The structure of a coadsorbed layer of thiophene and CO on Cu(1 1 1)

The influence of coadsorbed CO on the structure and bonding of thiophene (C4H4S), an aromatic heterocycle, on Cu(1 1 1) has been investigated using a combination of near edge X-ray absorption fine structure spectroscopy, normal incidence X-ray standing wavefield absorption (NIXSW) and temperature programmed desorption. The coadsorption of CO does not induce an ordering of the disordered chemisorbed thiophene layer. This lack of ordering contrasts with the behaviour of related benzene coadsorption systems where CO induces ordering of disordered layers. The results of NIXSW measurements show that CO and thiophene both adopt atop adsorption sites within the coadsorbed overlayer, the same site adopted by both in pure layers. The orientation of the thiophene within the coadsorbed layer is also similar to that adopted in a pure layer. However, the CO molecules within the coadsorbed layer are tilted, which contrasts with a linear geometry observed in pure CO layers of a similar coverage. It is suggested that the lack of any significant cooperative effects between the CO and thiophene within the coadsorbed overlayers is due to the relatively weak adsorbate–substrate interactions.