Practice of Kinetics (Comprehensive Chemical Kinetics, Volume 1)

2 CLASSICAL SPECTROSCOPY 287Dalby and Bennett63-67 which has given accurate probabilities for a series oftransitions. The technique is described briefly on pp. 291-2. Accurate determinationof concentrations may still be hindered by self-absorption of the radiation, particularlyin the case of the hydroxyl radical. Penner and co-workers have overcomethe difficulty by the use of a double path technique, and are able to determine therotational temperature and concentration of hydroxyl radicals in-both flame” andshock-tube3’ studies. The single and double path emissivities are compared simultaneously,the double path beam being chopped to give modulation at about 5 psecintervals3’. The method of correction for line widths and Doppler broadening isdiscussed30.A further requirement for measurement of absolute concentrations of excitedspecies in flames is that the volume from which emission is collected be known. Thesimplest experimental arrangement for flames at atmospheric pressures is to focusthe radiation from the flame onto the entrance slit of a spectrograph. Reasonableassumptions can be made about the thickness of the emitting layer, and Ausloosand van Tiggelen3’* have used the arrangement successfully in semi-quantitativedeterminations of excited OH, NH, NO and NH2 in flames emitting the bands ofthese species.Studies of low-pressure flames offer several advantages. In particular, the flamecan be maintained flat, and the light from different parts of the reaction zonestudied separately; the reaction volume from which light is collected is determinedwith much greater accuracy for such flames. At low pressures, chemiluminescentprocesses are more important than thermal excitation, collisional quenching ofexcited species is reduced, and self-absorption is diminished. A typical investigationof the low pressure flame is that of Gaydon and W~lfhard~~: quantitative measurementsof the Cz emission were made.So far, we have been concerned mainly with emission of radiation from electronicallyexcited states. Emission may also arise from vibrational transitions in variousreaction systems. The species HOz has long been postulated as an important chaincarrier in combustion reactions, although emission from electronically excited HOzhas yet to be demonstrated unequivocally. However, Tagirov3’ has observed radiationin flames at a frequency of 1305 cm-’ which he ascribes to transitionsfrom vibrationally excited HOz. Investigations of vibrational quenching processesare of great interest, and if the vibrationally excited species emit infrared radiation,then emission spectrometry may be the most satisfactory way of following thereaction. Davidson et a1.36*37 describe a shock-tube study of the relaxation ofvibrationally excited carbon monoxide, in which the overtone 2 + 0 emission at2.335 p was followed. Hooker3* presents a similar study of the 3287 cm-’ parallelIband of acetylene.No discussion of emission spectroscopy would be complete without mention ofthe “atomic flame” studies of Michael Polanyi. In these now-classical investigations,Polanyi and his co-workers (ref. 39 and the references cited therein) ob-References pp. 336-342