Two-Color Photodetachment Study of the A3Π-X3Σ- Origin Band of C5H-

The origin band in the electronic transition to the dipole bound excited state of C5H(D)(-) anions was measured using two-color photodetachment spectroscopy. The rotational analysis of the partially resolved contour is consistent with a linear structure of the anion in both the ground X-3 Sigma(-) and excited A(3)Pi electronic states, in contrast with an earlier interpretation. The following spectroscopic constants are inferred for C5H-: T-00 = 19248.0(1), B' = 0.0835(1), B '' = 0.0826(2), A(SO)' = 11.96(1), lambda(SS)' = 1.97(8) lambda(SS)'' = 0.24(15). Ab initio calculations at the UHF-MP2 level support the conclusion that C5H- is linear in the ground state. The experimentally determined ground state rotational constant can be used in the search for the millimeter wave spectrum of C5H-.