Incoherent broadband cavity-enhanced total internal reflection spectroscopy of surface-adsorbed metallo-porphyrins

An improvement of conventional attenuated total reflection (ATR) spectroscopy is demonstrated by applying an incoherent broadband light source (short-arc Xe-lamp) in a cavity-enhanced evanescent-wave absorption method. With this novel approach the absorption spectra of several metallo-octaethyl porphyrins (palladium (PdOEP), platinum (PtOEP) and zinc (ZnOEP)) in thin acetone solution layers and on a fused silica (FS) surface are studied between approximately 390 and 625 nm. The time dependence of the evaporation process of the solution on the FS surface is described. The maximum sensitivity of the setup is estimated at approximately 2 x 10(-5) per pass, which translates into a minimal detectable surface density of less than 2 x 10(-3) monolayers for the porphyrins studied (based on the strong absorption in the Soret bands). Changes of surface and solution spectra are characterised and discussed on the basis of observed band broadenings and spectral shifts. For Pd- and PtOEP the changes of spectral feature can be interpreted with respect to J-aggregate formation supported by polarization dependent measurements. The reason for an observed blue-shift of ca. 10 nm for the Soret band in ZnOEP in combination with a large red-shift of the Q-bands (11 nm for Q1 and 18 nm for Q2) is discussed.