  We study the natural excitation of molecular systems, applicable to, for
example, photosynthetic light-harvesting complexes, by natural incoherent
light. In contrast with the conventional classical models, we show that the
light need not have random character to properly represent the resultant linear
excitation. Rather, thermal excitation can be interpreted as a collection of
individual events resulting from the system's interaction with individual,
deterministic pulsed realizations that constitute the field. The derived
expressions for the individual field realizations and excitation events allow
for a wave function formalism, and therefore constitute a useful calculational
tool to study dynamics following thermal-light excitation. Further, they
provide a route to the experimental determination of natural incoherent
excitation using pulsed laser techniques.
