Practice of Kinetics (Comprehensive Chemical Kinetics, Volume 1)

282 DETECTION AND ESTIMATION OF INTERMEDIATESRarefaction waves have been used in shock tubes to produce rapid quenchingof a reaction’. Fig. 2 shows the usual experimental arrangement. The rupture ofdiaphragm 1 forms the initial shock and leads to the ordinary temperature risein the reaction chamber. The temperature is increased to the required value by thereturn of the reflected shock wave from the end-wall. Diaphragm 2 is then rupturedmechanically at about the time that the rarefaction wave arrives; this ruptureproduces a stronger expansion wave which rapidly cools the reacting gases.1.2 FREE RADICALS AND ATOMSOf those intermediates susceptible to quantitative determination, perhaps themost important class is made up of free radicals and atoms. Although most membersof the class have one unpaired electron and are therefore in doublet states,such species as atomic oxygen (3P in the ground state, ‘D in the first electronicallyexcited state) and methylene (also a triplet in the ground state) belong, generically,in the same category.Free radicals and atoms appear as intermediates in a wide range of processes.Almost all photochemical reactions initiated by radiation in the visible or near ultravioletregions of the spectrum are propagated by radicals or atoms (but see Section1.4 for the photochemical formation of ions). Similarly, combustion processes arebelieved to occur predominantly by a radical mechanism, as are a large number ofother thermal reactions.1.3 EXCITED SPECIESMolecules, free radicals, atoms, and, indeed, ions, may possess excess energy byvirtue of excitation in electronic, vibrational, rotational or translational modeswhere such exist. A number of reactions are known in which such energy-richspecies participate. Much interest lies in the fundamental problem of how theexcess energy carried by a reactant can overcome the energy barrier to reaction.The energy of excitation may arise from chemical reaction, by absorption oflight, or by thermal excitation. Chemical reaction in flames gives rise to electronicallyexcited species, and it is the emission from these excited states that give rise tothe characteristic “flame bands” ’.Ozone enters into a number of processes in which excited species play a part.The reaction of atomic hydrogen with ozone gives rise to hydroxyl radicals of whichan appreciable fraction may be vibrationally excited6H+03 + OH’+O,