Practice of Kinetics (Comprehensive Chemical Kinetics, Volume 1)

290 DETECTION AND ESTIMATION OF INTERMEDIATESyield valuable information about the species excited, their energy states, and therates at which they may radiate or be quenched. Nevertheless, it seems inappropriatein the present context to do more than point out the importance of suchinvestigations. There is, however, one special use of fluorescence which is pertinentto our thesis, and it is well illustrated by experiments of von Hartel and Polanyi”.These workers, investigating the reactions of atomic sodium, measured the atomconcentrations by study of the resonance fluorescence from the reaction mixtureexcited by the D lines of a sodium discharge lamp.The discussion of emission spectroscopy will be concluded by a description of arather unusual application. Bay and Steiner’ have measured atomic hydrogenconcentrations in the presence of molecular hydrogen by microwave excitation ofthe atomic hydrogen line spectrum. With low power fed into the gas (ca. 5 watts),there is not enough energy available for the dissociation of molecular hydrogen andsubsequent excitation. Thus the measured intensities of the atomic hydrogen linescorrespond to the concentrations of atoms already present in the reaction mixture.The method is curiously similar to that adopted to detect atoms and free radicalsby mass spectrometry (see Section 3).2.2 ABSORPTION SPECTROSCOPYThe difficulties associated with the use of absorption spectroscopy as a methodfor investigating reaction intermediates have already been touched upon. Evenwhere the concentration of intermediate is high, and the spectroscope resolutionadequate, the experimental conditions may be unfavourable to absorption techniques.Thus in flames, at atmospheric pressures, the concentration of intermediatesmay be relatively high, but the reaction zone is very thin and absorption willbe weak. The light beam used must also be sufficiently narrow to keep it withinthe reaction zone, and a complication arises in that the gradient in refractive indicesof the high-temperature reaction gases tends to deflect the beam. Further, itmay be difficult to obtain a background source of higher brightness than the flame.Notwithstanding the obstacles, however, some absorption studies of combustionprocesses have been made. Molecular intermediates, such as aldehydes and acids,have been identified in the slow combustion of propane5’. Hydroxyl radicals canbe observed in the absorption spectra of several flames5*. The greatest success inthe application of absorption spectroscopy to flame studies has been in investigationsof diffusion flames. Wdfhard and Parker59.60 studied the diffusion flamesin oxygen of hydrogen, ammonia, hydrocarbons and carbon monoxide. In everycase they were able to observe absorption by hydroxyl radicals, and they observedalso the absorption of NH in the ammonia flame (NH2 appeared in emission only).Molecular oxygen, and in suitable cases the reactants, could be detected by theirabsorption spectra, so that a clear picture of the structure of the diffusion flame