He∗ interaction with soft matter surfaces: Ultra thin l-cysteine films

We report first results about the interaction of a He∗ beam with nanometer thick layers of l-cysteine, a fundamental amino-acid, deposited under ultra high vacuum conditions on Cu and Au surfaces. Electronic properties of adsorbed molecules have been investigated by metastable deexcitation spectroscopy (MDS). Deexcitation of He∗ on molecular layers gives rise to well defined Penning ionization spectra. In these spectra, we distinguish contributions of the molecules strongly bound to the surface through the thiol group and of molecules weakly bound to the surface, as for example those belonging to multilayers. The beam (whose intensity on the sample is of the order of 1011 at/s with a spot diameter of 1 mm) induces slow rate desorption of weakly bound molecules. Mild annealing to 330 K induces the desorption of weakly bound molecules as well. Cleavage of intra-molecular bonds is observed for annealing temperatures above 420 K. Beam-induced damage of first layer molecules appears not significant on the time scale (tens of seconds) of the MDS experiment. The data demonstrate the full applicability of MDS to the investigation of self-assembled monolayers of small biomolecules.