Rotational structure of the origin band in the (1)A ' <- X-1 Sigma+ electronic transition of C4H- and C4D-

The rotational structure of the origin band for the (1)A' <-- X (1)Sigma(+) electronic transition, lying just below the electron affinity of C4H, was recorded by means of a two-colour resonant photo-detachment technique. This allowed a determination of the rotational constants in the X (1)Sigma(+) ground and (1)A' dipole bound excited state. The low lying A (2)Pi excited state of C4H is inferred to be the parent of the dipole bound state. The excited electronic state is deduced to have a nonlinear planar structure whereas the ground is linear according to the spectral analysis. The rotational constants have been obtained: B = 0.1552(2) cm(-1) for the X (1)Sigma(+) state, and A' = 30.73(1), B' = 0.1587(2), C' = 0.1581(2) cm(-1) for the (1)A' state.