Focusing of electromagnetic waves into a biaxial crystal, experimental results

We present experimental results for focusing of an electromagnetic wave from air into a biaxial crystal. These results are compared with numerical results, and good agreement is found. The experimental results show distinct tangential and sagittal foci, whose positions agree with numerical predictions. The theory is outlined both for the exact solution, which includes extra-paraxial geometries and double refraction, and for the paraxial solution, in which double refraction is ignored. We also present a simple ray-optical interpretation of the results, which allows us to predict the positions of the sagittal and tangential foci.     

The synthesis of methanol and the reverse water-gas shift reaction over Zn-deposited Cu(100) and Cu(110) surfaces: comparison with Zn/Cu(111)

The catalytic activity of Zn vapor-deposited Cu(100) and Cu(110) surfaces for methanol synthesis by the hydrogenation of CO₂ and the reverse water-gas shift reaction were studied using an XPS apparatus combined with a high-pressure flow reactor (18 atm). At a reaction temperature of 523 K, no promotional effect of Zn was observed for the methanol synthesis on both Zn/Cu(100) and Zn/Cu(110). The results were quite different from those for Zn/Cu(111), on which a significant promotion of methanol synthesis activity appeared to be due to the deposition of Zn, indicating that the promotional effect of Zn was sensitive to the surface structure of Cu. However, hysteresis was observed in the catalytic activity for methanol synthesis over the Zn/Cu(110) surface upon heating above 543 K in the reaction mixture. The activity became twice that measured before heating, which was close to the methanol synthesis activity of Zn/Cu(111) at the same Zn coverage. On the other hand, no such hysteresis was observed for the reverse water-gas shift reaction on Zn/Cu(110), indicating that the active site for methanol synthesis was not identical to that for the reverse water-gas shift reaction. In the post-reaction surface analysis, formate species was detected on both Zn/Cu(100) and Zn/Cu(110), whose coverage increased with increasing Zn coverage at 0<Θ_Zn<0.2. No correlation between the formate coverage and the methanol synthesis activity was obtained, which was in contrast to the results for Zn/Cu(111). Thus, the structure sensitivity observed in the catalytic activity of methanol synthesis over Zn-deposited Cu surfaces is ascribed to the significant difference in the reactivity of the formate intermediate.