Very-low energy vibrational modes as a fingerprint of H-bond network formation: L-Cysteine on Au(111)

The ultrahigh vacuum adsorption of cysteine layers on the Au(111) surface has been studied by means ofX-ray photoelectron (XPS) and high-resolution energy loss spectroscopies (HREELS). Room-temperaturedeposition determined the formation of a quite heterogeneous first layer, where both weakly and stronglybound molecules coexist. Deposition at a slightly higher temperature (330 K) led instead to the formation ofa homogeneous, self-assembled monolayer made of molecules chemisorbed through a thiolate bond. In thelatter case, HREELS measurements have been interpreted in terms of a well-organized H-bond network madeof zwitterionic molecules. Two vibrational modes, denoted as N and H modes, respectively, have been identifiedas distinguishing features of the homogeneous monolayer obtained at 330 K. The N mode lies at 3350 cm-1and is attributed to a stretching vibration of the N-H· · ·O bond. The H mode, observed at 74 cm-1 for fullmonolayer coverage, is assigned to a collective vibration of the two-dimensional H-bond network. At halfmonolayercoverage, the H mode has been observed at 55 cm-1. This red-shift indicates a coverage dependenceof the H-mode frequency, which clearly supports its intermolecular origin. This finding is a nice example ofthe extreme sensitivity of low-frequency vibrational modes to the details of molecule-molecule interactions.