Practice of Kinetics (Comprehensive Chemical Kinetics, Volume 1)

2 CLASSICAL SPECTROSCOPY 29 1emerged. Garton and Broida6’ have made vacuum ultraviolet studies of flamesburning in a nitrogen atmosphere, but without definite result.One of the first radicals to be investigated by absorption spectroscopy was thehydroxyl radical. Bonhoeffer and Reichardt6’ detected the absorption of theA’Z-X’ZI transition of OH in water heated to 1600” C. Oldenburg and his collaboratorsmade an exhaustive study of the absorption by OH resulting from electricalor thermal dissociation of water vapour and hydrogen peroxide. An absolutevalue for the oscillator strength of the transition is derived in the paper by Dwyerand OldenburgZ6, and the paper gives references to Oldenburg’s earlier work.Direct calculation of extinction coefficients (and hence oscillator strengths) requiresa knowledge of both intensity of absorption and concentration of absorbingspecies. For the transition described above, concentrations of OH were calculatedfrom the known fractional dissociation of water at elevated temperatures. In a fewother instances, it may be possible to estimate the concentration of the intermediatefrom chemical considerations. Thus, Lipscomb et ~ 1.’~ were able to calculate theextinction coefficient at L = 2577 A of the radical C10 from flash photolysis studiesof chlorine dioxide. It was shown that the disappearance of C10 obeyed a secondorder rate law, so that---- +-1 1 ktD Do Ewhere D is the optical density (= ~[Clo]) at time t and Do the optical density att = 0. The optical density of the reaction mixture was determined by plate photometryat a number of delay times, and the value of Do obtained by extrapolationof the results. The assumption was made that, apart from some C10, (for whichcorrections were applied), there was material balance between ClO and ClO,, andthat [ClO], could be calculated from the amount of C102 destroyed. The valuesof Do and [ClO], lead immediately to an estimate of E.Extinction coefficients obviously may be determined for suitable absorptions ofspecies whose concentration is already known from non-spectroscopic investigations.The results obtained in this way are limited by the accuracy and availabilityof other methods for the estimation of the species in question. For many reactionintermediates, direct calculation of Concentration is not possible, and until recentlyonly theoretical estimates were available for the oscillator strengths of absorptionsinvolving such species.Dalby and Bennett have now calculated oscillator strengths from spontaneousradiative lifetimes of several excited species. Excited unstable species are producedby electron bombardment of suitable parent molecules. After the electron beam isturned off the intensity of radiation decays, and from observations of the decayrate the absolute lifetime for the radiative transition may be calculated. The experimentalmethod and theory are given in the first paper6’. Among the oscillatorReferences pp. 336-342