Ronald Cooper

Fast excited state formation and decay in the pulse radiolysis of gaseous argon–iodine systems

Processes responsible for the formation and decay of excited states of I2 emitting in the 320–345 nm region in the pulse radiolysis of gaseous argon–iodine systems are investigated using an electron pulse of half‐width less than 40 psec and an analyzing system with a time resolution slightly better than 1 nsec. The observed emission behavior is interpreted on the basis of a competition between light emission from vibrationally excited levels of an excited electronic state and vibrational deactivation of these levels by collision with argon. Vibrational deactivation rate constants in the range of 2×10−11 cm3 sec−1 are observed. The lifetime τ (=1/k) for the emission at the longer wavelength end of the emission region is 6.7 nsec; the pressure independence of this value indicates that it is the radiative lifetime. Possible processes responsible for the formation of the excited I2 are discussed. We conclude that an excited state of I2 is produced on a subnanosecond time scale via a direct interaction of energetic electrons with iodine molecules in their ground electronic state. Collisions of these excited I2 molecules with argon are necessary to reach the excited states of I2 which emit in the 320–345 nm region. The processes investigated in this study occur too rapidly to be connected with ion‐recombination processes or collisional transfer of energy to I2 from excited argon atoms or Ar2 excited dimers.

Time resolved luminescence spectroscopy of alkaline earth oxides after pulsed electron beam irradiation. II. Spectra and thresholds of MgO

Time‐resolved luminescence spectroscopy has been used to study the emissions occurring in MgO in the time range 100 ns to 100 μs after irradiation with nanosecond duration pulses of electrons of energy between 0.2 and 3.0 MeV. An emission at ∼380 nm with a threshold energy of 0.28±0.01 MeV is attributed to processes resulting from the displacement of oxygen anions and the formation of F+ centers. An emission at ∼240 nm is attributed to processes resulting from the displacement of magnesium cations, with a threshold energy of 0.32±0.01 MeV. An emission at ∼470 nm at an electron energy of 3.0 MeV is attributed to processes resulting from the formation of aggregates of F+ centers, such as the F2+2 center. Displacement energies of 49±2 and 38±2 eV were estimated for oxygen anions and magnesium cations, respectively.

The reactions of the halomethyl radicals CCl3 and CF3 with oxygen

Abstract The pulse radiolysis of CCl4 and CF3Cl) has been used to produce the CCl3 and CF3 radicals, respectively. Their reactions with oxygen were followed by the spectrophotometric detection of the product peroxy radicals. The absorption spectra of the methyl peroxy radicals in the gas phase are reported and follow the general features observed for the spectra of peroxy radicals in solution. Kinetics of formation of the peroxy radicals were studied at room temperature and the limiting second order rate constants were determined as follows; k(CCl3+O2) = (3.09 ± 0.3) × 109dm3mol−1sec−1; k(CF3+O2) = (6.02 ± 0.2) × 109dm3mol−1sec−1.

Optical emission due to ionic displacements in alkaline earth titanates

Abstract Optical emission spectra in the 300–700 nm range were collected from single crystal CaTiO 3 , SrTiO 3 and BaTiO 3 , and polycrystalline CaTiO 3 samples, that were irradiated at room temperature using a Febetron 706 variable energy pulsed-electron-beam generator. The long-lived emissions (up to microseconds after the electron pulse) consist of broad (halfwidths ∼100 nm) bands centred around 380, 425, and 445 nm for CaTiO 3 , SrTiO 3 and BaTiO 3 , respectively. These emission bands are similar to cathodoluminescence emissions from 25 keV electron irradiation attributed by others to direct conduction-valence band transitions in unreduced samples and oxygen vacancies in reduced samples. CaTiO 3 , SrTiO 3 and BaTiO 3 all have emission thresholds of 0.26±0.02 MeV. This corresponds to a threshold displacement energy for oxygen, E d of 45±4 eV.

Pulse radiolysis studies of ion-electron recombination in helium. Pressure and temperature effects

Using pulse radiolysis techniques, rates of recombination of electrons with He+2 ions in the presence of helium over a wide pressure range (40–900 Torr) and at temperatures of 200, 235, 275, and 295 K were measured. Two‐ and three‐body recombination processes were resolved, and a temperature dependence for the three‐body recombination rate constant of T(−2.9±1.2)gas observed. This result agrees well with theoretical predictions of a temperature exponent of −2.5 by Bates and Khare and Pitaevskii, and remarkably well with recent work by Cao and Johnsen which gave a temperature exponent of −2.9 for the rate constant for three‐body recombination of electrons with simple molecular ions in the presence of helium.

