The photo-fragmentation spectrum of protonated fluorene has been recorded in
the visible spectral region, largely red shifted as compared to the first
excited state absorption of neutral fluorene. The spectrum shows two different
vibrational progressions, separated by 0.19 eV that are assigned to the
absorption of two isomers. As in protonated linear PAHs, comparison with
ab-initio calculations indicates that the red shift is due to the charge
transfer character of the excited state
