Orbital tomography of hybridized and dispersing molecular overlayers

With angle-resolved photoemission experiments and ab initio electronic structure calculations, the pentacene monolayers on Ag(110) and Cu(110) are compared and contrasted, allowing the molecular orientation to be determined and an unambiguous assignment of emissions to specific orbitals to be made. On Ag(110), the orbitals remain essentially isolated-molecule-like, while strong substrate-enhanced dispersion and orbital modification are observed upon adsorption on Cu(110). We show how the photoemission intensity of extended systems can be simulated and that it behaves essentially like that of the isolated molecule modulated by the band dispersion due to intermolecular interactions.

Figure 1

Sketch of an ARUPS experiment declaring the geometry of the experimental setup. The incoming photon with energy $h\nu$, the incidence angle $\chi$, and the vector potential A excite an electron from the initial state ${\psi }_i$ to the final state ${\psi }_f$. This final state is characterized by the kinetic energy $E_{\mathrm{kin}}$ and the momentum vector k, and the outgoing photoelectron is detected as a function of $E_{\mathrm{kin}}$ and emission direction, defined by the polar angle $\theta$ and the azimuthal angle $\varphi$. Forward emissions are denoted by a positive parallel momentum, and backward emissions are denoted by a negative parallel momentum.

Figure 2

Panels (a) and (e) show LEED images of 5A monolayers on Ag(110) and Cu(110), respectively. Panels (b) and (c) show ARUPS momentum maps of 5A/Ag(110) at binding energies 0.15 and 1.20 eV (exp), respectively, compared to simulated LUMO and HOMO maps of the isolated 5A molecule (sim). Panels (f) and (g) show the corresponding data for 5A/Cu(110) at binding energies of 0.80 and 1.50 eV. Panels (d) and (h) show structural models of 5A/Ag(110) and 5A/Cu(110), respectively, as deduced from LEED and the momentum maps. In (h) the orbital structure of the LUMO is overlaid.

Figure 3

Density of states of a monolayer of pentacene on Ag(110) projected onto orbitals of the free pentacene molecule using PBE-GGA (a) and the hybrid functional HSE06 (b). Projections onto the LUMO (blue), the HOMO (red), the HOMO-1 (orange), and the HOMO-2 (green) are shown. For comparison, panel (c) shows experimental ARUPS data along the [001] azimuth (black solid line) and an azimuthal direction ${45}^{\circ }$ between [001] and [1-10]. The colored curves are obtained by $k_x{k}_y$-integrating experimental momentum maps identifying the orbital character of the emissions. Panels (d)–(f) show the corresponding data for a monolayer of pentacene on Cu(110).

Figure 4

(a) Band map of a 5A monolayer on Ag(110) in the [001] azimuth. Panels (b) and (c) show band maps in the [1-10] azimuth of a half and a complete monolayer of 5A on Cu(110), respectively. Vertical lines indicate the Brillouin zone boundaries (yellow) and the $\Gamma$ points (purple) of the 5A overlayer.

Figure 5

(a) Color-coded two-dimensional dispersion $E(k_{\left[001\right]},k_{[1-10]})$ of the LUMO for a freestanding 5A monolayer with the structure of Fig. 2. Superimposed in white are the Brillouin zones of Cu(110) (rectangle) and the 5A monolayer (rhombus), and in black an intensity isoline of the calculated isolated 5A LUMO momentum distribution. (b) Close-up of the dispersing LUMO band with red, green, and blue lines indicating the energy positions at which momentum maps are extracted for comparison with the calculated photoemission momentum distributions in (c) the top, (d) the middle, and (e) the bottom of the band. Note that in panels (c)–(e), simulated momentum maps (sim) are compared to experimental maps (exp) as detailed in the text.

Figure 6

(a) Calculated LUMO of an isolated 5A molecule in real space; scale bars mark characteristic dimensions as detailed in the text. Panels (b) and (c) are simulated momentum maps for a free 5A molecule and the monolayer 5A/Cu(110), while (d) and (e) show experimental ARUPS maps of 5A/Cu(110) and 5A/Ag(110), respectively. The red and green crosses indicate the position of the maxima of the LUMO's major and minor lobe, and the horizontal green line is a guide for the eye.