photon incident on a sample is absorbed, one or more
electrons are excited from their equilibrium state in
normal conditions, the ground state Ψ0, to one of
the system’s electronically excited states Ψn. The
character of the excited state can be quite different
from the ground state, e.g. in the way atoms are
arranged or in its response to external perturbations.
The ability to account quantitatively for the physics
of electronic excitations is important for the
interpretation of experimental spectroscopic data and
for the prediction of complex phenomena occurring
in catalytic processes, solar energy conversion and
radiation damage in materials and biological systems,
like proteins and DNA in living cells.