Helium metastable (He*) deexcitation electron spectroscopy (MDS) allows one to obtain informations about the valence band structure of clean and adsorbate covered surfaces with high surface sensitivity. Further, spin-polarized He* beams provide surface selected information on magnetic properties.
The application of MDS to ultra thin biological films is very attractive because MDS shows well defined UPS-like Penning peaks related to molecular obitals and proved less invasive than photon-in techniques [1].
We will show MDS results obtained on ultra-thin L-Cysteine films grown, in ultra high vacuum conditions, on some noble and transition metal surfaces, i.e. Au(111), Au(110), Cu(100), Ag(100) and Fe(100).
L-Cysteine (Cys, HS-CH2-CH(NH2)-COOH) is a fundamental chiral aminoacid playing a key role in the immobilization of several metallo-proteins on gold. Moreover, investigations on Cys self-assembled monolayers (SAMs) allow one to study the influence of chirality on inter-molecular interactions affecting the SAM long-range order properties [2,3].
During molecular beam deposition, monitored in real time, the rise of well defined Penning peaks demonstrates a similar growth process on Au(110), Cu(100), Ag(100) and Fe(100) surfaces. Assignment of the peak, carried out by comparison with molecular orbital calculations, indicates the formation of a thiolate compact monolayer, followed by adsorption of weakly bound molecules typical of multilayer growth. On the contrary, on Au(111), the behaviour appears significantly different: in fact, already at submonolayer coverage, the measurements demonstrate the simultaneous presence of molecules either strongly or weakly bound to the substrate. These results are consistent with XPS measurements on Au(111) [4].
We will also show first results on spin-polarized MD spectra on Cys films deposited on Fe(100): these spectra show a well defined spin coupling of molecular orbitals to the underneath magnetic substrate.
[1] M. Canepa et al. NIM B, 2007, in press
[2] A. Kühnle et al. Nature 2002, 415, 891
[3] A. Cossaro et al. Langmuir 2006, 22, 11193
[4] L. Lavagnino, V. De Renzi, V. Corradini, R. Biagi ,L. Mattera, U. Del Pennino, M. Canepa in preparation
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