| Popis: |
The extreme ultraviolet (EUV) optical constants ( ) and ( ) of amorphous carbon were determined on the basis of transmission measurements at =18-450 eV, the first-principles calculation of the dielectric tensor at < 25 eV, and the Kramers-Kronig calculation of ( ). Our optical constants generally agree with the CXRO data, excluding the vicinity of the K-edge. First-principles analysis shows that two thresholds of absorption (at 284 eV and 291 eV) f ound in the present study are caused, respectively, by the - and - bonds. Their weights are controlled by an orientation of graphene sheets in a-carbon. Keywords: optical constants, anisotropic EUV absorption, electronic structure, graphene sheets. 1. INTRODUCTION. Analysis of carbon-based materials is important for many ecological, biological, and industrial applications. Electron energy loss spectroscopy (EELS) and near edge X-ray absorption fine structure spectroscopy (NEXAFS) provide valuable information about the arrangement and bonding of atoms, as well as about the electronic structure of carbon materials. Numerous examples of such investigations can be found in Refs. [1-3]. In these investigations the attention of researchers is focused on the shape of the carbon K-edge, while the absorption spectrum in a wide energy region, as well as the optical constants, remain unstudied. A different situation takes place for the applications of carbon-based materials in soft X-ray multilayer optics. In this case both optical constants ( ) and ( ), which characterize the complex refractive index n( )=1- ( )+i ( ), are important for multilayer design. In the present study we concentrated on the measurements of the optical constants of amorphous carbon (a-carbon). This material is widely used in soft X-ray multilayer optics at short wavelengths ( =4.4-6.0 nm) because of its transparency and low roughness. In particular, the highest normal incidence reflectivity 15 % has been obtained at these wavelengths for a Co-C multilayer mirror [4]. To further improve multilayer optics based on a-C, precise information about its ( ) and ( ) is required. Currently available optical constants of a-carbon are only those from the atomic scattering factors of elements [5]. Despite many advantages of the CXRO data [5], they are not precise near the absorption edges, where the effect of interatomic bonding is of importance. This fact is disappointing, as the working interval of reflective multilayers is normally placed just below the absorption edge. The present study provides the optical constants of amorphous carbon in a wide EUV region from 40 eV to 450 eV. The determination of ( ) and ( ) is based on our transmission measurement, which is supplemented by the first-principles calculation of the dielectric tensor at 25 eV. Particular attention is given to the spectral region near the K-edge of carbon. To interpret our transmission spectra in this region, we invoke the results of our first-principles calculation that allows connection of the shape of the absorption K-edge with the atomic bonding in a-carbon. |
