Molecular tagging techniques are promising tools for providing local measurements of pressure, temperature and velocity fields in rarefied gas flows. These techniques are based on the optical collection of photons emitted by a molecular tracer, such as acetone or diacetyl vapor, when excited by UV light. The measurement of the gas properties of interest is carried out by exploiting the intensity, the lifetime and/or the spatial displacement of the emitting tracer. A key point for the success of these techniques resides on the ability of accurately extracting the information from the distribution of the emitted light. In this work, a simple statistical model for describing the photoluminescence process is proposed and implemented in the framework of the Direct Simulation Monte Carlo method. New experimental data on acetone and diacetyl emission are compared to the numerical simulations.