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) Sigma <-- 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 observed in neon matrices. Absorption spectra were measured after mass-selected deposition of the anions with excess of neon at 5 K, The assignments are based on mass-selection, wavelength dependence of the 0(0)(0) transition on size, photobleaching behavior, and ab initio calculations. Vibrational frequencies in the excited electronic states have been 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 to be the (2)(II)-I-2 <-- X (II)-I-2 transition of their anions. Another 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 nonlinear geometries system of Cio but none were discernible for C-10. (C) 1997 American Institute of Physics.