Adsorption ofα-sexithiophene on Au(001): Molecule-induced partial lifting of the substrate reconstruction

Ultrathin films of the organic molecule $\ensuremath{\alpha}$-sexithiophene (6T) on Au(001) have been studied by variable-temperature scanning tunneling microscopy. Upon adsorption at room temperature, the monolayer forms two highly ordered and enantiomeric pure structures of flat-lying molecules arranged in rows. While one structural phase is related to quenching of the underlying substrate reconstruction, the quasihexagonal Au(001) reconstruction is conserved for the other one. This leads to two different substrate symmetries for the two molecular structures. Detailed analysis reveals a commensurate unit cell identical to a 6T monolayer on Ag(001) for one structure and an incommensurate 6T arrangement for the other structure strikingly similar to 6T on Au(111). In situ measurements at elevated temperatures show a thermally induced order-disorder transition due to molecular diffusion processes at approximately 400 K.