Unusually large absolute Raman scattering cross section of hydrogen vibrations on Si(111)

We have measured the absolute Raman scattering cross section of the terrace vibration mode of silicone-monohydride on a Si(111) surface that was prepared by chemical etching in a N${\mathrm{H}}_4$F solution. The measured cross section at the incident wavelength of 4880 Å is $\left(\frac{d{\sigma }_{\mathrm{zz}}}{d\Omega }\right)=(8.37\mathrm{}\pm 0.32)\times {10}^{-28\mathrm{}}$ ${\mathrm{cm}}^2$/(sr line Si-H bond). It is about 74 times the cross section of the symmetric stretching mode of the gaseous Si${\mathrm{H}}_4$ molecule. The measured excitation profile of the cross section in the range of ${\lambda }_{\mathrm{in}}=4579-5145\mathrm{}$ Å has a ${\omega }^4$ dependence indicating that there is no resonant enhancement of the cross section. We estimate that the Raman transition dipole matrix elements are about three times larger for the Si(111)/H monolayer system than for the free Si${\mathrm{H}}_4$ molecule.