Electronic absorption spectra of carbon chain anions C2n− (n=4–7) in neon matrices

Abstract

Electronic transitions, (2)2Π←X 2Π (n=4–7), 2Σ←X 2Π (n=4,5) in addition to the known one C 2Π←X 2Π (n=4–7) of linear carbon anions C2n− 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 000 transition on size, photobleaching behavior, and ab initiocalculations. Vibrational frequencies in the excited electronic states have been inferred. The band system attributed previously to the 3Σu−←X 3Σg− transition of C10, C12, and C14 is shown to be the (2)2Π←X 2Π transition of their anions. Another set of weak bands is assigned to the 3Σu−←X 3Σg− system of C10 but none were discernible for C12 or C14. This is experimental evidence for the predicted change from linear to nonlinear geometries for the even numbered carbon species larger than C10.