No abstract

Electronic absorption spectra of linear HC 2n+1 H + ͑n =2-7͒ were recorded in 6 K neon matrices following their mass-selective deposition. Four new electronic band systems are identified; the strongest Ẽ 2 ⌸ g/u ← X 2 ⌸ u/g lies in the UV and the second most intense C 2 ⌸ g/u ← X 2 ⌸ u/g is located in the visible range. The known à 2 ⌸ g/u ← X 2 ⌸ u/g absorption is an order of magnitude weaker than C 2 ⌸ g/u ← X 2 ⌸ u/g . Transitions to the B and D states are also discussed. The wavelengths of the HC 2n+1 H + ͑n =2-7͒ electronic systems obey a linear relation as a function of the size of the cations, similar to other carbon chains. The B 3 ⌺ u − ← X 3 ⌺ g − transition in the UV of neutral HC 2n+1 H ͑n =4-7͒ has also been identified upon photobleaching of the cations trapped in the matrices.

Alpha- and beta-protonated naphthalenes (α- and β-HN(+)) were investigated by electronic absorption and fluorescence spectroscopies in 6 K neon matrixes using a mass-selected C(10)H(9)(+) ion beam. The absorption spectra reveal S(1)/S(2) ← S(0) transitions with onsets at 502.1 and 396.1 nm for α-HN(+), and 534.5 and 322.3 nm in the case of β-HN(+). Wavelength-dispersed fluorescence was detected for α-HN(+), starting at 504.4 nm. Light-induced α-HN(+) → β-HN(+) isomerization was observed upon S(2) ← S(0) excitation of α-HN(+), whereas β-HN(+) relaxed back into the more stable alpha form either upon excitation to S(1) or via thermal population of the ground state vibrational levels near the top of the energy barrier between the two isomers. The intramolecular proton transfer leading to the α-HN(+) ↔ β-HN(+) photoisomerization is fully reversible. The observations are explained with the support of theoretical calculations on the ground- and excited states of the isomers, vertical excitation and adiabatic energies, minimum-energy pathways along the relevant reaction coordinates, and conical intersections between the electronic states.

Electronic spectra of three isomers of protonated anthracene and five isomers of protonated phenanthrene have been detected in 6 K neon matrices following deposition of mass-selected m/z = 179 cations produced from dihydro-anthracene or-phenanthrene. The cations exhibit moderately intense band systems in the 400-550 nm range. Corresponding neutrals have been observed in the UV. The absorptions are assigned to specific isomers of the protonated species on the basis of time-dependent density functional theory calculations. The astrophysical relevance of protonated anthracenes and phenanthrenes as candidates for carriers of diffuse interstellar bands is discussed.

Protonated pyrene and coronene have been isolated in 6 K neon matrixes. The cations were produced in the reaction of the parent aromatics with protonated ethanol in a hot-cathode discharge source, mass selected, and co-deposited with neon. Three electronic transitions of the most stable isomer of protonated pyrene and four of protonated coronene were recorded. The strongest, S(1) ← S(0) transitions, are in the visible region, with onset at 487.5 nm for protonated pyrene and 695.6 nm for protonated coronene. The corresponding neutrals were also observed. The absorptions were assigned on the basis of ab initio coupled-cluster and time-dependent density functional theory calculations. The astrophysical relevance of protonated polycyclic aromatic hydrocarbons is discussed.