ELECTRONIC ABSORPTION-SPECTRA OF CARBON-CHAIN ANIONS C-2N(-) (N=4-7) IN NEON MATRICES

Electronic transitions, (2)(II)-I-2 <-- X (II)-I-2 (n = 4 - 7), (2) Si gma <-- X (II)-I-2 (n = 4,5) in addition to the known one (CII)-I-2 <- - X (II)-I-2 (n = 4 - 7) of linear carbon anions C-2n(-) have been obs erved in neon matrices. Absorption spectra were measured after mass-se lected deposition of the anions with excess of neon at 5 K, The assign ments are based on mass-selection, wavelength dependence of the 0(0)(0 ) transition on size, photobleaching behavior, and ab initio calculati ons. Vibrational frequencies in the excited electronic states have bee n inferred. The band system attributed previously to the (3) Sigma(u)( -) <-- X (3) Sigma(g)(-) transition of C-10, C-12, and C-14 is shown t o be the (2)(II)-I-2 <-- X (II)-I-2 transition of their anions. Anothe r set of weak bands is assigned to the (3) Sigma(u)(-) <-- X (3) Sigma (g)(-) system of C-10 but none were discernible for C-12 or C-14. This is experimental evidence for the predicted change from linear to nonl inear geometries system of Cio but none were discernible for C-10. (C) 1997 American Institute of Physics.