Richard W. Fessenden

Multiple time scales in pulse radiolysis. Application to bromide solutions and dipeptides

Abstract A description is given of how multiple time scales are incorporated into a new computer program for running pulse-radiolysis experiments on a linear accelerator. For any kinetic trace, the data can be presented on ten distinct time scales. The algorithms for doing this are presented. The utility of this procedure is illustrated with the radiolysis of bromide solutions. The kinetic traces are discussed in terms of formation of a BrOH − adduct and its approach to equilibrium with Br − 2 . A second example, with the radiolysis of a dipeptide Met–Met, illustrates the ease of simultaneously determining the yield and decay behavior.

Thermal electron attachment to oxygen and van der Waals molecules containing oxygen

Thermal electron attachment to O2 has been studied for pure O2 (16O2 and 18O2), O2–N2, O2–CO, and O2–n‐C4H10 (16O2 and 18O2) systems at temperatures from ∼330 down to 78° K using pulse radiolysis and microwave conductivity. For pure O2, O2–N2, and O2–CO mixtures, the electron attachment rates showed three‐body pressure dependences at all temperatures over the pressure range studied (PO2<10Torr, PN2<60 Torr, PCO<40 Torr). The three‐body rate constant of 16O2 decreases from ∼2.4×10−30 cm6 molecule−2u2009sec−1 at 330u2009°K to about 0.9×10−30 cm6u2009 molecule−2u2009sec−1 at ∼ 140u2009°K but unexpectedly increases again to about 1.7×10−30 cm6u2009molecule−2u2009 sec−1 at 79u2009°K. Similarly, the three‐body rate constant of 18O2 decreases from 5.1×10−30 cm6u2009 molecule−2u2009sec−1 at 300u2009°K to 1.8×10−30 cm6u2009molecule−2u2009sec−1 at ∼110u2009°K but increases to 2.3×10−30 cm6u2009molecule−2u2009sec−1 at 80u2009°K. The three‐body rate constant of N2 shows a more dramatic monotonic increase from 0.9×10−31 cm6u2009molecule−2u2009sec−1 at 300u2009°K to 9.4×10−31 cm6u2009molecule−2u2009sec−1 ...

Photosensitized charge injection into TiO2 particles as studied by microwave absorption

Abstract The fluorescence of rose bengal is quenched with 96% efficiency when this dye is adsorbed on TiO2 particles. Microwave measurements on suspensions of these dye-coated particles have shown large absorption signals when the suspension is irradiated with pulses of 532 nm light from a laser. The microwave absorption is attributed to mobile charge carriers injected into the TiO2 from the excited singlet state of the dye. The observed rise time of the signal indicates no intermediate surface state of greater than 2 ns lifetime. Various characteristics of the observed signals are compared with those from TiO2 single crystals.

Time-resolved resonance raman spectra of dihalide radical anions in aqueous solution☆

Abstract Time-resolved resonance Raman spectra of pulse radiolytically produced dihalide radical anions in aqueous solution are reported. The harmonic frequency constants (ωc) for , and are found to be 273, 167 and 115 cm−1. The observated anharmonicities are low, implying bond dissociation energies of 1.6, 1.3 and 0.9 (±0.2) eV, respectively.

Electron spin‐lattice relaxation times of transient free radicals

The saturation recovery method has been used to measure the electron spin relaxation times of short‐lived free radicals in solution. The radicals were produced by continuous in situ radiolysis with a 2.8 MeV electron beam (except for two cases where stable radicals were formed by air oxidation). The ESR spectrometer used broad‐band amplification of the detected ESR signal (time constant of 100 ns) and signal averaging by means of a transient recorder and minicomputer. The magnetic field was stepped off the ESR line for alternate saturating pulses; the phase of the microwave saturating pulse could be changed by 180u2009° to allow separation of the saturation recovery and free induction decay. The sensitivity was such that T1 values greater than 1 ms could be determined for samples of 1014 spins (i.e., 3×10−6 M radical in 50 ml) if the ESR linewidth was 100 mG and transient curves from 40u2009000 saturating pulses were averaged. Somewhat smaller values of T1 could be determined with higher radical concentrations. T...

