Practice of Kinetics (Comprehensive Chemical Kinetics, Volume 1)

2 CLASSICAL SPECTROSCOPY 289‘atmospheric band’ system in hydrocarbon flames, and the same radiation has beendetected in carbon monoxide-oxygen explosion flames4’. The singlet oxygen moleculesmay also be produced in discharge-flow systems operated at low pressures1’,and measurement of the intensity of radiation has given quantitative data aboutthe rates of reaction of the metastable molecules. Transition probabilities areknown for both the 1Ag-3Zi (ref. 46) and 1Z:-3Xi (ref. 47) systems (whose (0,O)bands’lie at 12700 A and 7619 A respectively), so that concentrations may becalculated from absolute intensity measurements4*. Measurement of absolute intensitiesin low pressure flow systems has been simplified by the determination ofthe absolute intensity-wavelength distribution curve for the “air afterglow” reaction4’O+NO -+ NO,+hv (14)A photomultiplier whose spectral sensitivity is known may be calibrated againstthe air afterglow produced from known pressures of atomic oxygen and nitricoxide. If the geometry of the apparatus is unaltered when the intensity of the “unknown”emission is measured, then the absolute intensity may be calculated withoutexplicit measurement of the emitting volume, photomultiplier acceptance angleand so on. In many cases, the desired kinetic information can be obtained fromrelative concentration measurements under differing experimental conditions.Such studies, using the emission of radiation from 02(lAg) and O,(’Z:), havebeen described by several w ~rkers~~-~ ’.Absolute intensity measurements have been made for the chemiluminescentreaction between nitric oxide and ozones4NO+O, -+ NO:+O,NO; + NO,+hvAn estimate for the relative rates of radiation and quenching may be made and therate constant of (15) calculated, It was shown that the pre-exponential factors forreaction (1 5) and for the non-radiative processwere similar and that the differences in rate of (15) and (17) could be ascribed tothe different, measured, activation energies.In the example just described, excited nitrogen dioxide is certainly a reactionintermediate, although perhaps not in the usual sense. However, if we are to admitto the scope of this discussion excited species which do not “react” further, thenwe should include also all fluorescence phenomena. Fluorescence studies do indeedReferences pp. 336-342