Photoelectron spectra of the C2nH- (n=1-4) and C2nD- (n=1-3) anions

Anion photoelectron spectra of the carbon monohydrides, C2nH- for n = 1 - 4 and C2nD- for n = 1 - 3, have been measured. The spectra were recorded at a wavelength of 266 nm (4.657 eV) and yield electron affinities for each species. The spectra are vibrationally resolved, and some of the vibrational modes in the neutral C2nH(D) radicals are assigned. In addition, photoelectron angular distributions allow one to distinguish between photodetachment transitions to the (2)Sigma(+) and (II)-I-2 states of the neutrals. The spectra confirm previous work showing that C2H and C4H have (2)Sigma(+) ground states, while C6H and C8H have (II)-I-2 ground states. In addition, we observe the low-lying (II)-I-2 or (2)Sigma(+) excited states for all four radicals. The photoelectron angular distributions also serve as a probe of vibronic coupling between the (2)Sigma(+) and (II)-I-2 states. These effects are particularly prominent in the C2H- and C4H- spectra. (C) 1998 American Institute of Physics.