Electronic absorption spectra of the protonated polyacetylenes H2CnH+ (n = 4, 6, 8) in neon matrixes.

Electronic absorption spectra of the protonated polyacetylenic chains H2CnH+ (n = 4, 6, 8) and the neutral H2C8H have been observed in 6 K neon matrixes after mass selection. The wavelength of the H2CnH+ electronic transitions depends quasi-linearly on n, typical of carbon chains. The origin band is at 286.0, 378.6, and 467.6 nm for n = 4, 6, and 8, respectively. Two ground-state vibrations of H2C4H+ in the IR absorption spectrum were also detected. On the basis of the spectroscopic trends and the assignment of the vibrational frequencies in the ground and excited electronic states, it is concluded that the H2CnH+ species are C(2v) linear carbon chains with one H atom on one end and two on the other.