  In this article we present a generalization of the electron localization
function (ELF) that can be used to analyze time-dependent processes. The
time-dependent ELF allows the time-resolved observation of the formation, the
modulation, and the breaking of chemical bonds, and can thus provide a visual
understanding of complex reactions involving the dynamics of excited electrons.
We illustrate the usefulness of the time-dependent ELF by two examples: the
pi-pi* transition induced by a laser field, and the destruction of bonds and
formation of lone pairs in a scattering process.
