Photoelectron spectroscopy of small antimony cluster anions: Sb−, Sb2−, Sb3−, and Sb4−

We report the 351 nm photoelectron spectra of Sb−, Sb2−, Sb3−, and Sb4−. The electron affinity of atomic Sb is measured to be 1.046(5) eV. The Sb2− photoelectron spectrum displays rich vibrational and electronic structure. Low-lying electronically excited states are observed for both the anion and the neutral. Several features in both the 351 and 364 nm photoelectron spectra of Sb2− cannot be explained as Franck–Condon processes, indicating that we are accessing autodetaching resonances of the negative ion at these wavelengths. The adiabatic electron affinity of Sb2 is determined to be 1.282(8) eV. For the photoelectron spectra of Sb3− and Sb4−, the observed electronic structure is explained in terms of recently reported ab initio calculations. The adiabatic electron affinity of Sb3 is estimated to be 1.85(3) eV, and an upper bound on the electron affinity of Sb4 is reported, EA(Sb4)≤1.00(10) eV. The vertical detachment energies of Sb3− and Sb4− to the neutral ground states are determined to be 1.90(2) and 1.57(5) eV, respectively. We report photoelectron angular distributions for all the observed spectra, and find that the autodetaching resonance causes unusual angular distributions for Sb2− photodetachment. Finally, electron affinity trends for group V atoms, dimers, and small clusters are discussed in light of the present study.