The photostablities of two dozen polycyclic aromatic
hydrocarbon (PAH) cations and a fullerene,
C60
+, have
been tested. The molecular radical cations have been produced by
both electron impact and laser desorption,
then trapped and mass-analyzed in a Fourier transform ion cyclotron
resonance mass spectrometer. The
PAH cations were subjected to visible and UV radiation (λ ≥ 185 nm)
from a Xe arc lamp. Fragmentation
patterns fall into four groups: photostable, hydrogen loss only,
hydrogen and carbon loss, and photodestroyed.
The fragmentation patterns are discussed with respect to prior
experimental and theoretical models. Implications
of these findings for the “PAH hypothesis” and the unidentified
interstellar infrared (UIR) emission bands
are discussed briefly.
A uniform, comprehensive theoretical interpretation of spectroscopic data is presented for 51 radical ion
species of polycyclic aromatic hydrocarbons (PAHs) with the aid of (Tamm−Dancoff) time-dependent density
functional theory (TDDFT). TDDFT is capable of predicting the transition energies to the low-lying excited
states of PAH ions with quantitative accuracy (the standard deviation from experimental results being less
than 0.3 eV) and their intensity patterns qualitatively correctly. The accuracy is hardly affected by the sizes
of PAH ions (azulene through dinaphthocoronene), the types of transitions (Koopmans or satellite transitions),
the types of orbitals involved (π* ← π, π* ← σ, or σ* ← π transitions), the types of ions (cations or anions),
or other geometrical or electronic perturbations (nonplanarity, sp3 carbons, or heterocyclic or nonbenzenoid
rings).
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