METAL-INSULATOR-TRANSITION OF VO2 - A XANES INVESTIGATION OF THE O-K-EDGE OF VO2

We have studied the metal-insulator (MI) transition in VO2 with angle resolved x-ray absorption spectroscopy. Above T-MI the oxygen K near e dge structure shows two resolved peaks, the intensities of which do no t change if the polarization vector (E) under bar is rotated within th e rutile (ab) plane, but vary by a factor of two when (E) under bar is rotated in the (ac) plane. Below T-Mf we observe a third peak which s hows strong angular dependence: it vanishes for (E) under bar perpendi cular to <(c)under bar(not)> and is largest for (E) under bar parallel to <(c)under bar(not)>, confirming earlier suggestions that this peak is due to the so called d parallel to band. We find analogous feature s in the M-2 and T phases of Cr doped VO2. From comparisons of these s pectra we conclude that shifts in the d parallel to band between the i nsulating and metallic phase are primarily due to the effect of correl ations and the other shifts to the formation of dimers; tilting of the dimers plays only a minor role.