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 of X-ray photoelectron (XPS) and high-resolution energy loss spectroscopies (HREELS). Room-temperature deposition determined the formation of a quite heterogeneous first layer, where both weakly and strongly bound molecules coexist. Deposition at a slightly higher temperature (330 K) led instead to the formation of a homogeneous, self-assembled monolayer made of molecules chemisorbed through a thiolate bond. In the latter case, HREELS measurements have been interpreted in terms of a well-organized H-bond network made of zwitterionic molecules. Two vibrational modes, denoted as N and H modes, respectively, have been identified as distinguishing features of the homogeneous monolayer obtained at 330 K. The N mode lies at 3350 cm−1 and is attributed to a stretching vibration of the N−H···O bond. The H mode, observed at 74 cm−1 for full monolayer coverage, is assigned to a collective vibration of the two-dimensional H-bond network. At half-monolayer coverage, the H mode has been observed at 55 cm−1. This red-shift indicates a coverage dependence of the H-mode frequency, which clearly supports its intermolecular origin. This finding is a nice example of the extreme sensitivity of low-frequency vibrational modes to the details of molecule−molecule interactions.