LAUSR

A combination of scanning tunneling microscopy, low-energy electron diffraction,and low-energy electron microscopy (LEEM) has been used to identify the structural phases formed by 7,7,8,8-tetracyanoquinodimethane (TCNQ) on Ag(111). These comprise a two-dimensional gas phase, a low-density commensurate (LDC) phase, and a higher-density incommensurate (HDI) phase. LEEM also shows the presence of an additional ``precursor-HDI'' phase with a surface unit mesh area only \ensuremath{\approx}3% less than the HDI phase. Normal incidence x-ray standing-wave measurements of the HDI phase yield almost identical structural parameters to the LDC phase for which a full structure determination has been previously reported. The results show TCNQ does not adopt the inverted bowl distortion favored in earlier density functional theory calculations of TCNQ on coinage metal surfaces, but the N atoms are twisted out of the molecular plane, an effect found for the LDC phase to be due to incorporation of Ag adatoms. The possible role of Ag adatoms in the HDI phase, and in the transition from the precursor-HDI phase, is discussed.