Physical Review B
The phonon widths of outermost core-electron photoemission spectra from (110)-oriented overlayers of Na, K, and Rb metals show the expected Debye behavior for the bulk atoms, but significant deviations for the surface atoms. The data indicate a softening of the surface vibrational mode above 200 K. This effect, which is weak in Na but strong in K and Rb, demonstrates that the vibrational mode normal to the surface is anharmonic.
"Anharmonic Surface Vibrations in Photoemission from Alkali Metals," G. K. Wertheim, D. M. Riffe, and P. H. Citrin, Phys. Rev. B 49, 2277 (1994).