In this paper we present a study of the response of the bacterial photosynthetic reaction center to femtosecond pulses. We demonstrate that an analysis based on the density matrix can be used to give a description of recent pump-probe experiments. It is shown that apart from the usual relaxation process corresponding to a unit change of vibrational quantum number, relaxation corresponding to a second-order process also has to be taken into account. This conclusion is based on the apparent absence of oscillations at the blue side of the spectrum, Assuming that the relaxation of the oscillations is dominated by van der Waals forces, it is shown that the values for the relaxation rates of this pigment-protein complex can be understood in terms of a disordered solid model. Comparing the first-order relaxation rate to the second-order relaxation rate, we obtain an approximate upper mass limit of the oscillator. This then yields an estimate of the mass-of the oscillator which is found to be about 10 au. The analysis proposed in this paper should apply to other systems as well, where again a mass estimate can be obtained.