Reflectance Difference Spectroscopy for Adsorption Analysis

Monitoring the adsorption on surfaces in-situ and in real time is important for controlling the deposition of thin films used for coating, protection or functionalization of solid surfaces. Various surface analytical tools are available to characterize the adsorption kinetics, the chemical composition as well as the structure and morphology of the growing film. In addition to the conventional surface analytical tools we want to introduce an optical method -- reflectance difference spectroscopy (RDS) -- as a new and extremely sensitive method for the study of the adsorption of molecules on metal surfaces. While RDS has been established as a powerful tool to characterize the growth of thin films, multilayers, and interfaces on emiconductor surfaces, only few studies have been performed on metal surfaces. Yet, preliminary studies in our laboratory reveal that the RDS signal from bare metal surfaces is particularly strong and extremely sensitive to adsorption of atoms and molecules. This enhanced sensitivity is due to the particular electronic structure of metal surfaces. In some cases, the detection limit of adsorbed molecules, surface defects or impurities is well below 0.01% of a monolayer -- much lower than for most conventional techniques. Besides monitoring the adsorption, RDS can also sense structural and morphological changes occurring in the adlayer. Finally, surface electronic states and their modification upon adsorption can be studied spectroscopically. A major problem with the RDS technique is the interpretation and the correlation between the observed changes in the RDS signal with the actual surface or adlayer structure. To improve our understanding and make RDS a powerful and quantitative tool for adsorption analysis, we propose to study selected model systems by RDS and compare the results to those obtained with complementary analytical techniques (STM, LEED, TPD, AES). The systems to be studied are the adsorption of CO on the Cu(110) surface, the molecular adsorption and dissociation of oxygen and the formation of the oxygen induced Cu(100)-(2x1)O reconstruction. Finally the possibilities of RDS in providing information on the electronic and optical properties of adsorbed molecules will be explored using simple organic molecules such as pentacene and perylene adsorbed on the Cu(110) surface.

The analysis of adsorption phenomena on surfaces demands methods with highest surface sensitivity. Up to now such methods were relatively complex or required special conditions (e.g. ultra high vacuum), which made their application difficult and strongly limited. Within the frame of this project we were successful to apply a method to the analysis of adsorption phenomena, which is based on the measurement of the optical anisotropy. This method is called Reflectance Difference Spectroscopy (RDS) and is especially in case of anisotropic surfaces of crystals with cubic structures (the structure of many metals) extremely surface sensitive. Unlike many other surface science techniques, RDS does not require vacuum and thus may be used in high pressure applications. Within this project, RDS has been applied to study the adsorption of CO, O 2 , N2 and organic molecules on Cu(110) and TiO2 . The analysis of the influence of the adsorption on the optical anisotropy of the substrate as well as the identification of adsorbate specific characteristics in the RD spectra contributed to the investigation of adsorption processes. The RDS data have been correlated to in-situ measurements by means of established surface science techniques. The results of the RDS analysis of adsorption phenomena fully met our expectations: RDS is extremely sensitive to minute amounts of CO on Cu(110). RDS allowed to identify superstructures and reconstructions induced by CO and O adsorption on Cu(110). Time dependent RD measurements at distinct photon energies allowed investigating kinetic processes during the adsorption. RDS revealed the orientation of p-6P molecules adsorbed on Cu(110) and Cu(110)-(2x1)O. It turned out that the presence of oxygen alters the orientation of the molecules by 90. RDS showed that p-6P adsorbes selectively on the Cu stripes of the Cu-CuO stripephase Thus, with this project we were successful in proving the applicability of RDS to the analysis of adsorption phenomena and to establish the method for further studies in the field of molecular adsorption. The expansion of the application of RDS to the investigation of organic heterostuctures as well as to the analysis of catalytic reactions on surfaces appears to be promising.