Ionization in the track of a high-energy electron in hydrocarbon liquids; study of positive and negative ion scavenging using nanosecond pulse radiolysis

Abstract

The lifetime distribution of the charged particles in the track of a high-energy electron in cyclohexane and isooctane has been considered. Results on the decay of biphenyl ions in solutions of biphenyl obtained by means of pulse radiolysis with nanosecond time resolution have been compared with lifetime distributions calculated from the concentration dependence of the steady state scavenging, using recently determined absolute rate constants for reaction of excess electrons and mobilities of the charged species. The discrepancy is relatively small for cyclohexane but large for isooctane. It is concluded that the scavengers considerably affect the initial spatial distribution of the charged species in the track. The kinetics of the reaction of the positive species in cyclohexane and isooctane with biphenyl, TMPD, and pyrene has been considered. It has been concluded that the mobile positive species in cyclohexane reacts with pyrene with a rate constant of 5×1011 mol−1⋅s−1, and has a lifetime of 50 ns. In isooctane no evidence for the existence of a mobile positive species has been obtained.

Radiation Chemistry of Organic Liquids: Saturated Hydrocarbons

“…This suggested a short life time for the solvent holes. The kinetic data on delayed fluorescence [49] and transient absorbance [6,20,25] in radiolysis of cyclohexane supported this conclusion. It was concluded that the lifetime of the solvent hole is only 20-30 ns [6,25], or 10 times shorter than the lifetime of the high-mobility cations observed in the conductivity experiments.…”

“…2.1 High mobility cations. Ionization of several cycloalkane liquids -cyclohexane, methylcyclohexane, trans-decalin and cis-decalin -produces cations whose drift mobilities are 5to-25 times greater than the mobilities of normally-diffusing molecular ions and (in some cases) thermalized electrons in these liquids [4][5][6][7][8]. These high-mobility cations are shown to be cycloalkane solvent holes with unusually long natural lifetimes (0.2 µs to 5 µs).…”

“…From conductivity studies, it is known that the cycloalkane holes rapidly react with various solutes, typically by electron or proton transfer [7][8][9][10][11][12][13][14][15][16][17][18][19]. These scavenging reactions establish the identity of the high-mobility cations as the solvent holes: Rapid generation of aromatic radical cations (A^+) in reactions of the holes with aromatic solutes (A) was observed using pulse radiolysis -transient absorption spectroscopy [4,5,6,20,[23][24][25] and, more recently, using pulse-probe laser-induced dc conductivity [26]. Rapid decay of the conductivity and transient absorbance signals from the cycloalkane holes was also observed .…”

“…It is generally believed that these excited species relax to the "ground" state on a picosecond time 6. scale or even faster [37,38,53].…”

“…Class (i) reactions were observed in all four cycloalkanes that exhibit long-lived highmobility holes [4][5][6][7][8]10,13,14,17]. These reactions were also observed in squalane [24,31] and cyclooctane [26].…”