The role of the evanescent part of the unoccupied complex band structure in photoemission from surface states is revealed. The frequency dependence of the emission intensity from two surface states on the (100) and (111) surfaces of Al in the photon energy range from 40 to 110 eV is explained within an ab initio one-step theory of photoemission. A novel embedding method to determine surface states is presented. A high sensitivity of surface states spectra to details of the surface potential barrier is predicted, which offers a way to efficiently monitor surface properties.