Picosecond pulse radiolysis studies of the primary processes in hydrocarbon radiation chemistry

Motais, B.C. Le; Jonah, Charles D.

doi:10.1016/1359-0197(89)90307-x

Cited by 8 publications

(19 citation statements)

References 61 publications

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“…Transient Spectra in Cyclohexane. Absorption spectra in the pulse radiolysis of neat cyclohexane have been reported by Menhert et al and by Jonah and LeMotais . The latter results were obtained using picosecond pulse radiolysis.…”

Section: Resultsmentioning

confidence: 59%

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High-Mobility Ions in Cyclohexane. A Transient Absorption Study

1996

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Transient absorption kinetics in radiolysis of N2O-saturated cyclohexane has been studied (0.1−100 ns; 300−800 nm). The spectra indicate the involvement of at least three cations (ions I, II, and III), only one of them having abnormally high mobility. Ion II is probably the cyclohexene radical cation, and ion III might be the dimer olefin ion. These two ions absorb as much as ion I at 450−500 nm. While ion II and ion III are scavenged by ethanol and triethylamine with a rate constant of ≈1010 mol-1 dm3 s-1, the scavenging of ion I proceeds with rate constants of ≈9 × 1010 and 2.3 × 1011 mol-1 dm3 s-1, respectively. The spectrum of ion I is similar to the spectrum of the radical cation of cyclohexane isolated in low-temperature matrices. We were not able to observe the absorption from ion I at delay times longer than 50 ns. A corresponding fast growth of the absorption from solute radical cations of pyrene and perylene was observed. The data (simulated using continuum-diffusion and Monte Carlo approaches) indicate that the scavenging constant is ≈4 × 1011 mol-1 dm3 s-1; the lifetime of the precursor of the aromatic radical cations is ≈30 ns. This short lifetime cannot be explained by a reaction with radiolytic products or by homogeneous recombination, and it seems to be incompatible with identification of the long-lived high-mobility ions observed in conductivity experiments as the radical cation of cyclohexane. A mechanism in which the mobile radical cation is in equilibrium with a normally-diffusing ion is examined in an attempt to resolve this conundrum.

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Section: Resultsmentioning

confidence: 59%

“…Other concerns arose from the picosecond pulse radiolysis experiments by Jonah and Le Motais . No ultrafast scavenging of transient absorption from cyclohexane ion was found.…”

Section: Introductionmentioning

confidence: 99%

High-Mobility Ions in Cyclohexane. A Transient Absorption Study

1996

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“…In neat alkanes, such as n-hexane, two radiolytic products are observed several nanoseconds after the 20 MeV electron pulse: trapped electrons, which absorb in the near-and mid-infrared (IR), and olefin cations, which absorb mainly in the blue and ultraviolet (UV) (the ππ * band) but also have a spectral extension to the visible (the σπ * band). [34][35][36] This TA signal can be observed most distinctively in CO 2 -saturated solution (see Fig. 2S(a), trace (i) in section 4.1) since the TA signal from trapped electrons is removed rapidly by e qf − scavenging (see below); the cation signal is enhanced (Fig.…”

Section: − { }mentioning

confidence: 99%

“…due to the slowing of geminate recombination (the mobility of CO 2 − << µ ). The mechanism for formation of these "satellite cations" (observed as early as 30 ps after the electron pulse) 36 is still uncertain. 34 Fragmentation of vibrationally excited solvent holes (that is, alkane radical cations) and intraspur reactions of these holes and olefins 10.…”

Section: − { }mentioning

confidence: 99%

Electron Trapping by Polar Molecules in Alkane Liquids: Cluster Chemistry in Dilute Solution

Shkrob

Sauer

2005

J. Phys. Chem. A

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Experimental observations are presented on condensed-phase analogues of gas-phase dipole-bound anions and negatively charged clusters of polar molecules. Both monomers and small clusters of such molecules can reversibly trap conduction band electrons in dilute alkane solutions. The dynamics and energetics of this trapping have been studied using pulse radiolysis-transient absorption spectroscopy and time-resolved photoconductivity. Binding energies, thermal detrapping rates, and absorption spectra of excess electrons attached to monomer and multimer solute traps are obtained, and possible structures for these species are discussed. "Dipole coagulation" (stepwise growth of the solute cluster around the cavity electron) predicted by Mozumder in 1972 is observed. The acetonitrile monomer is shown to solvate the electron by its methyl group, just as the alkane solvent does. The electron is dipole-bound to the CN group; the latter points away from the cavity. The resulting negatively charged species has a binding energy of 0.4 eV and absorbs in the infrared. Molecules of straight-chain aliphatic alcohols solvate the excess electron by their OH groups; at equilibrium, the predominant electron trap is a trimer or a tetramer, and the binding energy of this solute trap is ca. 0.8 eV. Trapping by smaller clusters is opposed by the entropy that drives the equilibrium toward the electron in a solvent trap. For alcohol monomers, the trapping does not occur; a slow proton-transfer reaction occurs instead. For the acetonitrile monomer, the trapping is favored energetically, but the thermal detachment is rapid (ca. 1 ns). Our study suggests that a composite cluster anion consisting of a few polar molecules imbedded in an alkane "matrix" might be the closest gas-phase analogue to the core of solvated electron in a neat polar liquid.

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“…Transient Absorption. Olefin radical cations have also been identified in electron-irradiated alkanes on the picosecond timescale by optical absorption (Le Motais and Jonah, 1989). This is a very challenging experiment because of overlap of broad, unstructured spectra of radicals, radical cations and excited states, which makes precise characterization of the complex early chemical processes in many alkanes by absorption impractical.…”

Section: Rh + + Rh--_mentioning

confidence: 99%