomer emissions. The luminescence is ascribed to ion recombination. It is only necessary to postulate two kinds of electron trapping, in the solvent or as solute anions, but there are five recombination processes giving glow peaks. If electron-attaching solutes are absent, the dominant process is thermal release of trapped electrons at ∼100 K. In solutions of biphenyl most electrons form anions; these recombine with biphenyl cations when large molecules become mobile at the glass transition (∼180 K). Close ion pairs can also decay by electron tunnelling: this produces the isothermal luminescence at 77 K. At intermediate temperatures, there is a peak ascribed to thermally assisted tunnelling; this process becomes predominant at very high concentrations. Oxygen appears to act as a carrier of electrons between anions and cations, giving an extra peak, with a high proportion of phosphorescence, below the glass-transition. Carbon dioxide and sulphur hexaflouride also have striking effects on the glow-curves. Scavenging at 77 K can be distinguished from scavenging in the melting glass.