The optical constants of potassium and caesium were determined for the solid and liquid states. Observations were made at temperatures above and below the melting points of the metals. The glass cell containing the alkali mirror was heated by means of a small electrical oven enclosing the mirror and containing suitable apertures for the incident and reflected light rays. The method of observation was that employed in previous investigations.
The optical constants were computed by means of Drude’s equations. When the metal changed from the solid to the liquid state, it was found that the angle of azimuth of restored plane polarization, as well as the phase difference of the components of the light vector parallel and perpendicular to the plane of incidence, changed by only a small amount. After due allowance for the effects of strains in the glass cells, it was concluded that the optical properties of the liquid alkali surface are closely identical with those of the solid alkali surface.
J. B. NATHANSON, "VARIATION WITH STATE OF THE OPTICAL PROPERTIES OF POTASSIUM AND CAESIUM," J. Opt. Soc. Am. 17, 343-346 (1928)