We have determined the dispersion of acoustic and optical surface phonon modes at the $2H\mathrm{\text{−}}{\mathrm{NbSe}}_2$ by inelastic x-ray scattering under grazing incidence conditions. Already, at room temperature, an anomaly is observed close to the charge density wave $Q$-vector position located at about one-third along the $\Gamma \mathrm{\text{−}}M$ direction of the Brillouin zone. Our results indicate that the anomaly for the surface mode occurs at a lower energy than that measured in bulk sensitive geometry in the same experiment, showing evidence of a modified behavior in the uppermost layers. We demonstrate that inelastic x-ray scattering in grazing incidence conditions provides a unique tool to selectively study either surface or bulk lattice dynamics in a single experiment.

• Received 20 January 2005

DOI:https://doi.org/10.1103/PhysRevLett.95.256104