Sources of excited cyclohexane in the radiolysis of cyclohexane

Abstract

4099of TMD is electronic quenching by the CH3F buffer gas. The data in Figure 6 were obtained by using 1 1.5 Torr of CH3F. Thus, if quenching is the mechanism for depletion of 3A* population, the quenching rate constant is approximately 5 X cm3/ (molecule s). This value can be compared to previously measured acetone triplet quenching rates in solution and in the gas phase. Rate constants in the range 10-16-10-12 cm3/(molecule s) have been measured for a variety of quenchers in s o l~t i o n .~~-~ In the gas phase, O2 has been shown to quench 3A* with a rate constant of 7.5 X cm3/(molecule s).~~ Further tests of the quenching postulate under our experimental conditions are difficult because variations in the CH3F pressure cause variations in the temperature of the reacting system. We have been unable to perform absorption studies using SF6 as the heating gas due to the technical problem of low reflectivity of quartz at 10.6 pm. Further studies (45) Costela, A,; Crespo, M. T.; Figuera, J. M. J . Photochem. 1986, 34, 165.of electronic quenching of directly excited gas-phase acetone would prove very useful. ConclusionThe high-temperature decomposition of gas-phase TMD has been studied by using LPHP to obtain quantitative values for the chemiexcitation yields. The observed emission is due entirely to 'A*, whose yield is measured to be as = 0.017 f 0.008. This value is somewhat higher than that observed at room temperature in solution. A lower limit to the triplet yield was found to be aT 2 0.10 f 0.03. This yield is comparable to that found previously in solution. The main difference observed in the present study is the very short lifetime of 3A* under our experimental conditions. This is postulated to be due to electronic quenching by CH3F with an estimated rate constant of 5 X Acknowledgment.Streak camera measurements of the relative yield of the excited state of cyclohexane (CH*) as a function of electron scavenger concentration (perfluoro-n-hexane, COz, and N20) strongly indicate that less than 10% of the CH* is formed by direct excitation. The effects of the scavenging of geminate electrons on the yield of CH*, in terms of the formulationf, = (cY[S])~/(I + (cY[S]~),wheref, is the fraction of geminate electrons scavenged, [SI is the scavenger concentration, and CY and n are constants (parameters), can be described as follows: for perfluoro-n-hexane, n = 1 and a = 35 M-]; for C 0 2 , n = 0.6 and a = 50 M-'; for N20, n = 0.6 and a = 40 M-'. The effects of static quenching of CH* by the electron scavenger are taken into account in the analysis and are estimated from the values measured for the quenching rate constants. We obtain quenching rate constants, corrected for the effects of time-dependent rates, 2.4 X 1O'O M-I s-l for N 2 0 and C 0 2 (3Z20% estimated uncertainty) and 0.71 X 10" M-' s-I for perfluoro-n-hexane ( 3~1 5 % estimated uncertainty). IntroductionWhen liquid hydrocarbons are subjected to ionizing radiation, excited states can be formed instantly by direct excitation and almost instantly (in ca. 100 ps...

High-Mobility Ions in Cyclohexane. A Transient Absorption Study

“…Nitrous oxide scavenges e s - with a rate constant of 3 × 10 12 mol -1 dm 3 s -1 . It also quenches the excited state of cyclohexane with a rate constant of 2.4 × 10 10 mol -1 dm 3 s -1 (the actual decay is 1.8 times faster due to the time dependence of the rate constant) . In N 2 O-saturated solution, e s - and c- C 6 H 12 * have lifetimes < 200 ps.…”

Radiation Chemistry of Organic Liquids: Saturated Hydrocarbons

“…Only the lower excited states have been considered, the fluorescing singlet S 1 state [91] and the putative, dissociative, triplet T 1 state [92]. These excited states are produced mainly via charge recombination; the singlet states may also be produced by direct excitation of the solvent by secondary electrons and Cerenkov light [93]. The S 1 state lives between 0.5 ns and 2 ns; this lifetime is longer (3-5 ns) for long-chain paraffins [92,94].…”

FDMR in Low-Temperature Solids. 2. Observation of Weakly Bound Electrons

“…Although excited solvent molecules are produced in reaction 7, the fluorescence from the solvent excited states is in the far UV, and the quantum yield is small. 18 In many instances decomposition of S* may preclude a transfer of the excess energy from the solvent to solute molecules. 18 Nevertheless, the resonance in the primary pairs may be observed via spin memory transfer.1•19 If the primary pairs live sufficiently long, their spin states mix, this mixing being dependent on the resonance conditions in the pair of 2S+ and e-.…”

“…18 In many instances decomposition of S* may preclude a transfer of the excess energy from the solvent to solute molecules. 18 Nevertheless, the resonance in the primary pairs may be observed via spin memory transfer.1•19 If the primary pairs live sufficiently long, their spin states mix, this mixing being dependent on the resonance conditions in the pair of 2S+ and e-. Due to spin momentum conservation in the course of reactions 2 and 4, the newly formed secondary pairs retain the spin mixing of the primary pairs.…”