Use of surface plasmons for manipulation of organic molecule quasiparticles and optical properties.

Our recently proposed theoretical formulation based on Bethe­Salpeter G(0)W(0) methodology is applied here to explore the quasiparticle and optical spectra of anthracene (C(14)H10) placed close to a metallic surface. Special attention is paid to explore how the energy shift and decay width of the low-lying anthracene bright excitons p, α and ß depend on the type of the adjacent surface (described by the Wigner Seits radius r(s)) and the separation from the surface. It is shown that p and α excitons weakly interact with surface excitations, but for r(s) ≈ 3 the intensive ß exciton hybridizes with surface plasmon considerably, resulting in its splitting into two optically active modes. The ß exciton decays extraordinarily fast (Γ ≈ 200 meV) to the electron-hole excitations in the metallic surface even for non-contact separations (z(0) ≈ 12 a.u.). For r(s) > 5 the ß exciton becomes infinitely sharp (Γ ≈ 0) and no longer interacts with the surface plasmon. Moreover, it is shown that HOMO and LUMO states near a metallic surface behave as statically screened rigid orbitals, with the result that the simple image theory arguments are sufficient to explain the HOMO­LUMO gap shift. Finally, it is demonstrated that the HOMO­LUMO gap shift dominantly depends on the position of the effective image plane z(im) of the adjacent surface.

Density functional theory based screening of ternary alkali-transition metal borohydrides: a computational material design project.

We present a computational screening study of ternary metal borohydrides for reversible hydrogen storage based on density functional theory. We investigate the stability and decomposition of alloys containing 1 alkali metal atom, Li, Na, or K (M(1)); and 1 alkali, alkaline earth or 3d/4d transition metal atom (M(2)) plus two to five (BH(4))(-) groups, i.e., M(1)M(2)(BH(4))(2-5), using a number of model structures with trigonal, tetrahedral, octahedral, and free coordination of the metal borohydride complexes. Of the over 700 investigated structures, about 20 were predicted to form potentially stable alloys with promising decomposition energies. The M(1)(Al/Mn/Fe)(BH(4))(4), (Li/Na)Zn(BH(4))(3), and (Na/K)(Ni/Co)(BH(4))(3) alloys are found to be the most promising, followed by selected M(1)(Nb/Rh)(BH(4))(4) alloys.

Surface plasmon and electron-hole structures in the excitation spectra of thin films.

Surface excitation spectra are calculated, including both collective and single-particle modes, and examined in detail. This is achieved by calculating the non-local dielectric function ε(p)(Q,z,z('),ω) of the thin jellium film within the random phase approximation (RPA) (using local density approximation wavefunctions which actually takes us beyond the RPA), from which we then derive the spectral function. The high precision of the calculations enables us to analyse not only the collective (surface plasmon) modes and their dependence on the film thickness, but also the intra-band electron-hole excitations, and for the first time oscillatory structures due to inter-band transitions. The spectra are then analysed with special attention to their dependence on the slab thickness, and the periodic peaks observed due to single-particle excitations in the spectra.