William A. Mulac

Some Aspects of the Photochemistry of Benzene

Three methods of measuring absolute gas‐phase‐emission yields from benzene are described. Two of the three proved to have large possible errors due to scattered light. One was used to give a value of Q2537=0.18 at a pressure of 10 torr at 25°C. This is in agreement with one value in the literature and slightly lower than the other although all three agree within the limits of error prescribed by the various authors. There is no significant variation with pressure but there is a marked wavelength effect. Below 2500 A the emission decreases rapidly and it is essentially zero at 2400 A and below in agreement with the findings of Poole. Variation at wavelengths longer than 2537 A may exist but seems to be small. By implication the emission yield of biacetyl vapor as determined by Almy and Gillette is probably accurate within the errors specified by those authors. By use of these data it is shown that there is now excellent agreement on the extent of cross‐over of excited‐singlet benzene molecules to the tripl...

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.

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.

Photochemical Primary Process in Biacetyl Vapor at 4358 A

At 4358 A at room temperature the quantum yield for the primary dissociation of biacetyl increases with intensity. It is thus implied that the primary process is second order in some active species. By use of the rotating sector it is shown that the species responsible for this effect has a mean life close to that ascribed to an excited triplet state for biacetyl. At higher temperatures this intensity effect disappears and one of the products strongly inhibits both the phosphorescence and the primary dissociation. At these temperatures the data may best be treated by assuming that the triplet state of biacetyl undergoes a unimolecular dissociation with an activation energy of about 15 kcal.

Studies of Light Emission in the Pulse Radiolysis of Gases: Electron‐Ion Recombination in Nitrogen

An experimental method for studying ion recombination processes using pulse radiolysis is described. The electron‐ion recombination coefficient in N2 (280–980 torr) has been determined to be 3± 0.6× 10−6cm3ion−1·sec−1 at 25°C by analysis of the light emitted by N2(C 3πu), which is produced in the neutralization reaction. This recombination coefficient obeys a T−1/2 dependence. The formation of O3 from an O2–SF6 system was used as a radiation dosimeter to obtain the concentration of ions produced by a pulse of 13 MeV electrons.

Pulse radiolysis studies of U(VI) complexes in aqueous media

The rate constants for the reaction between the hydrated electron and dioxouranium(VI) complexes with carbonate IMDA, NTA, oxalate and chloride ion range from 2 to 4 × 1010 M−1 sec−1. The insensitivity of the reaction rate to formal charge, potential, and number of water molecules in the equatorial coordination sphere of uranium is interpreted in terms of an electron tunneling mechanism for the reaction. Spectroscopic and structural data are cited to provide evidence consistent with such a mechanism.

Improved TV-scanning method for recording time-resolved optical spectra of transients

An improved method of recording a time‐resolved transient absorption or emission spectrum produced by a single pulse of radiation is described. A streak image of the spectrum is produced by an image converter camera. The image is scanned by a TV camera, stored in a video‐disk recorder, and transferred line by line to a computer. The computer can produce three‐ dimensional plots of absorbance or emission versus time and wavelength containing 100×100 data points.

Electron transfer and dimerization of viologen radicals on colloidal TiO2

The reaction of several viologen radical cations with colloidal TiO2 particles of 70 A radius has been studied in detail. The electron-transfer reaction from methyl viologen radicals (MV+) to the TiO2 particles is controlled by the rate of the heterogeneous electron-transfer step. Protonation of the reduced particle follows the electron-transfer reaction in a temporally well separated reaction. Analysis of the subsequent equilibrium stage, but before protonation occurs, allows an estimate of the charge carrier density in the colloid. The effect of added Pt, either as a separate colloid or by photodeposition on the TiO2 particle, has also been studied. Kinetic analysis indicates that a mixture of Pt and TiO2 colloids results in adsorption of the Pt particle on the TiO2 colloid. However, the adsorbed Pt colloid reacts independently of the TiO2 particle.Heptyl viologen radicals (HV+) catalytically dimerize in the presence of TiO2 particles in a heterogeneous reaction between an absorbed HV+ and a dissolved radical. The competition between dimerization and hydrogen evolution can be directed towards the latter reaction either by reducing the pH or by loading the TiO2 particles with Pt. The unsymmetric C14MV+ viologen radicals aggregate even in the absence of any colloid. At low pH values and high Pt loadings they can, however, be directed towards hydrogen evolution.

The hydrolysis of neptunium(VI) and plutonium(VI) studied by the pulse radiolysis transient conductivity technique

Abstract The first hydrolysis constant of the NpO2+2 cation has been determined by either oxidizing Np(V) to Np(VI) with radiation-produced hydroxyl radicals, or reducing Np(VI) to Np(V) with hydrated electrons, and studying the pH dependence of the asymptotic conductivity signals. The method is described in detail. The values obtained by both methods agree within experimental errors. Extrapolated to infinite dilution, pKa(NpO2+2)=5.45±0.1. By reduction of Pu(VI) to Pu(V) with e-aq, we determined pKa(PuO2+2)=6.3±0.1. No evidence was found for a reduction of Np(VI) or Pu(VI) by H atoms. From the conductivity measurements, one obtains an upper limit for the rate constants of both H atom reactions, of 107 M−1 s−1. With the optical pulse radiolysis technique, we could establish k(H+NpO2+2)

Properties of copper(II) hydride formed in the reaction of aquacopper(I) ions with hydrogen atoms. A pulse radiolytic study. [Electron beams]

The spectrum of Cu-H/sup +/(aq), formed via Cu/sup +/ + H ..-->.. Cu-H/sup +/(aq) in aqueous solutions, is reported. In neutral solutions it decomposes via Cu-H/sup +/(aq) + H/sub 2/O ..-->.. Cu/sup 2 +/(aq) + H/sub 2/ + OH/sup -/(aq) with a rate of 4 x 10/sup 3/s/sup -1/. The properties of Cu-H/sup +/(aq) are discussed and compared with those of similar Cu/sup II/-R compounds.