Photoelectron imaging and theoretical investigation of bimetallic Bi(1-2)Ga(0-2)(-) and Pb(1-4)(-) cluster anions.

We present the results of photoelectron velocity-map imaging experiments for the photodetachment of small negatively charged Bi(m)Ga(n) (m=1-2, n=0-2), and Pb(n) (n=1-4) clusters at 527 nm. The photoelectron images reveal new features along with their angular distributions in the photoelectron spectra of these clusters. We report the vertical detachment energies of the observed multiple electronic bands and their respective anisotropy parameters for the Bi(m)Ga(n) and Pb(n) clusters derived from the photoelectron images. Experiments on the BiGa(n) clusters reveal that the electron affinity increases with the number of Ga atoms from n=0 to 2. The BiGa(2)(-) cluster is found to be stable, both because of its even electron number and the high electron affinity of BiGa(2). The measured photoelectron angular distributions of the Bi(m)Ga(n) and Pb(n) clusters are dependent on both the orbital symmetry and electron kinetic energies. Density-functional theory calculations employing the generalized gradient approximation for the exchange-correlation potential were performed on these clusters to determine their atomic and electronic structures. From the theoretical calculations, we find that the BiGa(2)(-), Bi(2)Ga(3)(-) and Bi(2)Ga(5)(-) (anionic), and BiGa(3), BiGa(5), Bi(2)Ga(4) and Bi(2)Ga(6) (neutral) clusters are unusually stable. The stability of the anionic and neutral Bi(2)Ga(n) clusters is attributed to an even-odd effect, with clusters having an even number of electrons presenting a larger gain in energy through the addition of a Ga atom to the preceding size compared to odd electron systems. The stability of the neutral BiGa(3) cluster is rationalized as being similar to BiAl(3), an all-metal aromatic cluster.