content_image/388422.jpg
<img src='content_image/388422.jpg'>

effects of molecular adsorption on the calculated PDOS spectra of a Ag (111) film. On the bare Ag (111) surface the electron energy structure is substantially modified as compared to the bulk Ag. Molecular adsorption on the crystal surface and atomic structure relaxation resulted in substantial overal increase of DOS related to the 5 s - and 4 p -orbitals as shows in Fig. 3. We observed at least two pronounced PDOS 5 s -peaks located near − 0 . 4 and − 1 . 6 eV (occupied states) on Ag (111) surface as well as energy increase of both 5 s - and 4 p -unoccupied states in the region between 0 and 4 eV. No substantial dif- ferences between surface and bulk of the 4 d -unoccupied PDOS has been observed. 

Analysis of the PDOS spectra shows that changes of molecular orientations on Ag (111) surface result in mod- ifications of mostly unoccupied p -electron states around 1.6 eV and higher. The Im ( ε ) function spectra calculated in visible and near infrared spectral regions are shown in Fig 4. 

In order to understand the effect of molecular dissoci- ation on optical losses of the Ag (111) surface we calcu- lated molecular configurations after the dissociation of a hydrogen atom from all molecules considered here. Ge- ometry optimization study indicates that the dissociated H-atom tends to interact much stronger with the surface. The same effect for water adsorbed on Ag (111), was re- ported earlier in Refs. 3 and 22. According to our to- tal energy minimization study, the dissociated hydrogen atom moved to the interstitial region. The adsorbed wa- ter pattern remarkably shifts while retaining the ice-like hexagonal structure. Dissociation caused enhancement of the interactions between unpaired oxygen and hydro- 

gen electrons. In all cases the dissociation of hydrogen leads to further substantial increase of Im ( ε ) function which means the increase of optical losses of Ag (111) in relevant spectral regions. 

Comparative analysis of the calculated PDOS spec- tra of Ag (111) surface with dissociated molecules and that of bulk Ag show strong modifications of the elec- tron density: appearance of new occupied states located at around − 1 . 7 to − 2 . 5 eV below Fermi energy and un- occupied states near 0.5 to 0.7 eV above the Fermi level. Consequently in dissociative configuration there are con- tributions to the Im ( ε ) function of electron transition - between occupied surface states and Fermi surface and between occupied and unoccupied surface states. The corresponding energy separations are between 1.5 to 3.2 eV. Contributions of the transitions between Fermi sur- face and unoccupied host states to the observed changes seem to be minor. Our results therefore indicate that chemical reactions between dissociated H- and O-atoms occur mostly with unpaired 5 s - and 4 p -surface electrons of the Ag-atoms. 

Predicted strong increase of optical losses in infrared and visible range apparently dominates over that calcu- lated in the near UV region (see Fig. 4). According to the PDOS spectra the d -electrons are involved in optical excitations in the region above 3.2 eV. In contrast to the infrared and visible region, the molecular adsorption in ultraviolet somewhat even reduces optical losses in the re- gion above 4.0 eV (see Fig. 4) due to the discussed charge redistribution in the near-the-surface region. These find- ings should be interesting for the optical engineering of the UV devices. 

