Practice of Kinetics (Comprehensive Chemical Kinetics, Volume 1)

312 DETECTION AND ESTIMATION OF INTERMEDIATESate molecules an interaction between the magnetic moment and the spin momentmay occur. The exact nature of the contribution depends on the coupling caseinvolved. If the coupling is described by the Hund (a) case, then microwave absorptionin the presence of a magnetic field can arise from transitions between thedifferent M, values (subject to the selection rule AM, = kl). However, somedecoupling in the magnetic field may occur: that is to say, there may be an incipientPaschen-Back effect. With magnetic fields large compared with the internal fieldof the species, the electronic spin and orbital angular momentum will be quantisedseparately along the direction of the field. Such a decoupling effect is observed inthe resonance spectrum of nitrogen dioxide‘74, but not at all with nitric oxide17’:molecular oxygen shows appreciable but not complete deco~pling”~. Hyperfinestructure of the resonance spectrum of gases may result from interactions bothwith nuclear spin and with molecular rotation. A number of resonances are observedfrom each main electronic level, depending upon the multiplicity of thespecies. Fig. 11 shows the transitions expected in the three sublevels 3P0, 3P1 and’P, of ground state atomic oxygen. It will be appreciated that while the gas-phaseresonance spectra may be more complex than those of condensed-phase systems,the information carried by the spectra is also more complete. In suitable cases, boththe chemical identity and the electronic state of the resonant species can be established.Absolute concentrations may be obtained by calculation from absoluteabsorption intensities, or, preferably, by calibration of the spectrometer with asuitable gas-phase standard (see below). Considerable effort has gone into relatingline shapes and absorption intensities with perturbing factors such as field modulation(see ref. 177).cE0$-1-2I T .-----Fig. 11. Zeeman energy levels of atomic oxygen in the aP states.V