Unusual C-O Bond Weakening on a Clean Metal Surface: CO on Cr(110)

A unique CO chemisorption mode (${\alpha }_1\mathrm{CO}$), with the lowest reported CO stretching frequencies (1150-1330 ${\mathrm{cm}}^{-1\mathrm{}}$) on any clean or promoted metal surface, has been identified on Cr(110) by means of EELS, ESDIAD, LEED, AES, and oxygen coadsorption experiments. A "lying down" binding configuration in twofold-symmetric hollow sites is proposed for ${\alpha }_1\mathrm{CO}$ molecules. For ${\theta }_{\mathrm{CO}}>0.25\mathrm{}$, a second mode (${\alpha }_2\mathrm{CO}$) bound perpendicular to Cr(110) is populated.