The electronic structures of the cluster compound
-Nb
3Cl
8 and the intercalated phase
'-NaNb
3Cl
8 ha
ve beenstudied by core le
vel and
valence band X-ray and ultra
violetphotoelectron spectroscopy (XPS and UPS), diffusereflectance spectroscopy, and charge-self-consistent molecular orbital(CSC-EH) and band structure (CSC-EH-TB) calculations. The crystal structures of the two compoundsconsist of layers of interconnectedNb
3Cl
13 units.XP and UP
valence band spectra as well as the band structurecalculations show well separated sets of Cl 3ple
vels at lower energy (higher binding energy) and Nb 4d le
vels athigher energy (lower binding energy), indicati
veof mainly ionic Nb-Cl bonding. The UP spectra of
-Nb
3Cl
8 re
veal a triple-peak structurefor the Nb 4d le
vels,corresponding to the 1a
1, 1e, and 2a
1metal-metal bonding orbitals of a se
ven-electron Nb
3cluster as suggestedby theory. The
valence band shapes are in good agreement with thetheoretical density-of-states cur
ve. Therelati
ve intensities in the XP
valence band spectra of
'-NaNb
3Cl
8 e
vidence an additionalelectron in the Nb 4dorbitals. The better resol
ved UP spectra, howe
ver, show a broadpattern for the Nb 4d le
vels which is neitherexpected from a single cluster model nor gi
ven by the theoreticaldensity-of-states cur
ve. Possible origins forthis discrepancy between experiment and theory are discussed.Electrostatic interactions between Na
+ andCl
-ions in
'-NaNb
3Cl
8 lead to a narrowingof the Cl 2p core le
vel and Cl 3p
valence band signals, the formerbeingshifted to higher binding energy in comparison to
-Nb
3Cl
8. Two rather narrowabsorptions in the optical spectraof
-Nb
3Cl
8 are assigned to thetransitions from the 1a
1 and 1e le
vels into the singlyoccupied 2a
1 orbital. Theseabsorptions are missing in the optical spectra of
'-NaNb
3Cl
8, in agreement with thetheoretical expectations foran eight electron Nb
3 cluster.