Radiation-induced dissolution of colloidal manganese oxides☆

Abstract The reductive dissolution of several well-characterized suspensions of manganese oxides (α-, δ-, γ-MnO 2 , and Mn 5 O 8 ) under steady γ-irradiation at pH 3 has been studied. Radiolytically produced reducing radicals (e.g., (CH 3 ) 2 COH, CH 3 CHOH, HO 2 , and viologen radicals) can be quantitatively captured by the oxide suspension to release Mn 2+ ions into the solution. The radiolytically produced H 2 O 2 similarly leads to quantitative dissolution of the oxide. The rate-determining step in the dissolution process is slower than the diffusion-controlled limit. The stoichiometry of the dissolution process under complete scavenging of the radicals is determined by the oxidation state of the oxide ( x in MnO x ).

Thermal and radiation control of the electrical double layer properties of silica and glass

Abstract High temperature or γ-radiation pretreatment of pyrex glass and vitreous silica has been found to change the isoelectric point of the oxide, and decrease, substantially, the magnitude of the zeta potential at any pH. The changes observed can be adequately explained in terms of a decrease in the number of surface hydroxyl groups existing on the metal oxide. Model calculations of the zeta potential as a function of the number of surface hydroxyl groups, indicate that the surface density of hydroxyl groups after pretreatment is of the order of less than one hydroxyl group per nm 2 This value is affirmed by independent contact angle measurements made on γ-irradiated quartz plates.

Kinetics of formation of krypton‐halogen atom exciplexes in electron beam irradiated gases

The formation of KrCl*, KrBr*, and KrF* exciplexes, in electron beam irradiated mixtures of krypton with either CF3Cl, CF3Br, SF6, or CF4, are observed to be formed by two distinct and kinetically resolvable time‐dependent processes. The rate of one process is total electron beam energy (dose per pulse) independent but dependent on the partial pressures of the halogen source. This process is attributed to a reaction between high electronic excited states of krypton and the halogen source. The second process is dependent on the total absorbed electron beam energy per pulse and is shown to be due to ion recombination processes. Specifically, the formation is due to a combination of krypton positive ions and halogen negative ions, the latter being formed by dissociative electron capture by the halogen source. This process produces KrF* in high vibrational levels.

A temperature effect in the luminescence emission from electron‐irradiated MgO

The temperature dependence of luminescence emissions from electron‐irradiated CaO and MgO single crystals has been studied by time resolved luminescence spectroscopy after pulsed nanosecond irradiation with 0.20 to 0.60 MeV electrons. Emissions from CaO at 375 nm at both 293 and 83 K, show similar threshold characteristics for atomic displacement. These have been attributed to the displacement of oxygen ions and subsequent electron trapping, resulting in the formation of F+ centers. The threshold energy of 0.32±0.01 MeV corresponds to an oxygen displacement energy of 58±2 eV. A 380 nm emission from MgO, also attributed to oxygen displacement and F+ center formation, shows no temperature effect, with a displacement threshold virtually identical to that for CaO. A 235 nm emission from MgO shows a significant temperature effect. The threshold energy at 293 K is 0.31±0.01 MeV, whilst at 83 K two thresholds are observed, 0.31±0.01 and 0.41±0.01 MeV.

Pulse radiolysis studies of ion‐electron recombination in gaseous argon

Using pulse radiolysis techniques, rates of recombination of Ar2+ ions with electrons in bulk argon were measured over a wide pressure range (150 to 1065 torr) at bulk gas temperatures of 295, 335, and 375 K. No pressure dependence for the total recombination rate constant was observed at any temperature studied. This observation is consistent with the model, whereby the encounter pair is stabilized by electron energy loss while in the Coulomb field of the cation. The presence of a Ramsauer minimum in the electron/argon atom momentum‐transfer cross section versus energy profile means that, at low energies, energy loss is very slow. The neutral‐assisted three‐body recombination mechanism in argon does not significantly enhance the total rate of recombination above the two‐body rate.