  The results are presented of a detailed combined experimental and theoretical
investigation of the influence of coadsorbed electron-donating alkali atoms and
the prototypical electron acceptor molecule TCNQ
(7,7,8,8-tetracyanoquinodimethane) on the Ag(100) surface. Several coadsorption
phases were characterised by scanning tunnelling microscopy, low energy
electron diffraction, and soft-X-ray photoelectron spectroscopy. Quantitative
structural data were obtained using normal incidence X-ray standing wave
(NIXSW) measurements and compared with the results of density functional theory
(DFT) calculations using several different methods of dispersion correction.
Generally good agreement between theory and experiment was achieved for the
quantitative structures, albeit with prediction of the alkali atom heights
being challenging for some methods. The adsorption structures depend
sensitively on the interplay of molecule-metal charge transfer and long-range
dispersion forces, which are controlled by the composition ratio between alkali
atoms and TCNQ. The large difference in atomic size between K and Cs has
negligible effects on stability, whereas increasing the ratio of K:TCNQ from
1:4 to 1:1 leads to a weakening of molecule-metal interaction strength in
favour of stronger ionic bonds within the two-dimensional alkali-organic
network. A strong dependence of the work function on the alkali donor-TCNQ
acceptor co-adsorption ratio is predicted.
