Fluorescence spectrometry is used to determine absolute cross sections of cluster-specific decay processes following photon excitation. Absolute values for selected processes in partially condensed supersonic jets are determined using atomic benchmarks. This method is applicable to a wide variety of processes in clusters in which a photon is emitted. In a proof-of-principle experiment, absolute cross sections of processes in two prototype systems were measured: (I) inner-valence excitation and ionization of Ar clusters and (II) resonant interatomic Coulombic decay (rICD) in Ne clusters. Example II is the first measurement of absolute cross sections of ICD whatsoever.