  Electronic transitions in molecules due to absorption or emission of light is
a complex quantum mechanical process. Their study plays an important role in
the design of novel materials. A common yet challenging task in the study is to
determine the nature of those electronic transitions, i.e. which subgroups of
the molecule are involved in the transition by donating or accepting electrons,
followed by an investigation of the variation in the donor-acceptor behavior
for different transitions or conformations of the molecules. In this paper, we
present a novel approach towards the study of electronic transitions based on
the visual analysis of a bivariate field, namely the electron density in the
hole and particle Natural Transition Orbital (NTO). The visual analysis focuses
on the continuous scatter plots (CSPs) of the bivariate field linked to their
spatial domain. The method supports selections in the CSP visualized as fiber
surfaces in the spatial domain, the grouping of atoms, and segmentation of the
density fields to peel the CSP. This peeling operator is central to the visual
analysis process and helps identify donors and acceptors. We study different
molecular systems, identifying local excitation and charge transfer excitations
to demonstrate the utility of the method.
