Practice of Kinetics (Comprehensive Chemical Kinetics, Volume 1)

3 MASS SPECTROMETRY 297where k, T are the Boltmann constant and absolute temperature of the electronbeam respectively.It will be realised that investigations of the kind described may be used in thecalculation of dissociation energies and ionization potentials; a discussion of suchwork is, however, outside the scope of this chapter.Several factors conspire to make the interpretation of the results obtained bymass spectrometry less simple than the preceding treatment might suggest. Theradical-ions, R’, may be produced with a lower appearance potential than V, bya number of processes other than (20), and the occurrence of these will depend notonly upon the reaction being investigated but also on the design of the spectrometer.Amongst these processes may be mentioned:(i) formation of R by the pyrolysis of reactants or products on the filament usedas the electron emitter;(ii) ionization of products giving rise to R+ with a low appearance potential;(iii) formation of R+ by ion-pair processes;(io) formation of radicals in the ionization chamber by electron impact, and thenionization in reaction (20).The presence of surfaces in the ionization chambers may lead to a decrease inconcentration of radicals, although in special circumstances wall decompositionof RX may occur, thus enhancing the radical concentration. LeGoff9’ has giventhe expressionto relate the observed radical concentration R to the initial concentration Ro in asystem with a ratio of wall area to openings in the ionization chamber of S and awall recombination efficiency of y. The value of an instrument with a low value ofS-such as the time-of-flight spectrometer-is clearly understood.Excess energy in RX may lead to a reduction in the ionization potential, so thatin experiments where “hot” species may be present due allowance must be madefor this reduction. This feature may, of course,-be turned to advantage in theinvestigation of excited species. Foner and Hudson9* investigated the appearancepotential for 0: ions in oxygen which had passed through a Wood’s dischargetube at a pressure of about 4 mm Hg. Their appearance potential curves for thedischarge on and off are reproduced in Fig. 5, and show clearly that some 0,species is formed with a lower ionization potential than ground state molecularoxygen (the decrease in ion intensity at higher energies when the discharge is onis accounted for in terms of 0, decomposition to 0 atoms). Analysis of the curvesshows that the energy separation of the two oxygen states is 0.93k0.1 eV, andReferences pp. 336-342