  A reliable photon source is required for many aspects of quantum technology.
Organic molecules are attractive for this application because they can have
high quantum yield and can be photostable, even at room temperature. To
generate a photon with high probability, a laser must excite the molecule
efficiently. We develop a simple model for that efficiency and discuss how to
optimise it. We demonstrate the validity of our model through experiments on a
single dibenzoterrylene (DBT) molecule in an anthracene crystal. We show that
the excitation probability cannot exceed 75\% at room temperature, but can
increase to over 99\% if the sample is cooled to liquid nitrogen temperature.
The possibility of high photon generation efficiency with only modest cooling
is a significant step towards a reliable photon source that is simple and
practical.
