Photofragmentation spectroscopy—the study of “half collisions” with polarized light of subdoppler line
width—opens a window to look into the structure of molecules. The energy partitioning among the
particular degrees of freedom of the products of the fragmentation reaction is described by the scalar
properties, the direction and magnitude of a particular type of motion is described by the vector
properties. The measurement of the scalar and vector properties allows a pictorial view of the intermediate
state. The forces which make the fragments fly apart or rotate and vibrate can be “seen” from the line
shapes. Information on the unstable intermediate state is gained from the stable fragments long after the
dissociation of the parent molecule. In particular, information on the “lifetime” of the intermediate on a
femtosecond time scale can be obtained.A number of molecules, mainly three and four atomic, have been studied by this technique. Hydrogen
peroxide has shown up as a textbook example. A complete analysis was possible including not only
correlation of different types of fragment motion but also a correlation of the two coincident particles
formed from the same parent molecule. The experimental results are in full agreement with recent
calculations of the dynamics of the fragmentation on newly obtained potential energy surfaces. Hydrogen
peroxide shows a strong dependence of its potential energy on the dihedral angle in the two electronic
states amenable to laser excitation. This experiment further demonstrates that an analysis is also possible
if two states are excited simultaneously.Another good example is the fragmentation of hydrazoic acid for which also coincident pair correlation
has been treated. Here again the results agree excellently with a qualitative picture which can be drawn
from recently calculated ab initio potential energy surfaces. The HN3 example is much more complicated
than the former one due to its higher structured upper potential energy surface. Strong rotational
excitation is observed in the N2 fragment leaving the NH fragment rotationally cold.The treatment of vector correlations in molecular photofragmentation is a powerful tool for the study of
the dynamics of molecular dissociation reactions.
Active learning of potential-energy surfaces of weakly bound complexes with regression-tree ensembles