  The electronic spectra of cold benzylium (C6H5-CH2+) and 1-phenylethyl
(C6H5-CH-CH3+)cations have been recorded via photofragment spectroscopy.
Benzylium and 1-phenylethyl cations produced from electrosprayed benzylamine
and phenylethylamine solutions, respectively, were stored in a cryogenically
cooled quadrupole ion trap and photodissociated by an OPO laser, scanned in
parts of the UV and visible regions (600-225 nm). The electronic states and
active vibrational modes of the benzylium and 1-phenylethyl cations as well as
those of their tropylium or methyl tropylium isomers have been calculated with
ab initio methods for comparison with the spectra observed. Sharp vibrational
progressions are observed in the visible region while the absorption features
are much broader in the UV. The visible spectrum of the benzylium cation is
similar to that obtained in an argon tagging experiment [V. Dryza, N. Chalyavi,
J.A. Sanelli, and E.J. Bieske, J. Chem. Phys. 137, 204304 (2012)], with an
additional splitting assigned to Fermi resonances. The visible spectrum of the
1-phenylethyl cation also shows vibrational progressions. For both cations, the
second electronic transition is observed in the UV, around 33 000 cm-1 (4.1
eV), and shows a broadened vibrational progression. In both cases the S2
optimized geometry is non planar. The third electronic transition observed
around 40 000 cm-1 (5.0 eV) is even broader with no apparent vibrational
structures, which is indicative of either a fast non-radiative process or a
very large change in geometry between the excited and the ground states. The
oscillator strengths calculated for tropylium and methyl tropylium are weak.
Therefore, these isomeric structures are most likely not responsible for these
absorption features. Finally, the fragmentation pattern changes in the second
and third electronic states: C2H2 loss becomes predominant at higher excitation
energies, for both cations.
