  Photosynthesis is the basic process used by plants to convert light energy in
reaction centers into chemical energy. The high efficiency of this process is
not yet understood today. Using the formalism for the description of open
quantum systems by means of a non-Hermitian Hamilton operator, we consider
initially the interplay of gain (acceptor) and loss (donor). Near singular
points it causes fluctuations of the cross section which appear without any
excitation of internal degrees of freedom of the system. This process occurs
therefore very quickly and with high efficiency. We then consider the
excitation of resonance states of the system by means of these fluctuations.
This second step of the whole process takes place much slower than the first
one, because it involves the excitation of internal degrees of freedom of the
system. The two-step process as a whole is highly efficient and the decay is
bi-exponential. We provide, if possible, the results of analytical studies,
otherwise characteristic numerical results. The similarities of the obtained
results to light harvesting in photosynthetic organisms are discussed.
