Fourth-order Raman spectroscopy of adsorbed organic species on TiO2 surface

We present the fourth-order Raman spectrum of an organic adsorbate on a solid surface. TiO2(1 1 0) surfaces covered by p-nitrobenzoate (pNB) and trimethyl acetate were observed. In addition to phonon modes of TiO2, a weak band at 572 cm−1 was observed for the surface covered by pNB, and assigned as a molecular vibration. The coverage of pNB was estimated to be 13% using XPS. The feasibility of fourth-order Raman spectroscopy is demonstrated in observing organic adsorbates of submonolayer quantity.