Practice of Kinetics (Comprehensive Chemical Kinetics, Volume 1)

2 CLASSICAL SPECTROSCOPY 293in elucidating a reaction mechanism, while the measures of the relative concentrationsof such species may yield useful kinetic data. Known transition probabilities,determined in the ways indicated previously, may be used to calculate absolute concentrationsof intermediate in suitable cases.Energy-rich species also may be observed by absorption spectroscopy in flashphotolyticexperiments. A brief survey of this work will be given since previouslyit has received less adequate notice than the more familiar studies of ground stateradicals. The flash photolysis of chlorine dioxide76, nitrogen dioxide76 andleads in each case to the appearance of molecular oxygen in high vibrationallevels of the ground electronic state (up to 21 vibrational quanta in the caseof ozone photolysis7'), which may be detected as an extension of the Schumann-Runge absorption spectrum to longer wavelengths (2000-4000 A). The excitedoxygen seems to be produced in each case by the reaction of atomic oxygen,formed in the primary photolysis, with a reactant molecule. Examination of therotational fine structure of the vibrational bands shows that the molecules are rotationallycold and that the reactions are isothermal. The importance of absolutemeasurements is emphasised by the recent determination by Fitzsimmons andBair79 of the transition probabilities for high vibrational levels of the Schumann-Runge system. Their probabilities, coupled with the observations of McGrath andN~rrish~~, suggest that the concentration of highly excited oxygen is small. Unfortunatelythis does not resolve the question as to whether the concentration issmall because the process leading to the production of excited molecules is a minorone, or whether, as McGrath and Norrish themselves propose, excited oxygen isconsumed by reaction with ozone. Other secondary reactions in photochemicalexperiments which give rise to vibrationally excited products are those of atomichalogens with ozone. Clot with 5 quanta of excitation, and BrOt with 4, have beenobserved in the flash photolysis of chlorine or bromine in the presence of ozones0.Primary photochemical processes also may yield vibrationally excited products.Flash photolysis" of cyanogen, cyanogen bromide and cyanogen iodide in eachcase yields a product whose absorption spectrum identifies it as a vibrationally hotCN radical with up to 6 quanta of excitation. Vibrationally excited nitric oxide inthe ground electronic state (detected by the A 'Z+-X'n absorption system) isproduced in the flash photolysis of nitric oxide itself'2 and of the nitrosyl halides".Relaxation studies have been particularly fruitful ia these systems, since it has beenpossible to make absolute concentration measurements. The 0-1 absorption bandappears in the ordinary absorption spectrum, and under equilibrium conditions[NO', u = 11 = [NO, u = o]e-hV'kT 'where v is the vibration frequency. Thus a calibration can be obtained for anychosen rotational line in the band and applied to the flash photolysis experiments.A similar calibration has been made for vibrationally excited carbon mon-References pp. 336342