VIBRATIONAL AND GEOMETRIC STRUCTURES OF NB3C2 AND NB3C2-FIELD IONIZATION ZERO ELECTRON KINETIC-ENERGY PHOTOELECTRON-SPECTRA AND DENSITY-FUNCTIONAL CALCULATIONS( FROM PULSED)

Vibrational frequencies of three niobium normal modes of triniobium di carbide neutral and cation have been determined from pulsed field ioni zation-zero electron kinetic energy photoelectron spectra. The niobium stretching mode has a frequency of 326 cm(-1) in the neutral and 339 cm(-1) in the ion. The two deformation modes have frequencies of 238 a nd 82 cm(-1) in the neutral, and a degenerate frequency of 258 cm(-1) in the ion. The geometry of the triniobium dicarbide has been establis hed by comparing the experimental spectra with theoretical calculation s, The cluster has a trigonal bipyramid geometry with carbon atoms cap ping on each face of the metal frame. The cation cluster has D-3h symm etry whereas the neutral cluster has lower symmetry resulting from a J ahn-Teller distortion. A second low-lying structure with doubly bridgi ng carbon atoms has been identified by the calculations but has not ye t been observed. (C) 1996 American Institute of Physics.