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By use of the new technique of electroreflectance in the infrared frequency range, a low-lying unoccupied surface state on the (100) single-crystal surface of Ag has been detected. Another absorption feature, due to excitation into a higher surface state, has been seen around 3.1 eV. The positions of the two surface states are in good agreement with the calculated values. A study of the Stark shift of these states has revealed that the electric field near the metal surface has a much more complex structure than hitherto recognized.PACS numbers: 78.30.Er, 73.20.Cw In situ reflectance spectroscopy in the visible frequency range has been a valuable tool to detect adsorbed species on the surface of metal electrodes. 1 ' 2 By applying a moderate bias potential (~ 1 V) on the electrode, it is possible to obtain an exceedingly high electric field (~ 10 7 v/cm) which is concentrated near the metal surface. Inside the metal the field is screened out within a Thomas-Fermi screening length. Therefore, only the surface electronic states are affected by this field. The electroreflectance (ER) experiment, which measures the relative reflectance change of the electrode surface as a function of the potential, should be a very effective method to investigate the surface states of the metal. In absence of specifically adsorbed species and chemical reactions, the ER spectra of (110) surfaces of noble metals often exhibit anisotropic polarization dependence, 3 " 8 and one possible source of this effect has been shown to be electronic excitations from the filled band states into a band of unoccupied surface states. 9 However, because the experiments were conducted in a narrow range of optical frequency, they could only provide indirect evidence of the existence of the surface states.In this Letter we report direct observations of two bands of unoccupied surface states on Ag(100) by extending the ER technique from the visible to the infrared frequency range. The positions of these surface states are in good agreement with 1921 the calculated values, also reported here for the first time. As expected, the energies of the surface states are very sensitive to the bias potential, and a detailed study of their Stark shifts leads to a new microscopic picture of the electric field distribution in the electrolyte immediately adjacent to the metal surface.The experiments were performed with silver single-crystal electrodes of (100) surface orientation prepared in the standard manner by chemical polishing. 10 The electrolyte was 0.5M NaF made of Suprapur chemicals and triply distilled water. All electrode potentials were measured with respect to the saturated calomel electrode (SCE). The optical measurements were done with linearly polarized light at near-normal incidence. The ER signal &R/R, i.e., the relative change in reflectance with potential modulation, was detected by a lock-in amplifier, yielding a sensitivity better than 5 x 10" 6 in AR/R. The signal was recorded at constant wavelength as a function of the electrode potential, which...

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