Vector Correlations in Molecular Photofragmentations

Photofragmentation spectroscopy—the study of “half collisions” with polarized light of subdoppler linewidth—opens a window to look into the structure of molecules. The energy partitioning among theparticular degrees of freedom of the products of the fragmentation reaction is described by the scalarproperties, the direction and magnitude of a particular type of motion is described by the vectorproperties. The measurement of the scalar and vector properties allows a pictorial view of the intermediatestate. The forces which make the fragments fly apart or rotate and vibrate can be “seen” from the lineshapes. Information on the unstable intermediate state is gained from the stable fragments long after thedissociation of the parent molecule. In particular, information on the “lifetime” of the intermediate on afemtosecond time scale can be obtained. A number of molecules, mainly three and four atomic, have been studied by this technique. Hydrogenperoxide has shown up as a textbook example. A complete analysis was possible including not onlycorrelation of different types of fragment motion but also a correlation of the two coincident particlesformed from the same parent molecule. The experimental results are in full agreement with recentcalculations of the dynamics of the fragmentation on newly obtained potential energy surfaces. Hydrogenperoxide shows a strong dependence of its potential energy on the dihedral angle in the two electronicstates amenable to laser excitation. This experiment further demonstrates that an analysis is also possibleif two states are excited simultaneously. Another good example is the fragmentation of hydrazoic acid for which also coincident pair correlationhas been treated. Here again the results agree excellently with a qualitative picture which can be drawnfrom recently calculated ab initio potential energy surfaces. The HN 3 example is much more complicatedthan the former one due to its higher structured upper potential energy surface. Strong rotationalexcitation is observed in the N 2 fragment leaving the NH fragment rotationally cold. The treatment of vector correlations in molecular photofragmentation is a powerful tool for the study ofthe dynamics of molecular dissociation reactions.