On the mechanism of solute singlet state formation in electron irradiated liquid alkanes

The naphthalene fluorescence induced by a 2 ns electron pulse was used to monitor the solute singlet state formation in liquid cyclohexane, decalin, n-heptane and n-decane. At solute concentrations smaller than 10-3 M a fast formation of the solute singlet was observed. Additionally, at higher naphthalene concentrations a slowly rising emission component was obtained. The fast emission seems to be mainly caused by radiative excitation transfer and the slower one is formed by different ion-recombination reactions. To explain the observed slow solute singlet formation by ion recombination two models were used: a phenomenological one and a model based on the solution of the rate equations using time-dependent rate constants. The latter model gives a reasonable description of the experimental results at room temperature as well as at low temperatures. Particulary the influence of the hole life-time which varies for the alkanes was studied. Energy transfer from the alkane singulet to the solute was found to be of minor importance under our conditions at room temperature. The effect of temperature on the kinetic behaviour of the observed solute emission was investigated in methylcyclohexane (MCH) and 3-methylheptane (MH). Decreasing the temperature of the irradiated system continuously from 295 K to about 120 K (solid) leads to a slower rise time of the slow part of solute fluorescence. In frozen solutions only a fast formation of solute emission was obtained. (author)