  We found direct experimental evidence for an orbital switching in the V 3d
states across the metal-insulator transition in VO$_{2}$. We have used
soft-x-ray absorption spectroscopy at the V $L_{2,3}$ edges as a sensitive
local probe, and have determined quantitatively the orbital polarizations.
These results strongly suggest that, in going from the metallic to the
insulating state, the orbital occupation changes in a manner that charge
fluctuations and effective band widths are reduced, that the system becomes
more 1-dimensional and more susceptible to a Peierls-like transition, and that
the required massive orbital switching can only be made if the system is close
to a Mott insulating regime.
