Circular dichroism in valence band photoemission of self- assembled chiral molecules on surface: alaninol on Cu(100)

Adsorption of molecules on surfaces is a well established field with application in optoelectronics, non linear optics, sensor technology and molecular recognition. In particular systems showing surface chirality attract special attention because of their special selectivity opening important applications for enantioselective reactions in catalysis and sensors fields. This contribution reports on the experimental results obtained on a self-assembled monolayer of alaninol on Cu(100) grown in ultrahigh vacuum (UHV) conditions and studied by low energy electron diffraction (LEED), scanning tunnelling microscopy (STM) and circular dichroism in valence band photoemission. The LEED investigation displays an ordered phase with a clear diffraction structure for D- and L- alaninol on Cu(100). The molecules arrange in a square structure represented in matricial form as (1,4|4,-1) and (-1,4|4,1) aligned 14° degrees clockwise and counter clockwise apart from [011] direction of the copper surface for D- and L-alaninol, respectively. Chiral template induced by adsorption has formed since the molecular growth direction destroys all the symmetry elements of the underlying metal, leading to a creation of a chiral surface that is non super-imposable on its mirror image. STM investigation clarify that the self-assembled structure is homogenously distributed on the surface and that only one domain is present on the sample and show that the diffraction cell contains four protrusions that can be interpreted as formed by four molecules. A fractional coverage of one molecule for seventeen copper surface metal atoms is obtained combining LEED and STM results. The asymmetry in circular dichroism of the angle resolved valence band for alaninol on Cu(100) were measured by changing the helicity of the light for the two systems D- and L-alaninol on Cu(100). Clear dichroic effects have been detected in the photoelectron spectra; the sign of the dichroism is opposite for the two enantiomers.