6 only slightly modifies the Im ( ε ) spectra mostly due to minor changes of atomic geometries. The most sub- stantial observed modifications of the Im ( ε ) spectra are caused by chemisorption: molecular dissociation and cre- ation of new inter-atomic bonds. Adsorption of molecules substantially enhances optical losses in infrared region. Comparison with PDOS spectra indicate redistribution of the charge associated with 5 s and 4 p silver orbitals (see Fig. 3). Results of this study clearly demonstrate strong con- tribution of the surface states to the optical losses on Ag (111) surface. Adsorption of organic molecules (ethanol and methanol) result in further increase of the losses on Ag (111) surface. Nature of the observed in- crease of the Im ( ε ) function of Ag (111) surface induced by molecular adsorption is substantial modifications of surface electron energy structure and dramatic enhance- ment of electron transitions between surface and bulk states. Equilibrium atomic structure, projected density of states, and imaginary part of optical dielectric function spectra of Ag (111) surface covered with water, methanol, and ethanol molecules were studied using first principle density functional theory. Strong contribution of the sur- face states on optical losses is observed. Substantial mod- ifications of optical functions of metallic nano-slabs in near infrared and visible spectral regions, caused by sur- face states and molecular adsorption is predicted. Our re- sults indicate increase of the imaginary part of dielectric function of the Ag (111) surface due to charge redistri- bution of the surface Ag-atoms accompanied by the en- hancement of electron transitions probabilities between surface and bulk states. IV. CONCLUSIONS V. ACKNOWLEDGMENTS FIG. 4. Imaginary part of calculated dielectric function of wa- ter (a), methanol (b), and ethanol (c) adsorbed on Ag (111)- √ 3 × √ 3 surface. The thin, dotted and dashed lines represent the undissociated configuration, while the bold solid lines cor- respond to dissociated molecules on the surface, respectively. Analysis of the data presented in Figs. 2 and 4 in- dicates several features related to the effect of molecu- lar adsorption on the Ag (111) surface. Physisorption Authors acknowledge helpful discussions with M. A. Noginov, V. M. Shalaev, and V. P. Drachev. This work is supported by NSF PREM DRM-0611430 and NSF NCN EEC-0228390 ∗ a.v.gavrilenko@nsu.edu 1 M. Methfessel, D. Hennig, and M. Scheffler, Phys. Rev. B 46 , 4816 (1992). 2 K. Morgenstern and J. Nieminen, Phys. Rev. Lett. 88 , 066102 (2002). 5 A. Pinchuk, U. Kreibig, and A. Hilger, Surf. Sci. 557 , 269 (2004). 6 B. N. J. Persson, Surf. Sci. 281 , 153 (1993). 7 B. E. A. Saleh and M. C. Teich, Fundamentals of Photon- ics , 2nd ed. (Wiley, Hoboken, New Jersey, 2007). 8 G. Zhu, M. Mayy, M. Bahoura, B. A. Ritzo, H. V. Gavrilenko, V. I. Gavrilenko, and M. A. Noginov, Optics Express 16 , 15576 (2008). 3 A. Michaelides, Appl. Phys. A 85 , 415 (2006). 4 F. Forster, A. Bendounan, J. Ziroff, and F. Reinert, Phys. Rev. B 78 , 161408 (2008).