RESUMO
The visible spectra of phenylacetylene+ and 4-fluorostyrene+ have been measured by laser photodissociation spectroscopy. The observed vibronic systems were assigned to the B2A'' <-- X2A'' and C2B1 <-- X2B1 electronic transition in the 4-fluorostyrene+ and phenylacetylene+ cations, respectively. Two methods were employed and compared: a resonant multiphoton dissociation scheme of the bare cations and a resonant photodissociation technique applied to the chromophore+-argon n=1,2 ionic complexes. The latter approach allowed the intrinsic profile to be resolved, revealing different intramolecular dynamical behavior. Their electronic relaxation has been rationalized in terms of an apparent energy gap law for the benzene derivative cations.
RESUMO
A novel laboratory technique is described, combining the use of supersonic expansion, laser excitation and small aromatic-rare gas van der Waals (vdW) clusters properties, which was developed to access the electronic absorption spectra of the polycyclic aromatic hydrocarbon (PAH) cations in the visible. It consists in preparing vdW complexes of the PAH molecule with a rare gas in a molecular beam, to photoionize it by resonant selective two-photon ionization, then to photodissociate this ionic complex by means of a delayed laser pulse in a time-of-flight mass spectrometer. The method is illustrated by presenting the visible spectra of the Naphthalene, Phenanthrene, Fluorene and Phenylacetylene cations. Such spectra can be unambiguously compared to the astronomical spectra of reddened stars, which exhibit the so-called diffuse interstellar bands (DIBs) in absorption. An interesting feature of the technique is its ability to measure the absolute absorption cross-sections. The large values of the oscillator strengths of the transitions, which are derived, are discussed in the astrophysical context which consists in considering that the PAH cations could be carriers for some of the DIBs.