PHYSICAL PRINCIPLES OF CHEMICAL REa'cTIONS 217 



ground state. Fluorescence spectra of a great many diatomic gases have 

 been studied (Pringsheim, 1949), but as no essentially different basic 

 ideas are involved they will not be discussed here. 



3-2e. Dissociation. An example of optical dissociation, in particular 

 the dissociation of a diatomic molecule by light absorption, has already 

 been discussed (Br2; cf. Sect. 3-2c). Here the light absorption is pre- 

 dominantly to the continuous portion of the potential curve of the upper 

 electronic state (Fig. 3-4), and each excited Br2 so formed dissociates at 

 once. By measurement of the long-wave-length limit of the continuous 

 absorption (which is joined by some band absorption because a small 

 latitude of variation in r is permitted by the Franck-Condon principle), 

 or even better by measuring the short-wave-length limit of the dis- 

 continuous band spectrum, it is possible to determine the heat of dis- 

 sociation of normal Br2; this is evident from Fig. 3-4, and makes use of 

 the separation of horizontal asymptotes of the two curves (Br ^Pi,^ — ^A^ 

 separation), which is known from measurement of the atomic spectrum 

 of bromine. 



Optical dissociation of diatomic molecules is quite generally connected 

 with continuous absorption (and in some cases even emission) spectra, 

 the final state of the transition usually having a repulsive potential curve 

 or an attractive curve of low stability. The only exception is the case in 

 which light absorption ionizes the molecule ; such spectra are always con- 

 tinuous, but may not always lead to dissociation (cf. Sect. 4-3b). 



An important example of an optical dissociation by hght emission is 

 provided by the familiar hydrogen continuum, a widely used source of 

 continuous ultraviolet radiation. Here the initial state of the radiative 

 transition is the ^2+ state, which has a stable potential curve and is 

 formed by union of one normal and one excited H atom. Molecules in 

 this state cannot be produced directly from the ground state by light 

 absorption because the transition is forbidden, but in an electric dis- 

 charge they are formed by impacts of slow electrons. Once formed, they 

 can lose energy, in the absence of collisions, only by radiating to the 

 ^'L'^ state, a purely repulsive state which leads at once to dissociation 

 (Fi^g. 3-3). 



Dissociation by electron impact is similar in nature to optical dissocia- 

 tion ; the molecule is transferred to an unstable excited state or to a point 

 on the potential curve of a stable state lying above the dissociation limit. 

 In the collision v/ith a slow electron, however, the number of possible 

 states which may be formed is greater. Thus in the case of H2, the 

 repulsive (^S,t) state may be formed directly. The case of the H2 con- 

 tinuous emission discussed above is an example of a more complex 

 sequence: electron impact, photon emission, dissociation. 



Both optical and electron-impact dissociation are extremely important 

 elementary processes in photochemical and radiation-chemical reactions. 



