Ionic geometries of acetylene in the X 2Πu and B 2Σu+ ionic states

Abstract Franck-Condon analyses are applied to the high-resolution photoelectron spectra of C 2 H 2 and C 2 D 2 . From the first photoelectron bands, the CH and CC bond lengths are found to be respectively 1.076 and 1.250 A for C 2 H 2 + and 1.080 and 1.247 A for C 2 D 2 + in the X 2 Π u state. Results obtained are compared with those by previous workers. As for the third photoelectron bands, simulated spectra of acetylenes agree well with observed ones. The CH and CC bond lengths thus deduced for the B 2 Σ u + state have values of respectively 1.271 and 1.191 A for C 2 H 2 + and 1.276 and 1.178 A for C 2 D 2 + . The long CH bond length obtained for the acetylene ions is discussed.