Practice of Kinetics (Comprehensive Chemical Kinetics, Volume 1)

284 DETECTION AND ESTIMATION OF INTERMEDIATESjustified since, in any case, elucidation of liquid-phase ionic mechanisms is normallyachieved by inference from the nature of the reactants and products and from theoverall kinetics of the process rather than by an explicit investigation of the natureand rate of reaction of the intermediate.Ionic intermediates in the gas phase are also of considerable importance, andmay be studied explicitly by the same sort of methods used for uncharged intermediates,as well as by special techniques relying on electrical properties. Manyflames are known to be conductors of electricity, and it is established that ions maybe formed in flames as a result of chemical reaction (chemi-ionization). Directthermal ionization is not expected to be significant in flames except for metallicelements of low ionization potentials (e.g. the alkali metals) and nitric oxide (whichhas the low ionization potential of 9.4 eV). In shock tubes, however, where muchhigher temperatures may be produced, considerable thermal ionization can occurin other materials. Photo-ionization may be produced by short wavelength radiation,and if the energy of the radiation is greater than that required for ionization,then excited states of ions may be produced. Ionic intermediates are also producedby electron or heavy particle bombardment of neutral reactants, and again theions may be in ground or excited states. The processes described lead to the productionof “primary” ions, which may themselves react with neutral molecules toform “secondary” ions. Such ion-molecule reactions have recently elicited muchinterest in so far as they are important in upper atmosphere photochemistry,problems of rocket exhausts and re-entry into atmospheres; radiation chemistry,plasma magnetohydrodynamics and processes within mass spectrometers.2. Classical spectroscopyThe use of the term spectroscopy in this section is taken to imply, perhaps somewhatarbitrarily, the study of electronic, vibrational and rotational transitions inreaction intermediates. In principle, spectroscopy is the ideal tool with which tostudy reaction intermediates, since not only unequivocal identification, but alsodetermination of the absolute concentration and energy state of the intermediateshould be possible. Both emission and absorption studies may be made, and thesewill be discussed separately. The small intensities absorbed or emitted also leave thesystem virtually unaffected by the investigation. However, a number of practicalproblems do arise which to some extent offset the theoretical advantages of spectroscopicstudy. Perhaps the most severe difficulty is that of obtaining a sufficientlystrong absorption or emission from the reaction intermediate. The difficulty mayresult from the operation of a number of factors. The oscillator strength may besmall for the transition, or, if a strong transition does exist, it may be in an inaccessiblespectral region. Thus the presence of relatively high concentrations ofintermediate may be needed before detection can be undertaken by spectroscopic