Three open-chain isomers of C6H4(+) and two cyclic ones were detected following mass-selective trapping in 6 K neon matrixes. The open-chain cations 5-hexene-1,3-diyne (CH2═CH-CC-CC-H)(+) and cis- (cis-HCC-CH═CH-CCH)(+) and trans-3-hexene-1,5-diyne (trans-HCC-CH═CH-CCH)(+), possess two absorption systems commencing at 609 and 373, 622 and 385, and 585 and 373 nm, respectively. They are assigned to the 1 (2)A" and 2 (2)A" ← X (2)A", 1(2)A2 and 2 (2)A2 ← X (2)B1, and 1 (2)Bg and 2 (2)B(g) ← X (2)A(u) electronic transitions of these cations. Two overlapping systems are detected at around 420 nm and tentatively assigned to the 1 (2)A" ← X (2)A" electronic transitions of propargyl cyclopropene and 2 (2)B1 ← X (2)A2 of o-benzyne cation structures. The assignment of the electronic transitions is based on theoretical vertical excitation energies calculated with CASPT2 and EOMEE-CCSDT methods for 12 isomers of C6H4(+). These have been carried out at the geometries optimized using several ab initio methods. Adiabatic excitation energies were calculated for the five identified isomers of C6H4(+).

Electronic absorptions of ferrocenium starting at 632.5 nm were measured in a 6 K neon matrix following massselective deposition of the ions. The spectrum shows clear vibrational structure and provides the best-yet resolved view of the electronic states of this cation. The absorption system is identified as the 1 2 E 1 ′ ← X 2 E 2 ′ transition (D 5h symmmetry) on the basis of vertical excitation energies and oscillator strengths calculated at the CASPT2 level. Vibrational bands in the spectrum are assigned with the aid of the ground-state frequencies calculated with the DFT method.

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Two electronic transitions at 512.3 and 250 nm of linear-C 5 H + are detected following massselective deposition of m/z = 61 cations into a 6 K neon matrix and assigned to the 1 1 Π ← X 1 Σ + and 1 1 Σ + ← X 1 Σ + systems. Five absorption systems of l-C 5 H with origin bands at 528,7, 482.6, 429.0, 368.5, and 326.8 nm are observed after neutralization of the cations in the matrix and identified as transitions from the X 2 Π to 1 2 ∆, 1 2 Σ − , 1 2 Σ + , 2 2 Π, and 3 2 Π electronic states. The assignment to specific structures is based on calculated excitation energies, vibrational frequencies in the electronic states, along with simulated Franck-Condon profiles. Published by AIP Publishing. [http://dx