The intrinsic optical absorption spectrum and photo-dissociation pathways of flavin adenine dinucleotide (FAD) mono-anions isolated in vacuo are probed using photo-induced dissociation (PID) action spectroscopy. The main photo-products are lumichrome and formylmethylflavin. Evidence is presented that the dissociation pathway leading to these products is non-statistical i.e. occurs during the excited state lifetime. This suggests that the stacking of the adenine and alloxazine chromophores, which enables ultra-fast quenching of the flavin excited state by photo-induced electron transfer in aqueous solution, is inhibited in vacuo. These results provide firm experimental confirmation that lumichrome formation from flavins proceeds via photo-induced, intra-molecular proton-coupled electron transfer.