Practice of Kinetics (Comprehensive Chemical Kinetics, Volume 1)

1 FLASH PHOTOLYSIS AND PULSE RADIOLYSIS 119charging the condensers through the flash tube with the help of either a spark gapor a smaller “trigger” electrode placed about halfway along the flash tube. The lightemitted by the flash tube is a continuum on which are superimposed the broadenedatomic spectral lines of the rare gas; it extends over the whole of the visible spectrumand well into the infra-red and ultra-violet. In the arrangement shown inFig. 1 , the light will enter the reaction vessel in a highly non-uniform manner; to helpincrease the uniformity of illumination, the reaction tube and the flash lamp aremounted in a hollow cylindrical vessel coated on the inside with magnesium oxide,which has a very high reflectivity towards light of most wavelengths. The durationtime of the photolysis flash depends on the energy to be dissipated. The two conflictingdemands of a short-lived but high intensity burst have to be balanced, andthe flash may have an energy of a few hundred joules spread over a few tens of microseconds.When a new reaction is investigated its course is normally followed in the firstplace by means of a second light flash-the “spectroscopic” flash. Another quartzflash tube, rather smaller than the photolysis lamp and with one of its ends transparent(the electrode may be built into the side of the tube), is mounted along theaxis of the reaction vessel. At the other end of the vessel is a spectrograph withplate camera which can cover the whole of the spectral range of interest. The spectroscopicflash is triggered electronically with a photocell which picks up light fromthe photolysis flash. The absorption spectra of the reaction mixture before and atknown times after photolysis are compared, and a suitable wavelength at which tostudy the reaction kinetically is chosen. One of the advantages of the method isthat, because of the comparatively large optical path-length, species of low extinctioncoefficient or present in very small concentration can be detected. However,it is sometimes very difficult to identify a transient species from its spectrum alone. Incertain cases it is possible to determine the spectrum of an intermediate on an absolutescale by assuming that that fraction of a reactant which is decomposed (and this canbe measured from the changes in its spectrum, for which extinction coefficients canbe readily measured) is converted completely to the intermediate. This gives theabsolute concentration of intermediate and thus, if the optical density is measured,the extinction coefficient. Such an approach is especially useful when an excitedmolecular state, e.g., a triplet, is formed. Where such an approach is not possible,8 relative concentration scale can be established and from this the reaction halflivesmeasured. If the concentration of the intermediate changes much more rapidlythan the pressure or temperature (which is often the case), the following assumptioncan be made. If the same intensity is found with a given system at differenttimes by using path lengths in the ratio a:b, then the concentrations at thesetimes are in the ratio b:a. This is a direct consequence of Beer’s Law. Provided certainrestrictions regarding the practical applicability of Beer’s Law’* are borne inmind, such a method is generally useful.When a suitable wavelength has been found at which to study the reaction kinet-References pp. 176-1 79