ABSORPTION-SPECTRA OF CONJUGATED HYDROCARBON CATION CHAINS IN NEON MATRICES

Absorption spectra for a series of mass selected C-k H-2(+) systems (k = 4-16) isolated in neon matrices at 5 K have been measured in the visible and near infrared region. Application of the free-electron molecular-orbital model reveals two homologous series corresponding to the deposition of systems containing even or odd numbers of carbon atoms. With the aid of previous gas-phase data, even-numbered carbon systems are identified as cations of polyacetylenic chains. Positions of the A (2) Pi <-- X (2) Pi origin bands as well as fundamental vibrational frequencies in the excited state for the systems H-(C=C)(n)-H+ (n = 4-8) have been obtained. For the deposition of ions containing an odd number of carbons, absorption transitions are tentatively assigned to HC2n+1H+, where n = 2-7.