Primary Processes, the Halogens Cl2, Br2, and

The photodissociation products of the homonuclear halogens in the visible and ultraviolet are now comparatively well established in view of the detailed spectroscopic studies that have been made. The strongest absorption system observed in this spectral region is associated with a transition to the 3II0u+ state which correlates with X / ) + X(2Pyz). Thus photoexcitation to the continuum associated with this state leads directly to the formation of an excited atom, while excitation to the banded region followed by predissociation will lead only to ground state atoms. 

Br(42Pi/2) being rapidly quenched by any Br2 present.76 While Cl(32iVt) would be expected from the photolysis of Cl2, the strongest absorption transition of the excited atom at 1351.7 A (Table IV) was obscured by the molecular spectrum of undissociated Cl2 and only an absorption transition of the ground state atom at 1335.7 A (Table IV) could be detected through a window in the vacuum ultraviolet molecular spectrum.29 

Tiffany78,78 has employed a tuned, pulsed ruby laser to excite Br2 to within 500-800 cm-1 below the dissociation continuum of the 3IIlu state (correlating with ground state atoms) and has observed the reaction of the bromine atoms resulting from the dissociation. By contrast with the collisional release mechanism, Tiffany has proposed a process in which the energy for dissociation for a small number of the Br IIm) molecules into ground state atoms is provided by collisions. 

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