The reddening of the laser beam is explained by fluorescence. This phenomenon can be
qualitatively described with the Jablonski diagram shown in figure 2: an incoming photon
with energy hνA excites an electron in its energy ground state E0 to an excited energy level E1.
Because of rotational degrees of freedom of the molecules making up the solution (cola), the
energy levels are subdivided into vibrational energy levels (numbered lines). The excited
electron may directly recombine back into the ground state thus re-emitting a photon of the
same energy as the incoming one. However, the electron can also non-radiatively decay
(internal conversion) within the excited energy level to energetically lower vibrational states,
and eventually recombine, emitting in the process a yet red shifted photon because of the
energy loss. The internal conversion happens at timescales much shorter than the direct
recombination, especially for solutions, which means that one is very unlikely to observe
direct recombination without energy loss. A more extensive review can be found in [5].