Practice of Kinetics (Comprehensive Chemical Kinetics, Volume 1)

294 DETECTION AND ESTIMATION OF INTERMEDIATESoxide in order to make quantitative studies of the relaxation of nitric oxide bycarbon monoxides4[NOt, u = l]+[CO, u = 03 [NO, u = O]+[COt, u = 11 (19)Absorption spectra of electronically excited states may be observed in flashphotolysis studies. Porters5 has established the existence of the triplet state in awide range of organic compounds in the liquid and gaseous phases. For example,the first triplet state of anthracene is populated by radiationless conversion from aphotochemically excited singlet molecule, and may be observed by the absorptionto the second triplet level. Absolute measurements of the triplet concentration maybe made by determinations, from the absorption spectra, of the depletion of thesinglet state. Similar results have been obtained with a variety of hydrocarbons,ketones, quinones and dyestuffs.Callear and Norrishs6 have used the technique to study the rise and decay ofmetastable Hg (6 3P0) atoms populated from Hg (6 3P1) atoms excited in mercuryvapour by the flash. A number of transitions in absorption from Hg (6 3P0) wereidentified, and the 6 3D, c 6 3P0 line at 2967 A was chosen for plate photometry.Herzberg and ShoosmithS7 have flash photolysed diazomethane in the presenceof a large excess of inert gas, and observed the absorption spectrum of excited,singlet, methylene (the state expected from photolysis of diazomethane). Absorptionfrom the triplet state is also observed, and it is favoured by long time delays or highdilutions, observations which suggest that the triplet is indeed the lower energystate. In the absence of excess inert gass8, the singlet CH,-which is highly excitedvibrationally-decomposes to electronically excited CH('C-), identified by its absorptionin the 'II t 'C- system at about 3000 A. After flash photolysis ofhydrazoic acid8', strong absorption of the lowest singlet state of NH, 'A, is observed,although in the photolysis of hydrazine only the 32-, ground state, of NHmay be detected.3. Mass spectrometry3.1 UNCHARGED SPECIESThe use of mass spectrometers suggests itself as a convenient method for theinvestigation of unstable reaction intermediates. The mass spectrometer can notonly measure the concentration of an intermediate but also establish its chemicalidentity. Eltent~n~~.~~first applied the technique to the study of free radicals, andfrom the earliest experiments useful results have been obtained. Thus Eltenton inhis first paperg0 showed that the pyrolysis of methane on a carbon filament leads tothe formation of methyl radicals, and not methylene as believed formerly, and he