Practice of Kinetics (Comprehensive Chemical Kinetics, Volume 1)

2 CLASSICAL SPECTROSCOPY 285methods. A second related problem is concerned with the resolving power of theinstrument employed. What may appear in absorption under low resolution as oneweak, fairly broad, band, may turn out at high resolution to be a series of strong,widely spaced bands. Any quantitative measurements made at low resolution, andassuming a weak transition, will be in error. Accurate quantitative estimation willrequire a comparison of line shapes and intensities using instruments capable ofresolqing fully the structure of the bands, and will take into account the effects onthe line shape of differing pressures and temperatures. Such considerations are ofparticular importance if the energy distribution in the intermediate is not the equilibriumdistribution. Determination of absolute concentrations of intermediatesrequires also a knowledge of the absolute transition probability, and this informationmay be difficult of access. Much of the discussion which follows in this chapteris therefore concerned with experimental techniques devised to overcome certainof the problems associated with spectroscopic investigations (see also Section 8).Kinetic spectroscopy is applicable to the study of non-stationary concentrationsof reaction intermediates. The greatest success of kinetic spectroscopy has been inthe investigation of reaction intermediates produced by flash photolysis. Flashphotolysis was developed by Norrish and in order to produce instantaneoushigh concentrations of reactive species. In its most usual form, the apparatusis arranged as shown diagrammatically in Fig. 3. The quartz cell is filled with thereaction mixture, and exposed to the intense flash from the “photolytic” flash tube.PhotocellCcndensers andcharging unitFig. 3. Apparatus for flash photolysis.After a definite delay, the duration of which is determined by the setting of theelectronic delay unit, the much weaker “spectroscopic” flash tube is fired. Thealmost continuous radiation from this source passes through the reaction mixture,the absorption spectrum of which may be recorded photographically in the spectrograph.Experiments with different delay times, but identical reaction mixtures,give information about the variation with time of the contents of the reaction cell(reactants, products and intermediates). One obvious modification to the techniqueReferences pp. 336-342