Structural and Bonding Properties of Al _\mathitn C _4 ^-/0 ( \mathitn =2 - 4) Clusters: Anion Photoelectron Spectroscopy and Theoretical Calculations

We measured the photoelectron spectra of Al _n C _4 ^- ( n = 2 - 4) clusters by using size-selected anion photoelectron spectroscopy. The structures of Al _n C _4 ^-/0 ( n = 2 - 4) clusters were explored with quantum chemistry calculations and were determined by comparing the theoretical results with the experimental spectra. It is found that the most stable structure of Al _2 C _4 ^- anion is a C_2 \rmv symmetry planar structure with two Al atoms interacting with two C _2 units. In addition, Al _2 C _4 ^- anion also has a D_\infty \rmh symmetry linear structure with two Al atoms located at the two ends of a C _4 chain, which is slightly higher in energy than the planar structure. The most stable structure of neutral Al _2 C _4 has a D_\infty \rmh symmetry linear structure. The most stable structure of Al _3 C _4 ^- anion is a planar structure with three Al atoms interacting with two C _2 units. Whereas neutral Al _3 C _4 cluster has a C_2 \rmv symmetric V-shaped bent structure. The global minima structures of both Al _4 C _4 ^- and neutral Al _4 C _4 are C_2 \rmh symmetry planar structures with four Al atoms interacting with the ends of two C _2 units. Adaptive natural density partitioning analyses of Al _n C _4 ^- ( n = 2 - 4) clusters show that the interactions between the Al atoms and C _2 units have both \sigma and \pi characters.