Gas-phase electronic spectra of C18 and C22 rings.

The electronic spectra of C(18) and C(22) in the 15 150-36 900 cm(-1) range have been detected in the gas phase by a mass-selective resonant two-color two-photon ionization technique coupled to a laser ablation source. The spectra were assigned to several electronic systems of monocyclic cumulenic isomers with a D(9 h) symmetry for C(18) and D(11 h) for C(22), based on time-dependent-density-functional calculations and reactivity with respect to H(2). The best cooling conditions were achieved with Kr as the buffer gas, and the origin of the A(1)A(2) (")<--X(1)A(1) (') transition of C(18) at 592.89 nm shows a pair of 1 cm(-1) broadbands spaced by 1.5 cm(-1). The next electronic transitions exhibited much broader, approximately 30 (in the visible) to 200 cm(-1) (in ultraviolet range), features. The spectrum of C(22) exhibits an absorption pattern similar to C(18), except that the narrow features to the red are missing; the oscillator strength of the A<--X transition is predicted to be low.