Electron attachment to N2O at pressures near one atmosphere

Electron attachment to N2O in pure N2O and in several mixtures has been studied by microwave conductivity at pressures to above 1 atm. Ionization was by pulse radiolysis. The values of effective two‐body rate constant exceed those predicted by the usual two‐step, three‐body mechanism and in several cases exceed the value of 5.5×10−13 cm3/moleculeu2009sec attributed to the first step of such a mechanism. This behavior is interpreted in terms of an additional mechanism that is only significant at higher pressures. It is suggested that this mechanism involves attachment to van der Waals complexes of N2O with the other molecules. A near zero value of activation energy supports this mechanism. Certain quantitative aspects are discussed.

Chlorophyll b‐modified nanocrystalline SnO2 semiconductor thin film as a photosensitive electrode

Nanocrystalline thin SnO2 semiconductor films (thickness≤1 μm) have been modified with chlorophyll b (Chl‐b) by electrodeposition and adsorption methods for use as novel photosensitive electrodes in photoelectrochemical cells. Excitation of Chl‐b with monochromatic light produced photocurrents with an incident photon‐to‐photocurrent efficiency of around 8.5%. The charge injection from excited Chl‐b into the conduction band of the semiconductor SnO2 crystallites has been probed by time‐resolved microwave absorption.

Flash photolysis of transient radicals. Benzophenone ketyl radical

A number of processes are found to follow excitation of the benzophenone ketyl radical. The lowest excited state of the radical absorbs, fluoresces and reacts with solvent. Rate constants for reaction of this state with cyclohexane and isopropanol are 4 × 107 and 7 × 108 M−1 s−1, respectively. The lifetime of 5.1 ns found in a solution containing 1% cyclohexane in acetonitrile at room temperature is longer than that reported previously.

Mechanism of thermal electron attachment in N2O and N2O–hydrocarbon mixtures in the gas phase

The attachment of thermal electrons to nitrous oxide at room temperature has been studied, following pulse radiolysis, by a microwave conductivity technique. For pure N2O at pressures from 10 to 300 torr, the results are explained by a combination of two‐body attachment followed by reactions leading to partial electron detachment, a two step three‐body process, and a process giving overall four‐body behavior. The results for mixtures of N2O with alkanes (C2H6, C3H8, n‐C4H10, iso‐C4H10, n‐C5H12, and neo‐C5H12) and butenes (1‐, 2‐cis‐, 2‐trans‐, and iso) are also explained in the same way, but with no electron detachment. Common values of 5×10−15 cm3/moleculeu2009sec for the two‐body rate constant and 4.6×10−33 cm6/molecule2u2009sec for the three‐body rate constant (with N2O as the third body) explain the data. The three‐body rate constants increase with molecular complexity (6×10−34 cm6/molecule2u2009sec for C2H6 to 1.55×10−31 cm6/molecule2u2009sec for neo‐C5H12). The four‐body rate constants range from ∼10−53 to ∼10−51 c...

TRIPLET STATE OF METALLOPORPHYCENES: ZnPCl, PdPC2, PtPC2, and NiPC2

We report on a study of the photoexcited triplet state, at low temperatures in ordered liquid crystals, and at room temperatures in the liquid phase, of some new metalloporphycenes: normal zincporphycene, ZnPCl; Pd(II), and Ni(II) complexes of 2,7,12,17‐tetra‐n‐propylporphycene, PdPC2, PtPC2, and NiPC2, respectively. The triplet state at low temperature is obtained by selective laser excitation and its magnetization response is detected by EPR in the 250 ns time scale. From triplet EPR line shapes and triplet spin polarization directions it is concluded that the zinc cation lies above the molecular plane and the palladium cation fits into the porphycenes cavity. Such a proposed structure implies a strong spin‐orbit interaction in PdPC2, thus resulting in an out‐of‐plane active spin state (z‐axis), whereas that in ZnPCl, having a smaller spin‐orbit interaction, in‐plane (x,y‐axes) are the active spin states. Laser photolysis of the metalloporphycenes gives rise to detectable triplets of ZnPCl, PdPC2 and PtPC2 with triplet lifetimes of 26, 5 and 0.2 µs, respectively. The sensitization experiment, using β‐acetonaphthone as a sensitizer to produce the triplet, results in detectable transients of only ZnPCl and PdPC2 with much longer triplet lifetimes of 85 and 20 µs, respectively. The short triplet lifetime of PtPC2 (and probably that of NiPC2) do not allow for triplet detection with the present sensitizer. However, the latter two metalloporphycenes quench the triplet lifetime of β‐acetonaphthone from 29 to 10µs.