Anthracene monomer signal σ_an as a function of nominal anthracene density (as calculated from anthracene flow rate, mixing ratio, and total flow density) for flows generated at the temperatures of 120, 235, and 470 K and sampled at different reaction times. The surviving anthracene fraction (right axis) is calculated by a simple nucleation kinetics model (red solid line). Nucleation affects at most a few percent of the monomers under our experimental conditions at these temperatures.