Neutral Tantalum−Carbide Clusters:  A Multiphoton Ionization and Density Functional Theory Study

Neutral tantalum−carbide clusters, Ta m C n ( m = 1−6, n = 1−7), have been formed in a supersonic expansion following the reaction of tantalum and acetylene in a laser ablation source. These clusters have been subjected to multiphoton ionization at 532 and 355 nm and the mass spectra recorded in a reflectron time-of-flight mass spectrometer. The cluster distributions show a drop off after m = 4 suggesting the stabilization of clusters containing four tantalum atoms. We have investigated the structures of these clusters using density functional theory and shown that the smaller clusters tend to form fragments of face-centered cubes, culminating in the formation of a distorted cube of stoichiometry Ta 4 C 4 . The structure of Ta 4 C 4 is shown to be a distorted tetrahedron, or “butterfly”, made up of four tantalum atoms with a carbon atom bonded to each face. The theoretical calculations also indicate that clusters containing three tantalum atoms form a triangle and that there is competition for the carbon atoms to bond to either the face or the edge of the tantalum triangle.