  We present the gas-phase infrared spectra of the phenyl cation, phenylium, in
its perprotio C$_6$H$_5^+$ and perdeutero C$_6$D$_5^+$ forms, in the 260-1925
cm$^{-1}$ (5.2-38 $\mu$m) spectral range, and investigate the observed
photofragmentation. The spectral and fragmentation data were obtained using
Infrared Multiple Photon Dissociation (IRMPD) spectroscopy within a Fourier
Transform Ion Cyclotron Resonance Mass Spectrometer (FTICR MS) located inside
the cavity of the free electron laser FELICE (Free Electron Laser for
Intra-Cavity Experiments). The $^1$A$_1$ singlet nature of the phenylium ion is
ascertained by comparison of the observed IR spectrum with DFT calculations,
using both harmonic and anharmonic frequency calculations. To investigate the
observed loss of predominantly [2C,nH] (n=2-4) fragments, we explored the
potential energy surface (PES) to unravel possible isomerization and
fragmentation reaction pathways. The lowest energy pathways toward
fragmentation include direct H elimination, and a combination of facile
ring-opening mechanisms ($\leq2.4$ eV), followed by elimination of H or
CCH$_2$. Energetically, all H-loss channels found are more easily accessible
than CCH$_2$-loss. Calculations of the vibrational density of states for the
various intermediates show that at high internal energies, ring opening is the
thermodynamically the most advantageous, eliminating direct H-loss as a
competing process. The observed loss of primarily [2C,2H] can be explained
through entropy calculations that show favored loss of [2C,2H] at higher
internal energies.
