Luminescence in High-Energy-Irradiated Alkanes at Low Temperature

Abstract

High-energy bombardment is well-known to produce luminescence in condensed systems which contain suitable scintillators. Such light emission has been observed for weak scintillators like benzene in sufficiently dilute solutions in aliphatic hydrocarbons and, more recently, even in pure liquid benzene. It is not observed in pure liquid alkanes. However, when suitable alkanesare reduced to sufficiently low temperature (e.g., 77 °K), irradiation with ⁶⁰Co gammas produces a persistent luminescence, the decay curves of which show peculiar behavior characteristics. This report reviews the decay characteristics and spectra of glassy 2-methylpentane and 3-methylpentane as well as of glassy and crystalline i-pentane, reexamines the kinetics and the mechanism of the luminescence process, interprets the applicability of the decay law I_o/I = 1 + αt, demonstrates that an excited state of the system (probably a charge-separated state) and not of an individual molecule is involved, presents evidence that the number of available (A and B type) luminescence sites is limited and depends on the history of the system and suggests that the nature of the emitting state may change with temperature. An important aspect of the theoretical considerations is that they suggest that at sufficiently low temperatures such luminescence can be observed even for hydrocarbons as simple as methane. The most recent work of Janssen, Lang, Hebert, and the authors shows that, although luminescence can be induced in hydrogen and the lower alkanes by irradiation of polycrystalline material at sufficiently low temperature, it appears only when the temperature is raised to certain definite temperatures characteristics of each of the alkanes. Some of these temperatures correspond to know transition points; others may correspond to other solid-state changes.