Probing the charge-transfer state of CO on Pt(111) by two-dimensional infrared-visible sum frequency generation spectroscopy

The lowest charge-transfer band of CO on Pt(111) was identified using two-dimensional infrared-visible sum frequency generation spectroscopy. The electronic resonance associated with the stretching mode of atop CO appeared at 2.51 eV with a bandwidth of 0.83 eV. The result is consistent with earlier calculations using CO-Pt cluster models indicating that the resonance is from $5d$ to $6sp$ orbital of Pt hybridized with orbital of CO. The measured transition energy suggests that the antibonding state of Pt $6sp$ and CO $5\ensuremath{\sigma}$ for atop CO on Pt(111) is $\ensuremath{\sim}2.2\mathrm{eV}$ above Fermi level.