A threehyphen;dimensional analytical Franckndash;Condon model is presented for the interpretation of intramolecular isotope effects in the photofragmentation of a symmetric triatomic system when the dissociation occurs on a repulsive surface with a single saddle point. The role of the geometry and force constants of the ground and excited surfaces, of the intermode coupling known as the Duschinsky effect, and of the initial vibrational energy content is discussed in relation with the dissociation cross sections and branching ratios measuring the competition between different isotope fragmentation arrangements. Recent ionndash;ion coincidence measurements on doubly charged deuterated water cations show that the formation of H+is five times more frequent than that of D+in twohyphen;body fragmentations. The interpretation of such an important hydrogenndash;deuterium isotope effect seems to be possible by the use of this analytical model.
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