The electronic absorption spectra of linear C6H+ and C8H+ were recorded in 6 K neon matrixes following mass selective deposition. The (1) 3Sigma- -X 3Sigma- electronic transition is identified with the origin band at 515.8 and 628.4 nm for l-C6H+ and l-C8H+, respectively. One strong (near 267 nm) and several weaker electronic transitions of l-C8H+ have also been observed in the UV. The results of ab initio calculations carried out for linear and cyclic C6H+ are consistent with the assignment.

The electronically excited doublet and quartet states of the linear (D 1h ) and cyclic (C 2v ) C þ 6 ion were studied using high-level ab initio methods. For the linear ion it is found that the three lowest excited 2 Å g states, which contribute to the n 2 Å g -X 2 Å u transitions between 1.88 and 2.73 eV, are strongly coupled and form avoided crossings if the bond distances are varied. This leads to a centrosymmetric double minimum potential of the 2 2 Å g state. For the cyclic C 2v structures the lowest states have 2 A 1 and 2 B 2 symmetries. At their equilibrium geometries both states are almost degenerate and their energies are 0.3 eV lower than the minimum of the linear X 2 Å u state. The excitation energies are strongly affected by geometry relaxation effects. The adiabatic excitation energies of the cyclic 2 2 A 1 and the linear 1 2 Å g states are predicted to be about 1.9 eV, i.e. close to the observed band origin. Several electronically excited states-2 2 A 2 , 2 2 B 1 , 2 2 B 2 -of the cyclic structure and the 2 2 Å g state of the linear structure were calculated between 2.3 and 2.5 eV, i.e. higher than the observed band origin at 2.17 eV. Due to strong electronic and vibronic couplings a reliable prediction of relative intensities is presently not possible, and therefore the calculated transition moments cannot be used for the interpretation of the experimental spectrum.

First-principle full valence complete active space calculations (CASSCF) are reported for the electronic states of the D2 h and C2 v planar isomers of B4, B4 + and B4 −. This approach is found to reproduce well the experimentally known transition energies for the two lowest excited triplet states of the most stable rhombic B4 isomer. The pattern of electronic transitions up to about 4.5 eV for B4 singlet and B4 + doublet states calculated in similar way should guide an UV spectroscopic search.

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