Characterization of adsorbate ensembles by X-ray linear dichroism microscopy

The applicability of the X-ray linear dichroism contrast mechanism in X-ray absorption spectromicroscopy for the investigation of adsorbate ensembles is demonstrated by using a monolayer of benzoic acid (BA) on the surface of a nickel polycrystal as a test system. The choice of this system was dictated by a requirement to have a two-dimensional molecular arrangement containing domains of oriented molecules with a characteristic length scale exceeding the lateral resolution of the available photoelectron microscope. Based on the spectroscopic measurements for BA on Ni(110), we have shown that the BA adsorbates should form such domains on the surface of the (110) microcrystallites incorporated into a nickel polycrystal. In the following spectromicroscopy experiments the photon energy was tuned to the excitation energy of the π∗ orbital of the phenyl ring of the BA molecule, and the C 1s→π∗ excitation was monitored by the carbon KLL Auger electrons. Well-resolved images of azimuthally aligned BA molecules on the surface of the (110) microcrystallites of a nickel polycrystal were acquired and the temperature dependence of this alignment was monitored. The sensitivity and the experimental requirements of X-ray linear dichroism microscopy for the study of adsorbate systems are considered.