Quantitative measurements of the second-harmonic response of Al(111), Al(100), and polycrystalline Al to 1.06-μm radiation under ultrahigh-vacuum conditions are presented. The results verify theoretical predictions that at low frequencies the second-harmonic response is sensitive to the charge-density profile of the surface region. The effects of band structure are clearly visible in the anisotropic second-harmonic response for appreciable field components parallel to the surface. The anisotropic response can be interpreted to give information about electronic surface states.

• Received 20 April 1989

DOI:https://doi.org/10.1103/PhysRevLett.63.318