Probing chiral monolayers of cysteine on Au(110) using reflection anisotropy spectroscopy and second-harmonic generation



Chiral synthesis is an important route to biologically useful chemicals. Linear optical techniques like circular dichroism are too weak to detect chirality in monolayers of small molecules at surfaces, unless the surface area is very high and, in addition, the chiral response is annulled in reflection geometry. Chiral second-harmonic generation (SHG) from monolayer quantities of larger, more polarizable organic molecules has been detected at silica surfaces, where the presence of electronic resonances enhances the signal. Non-resonant chiral SHG has also been reported from multilayers of cysteine adsorbed on silica and silicon surfaces, where the strength of the response indicated potential monolayer sensitivity. Here, the chiral response of cysteine monolayers adsorbed on atomically clean, well-ordered Au(110) under ultra-high vacuum conditions is explored. Reproducible differences in the linear and nonlinear optical response from (R)-(+)-cysteine and (S)-(−)-cysteine monolayers are observed. Possible origins of the differences are discussed. Reflection anisotropy spectroscopy and SHG, in principle, provide complementary information on chiral structures at surfaces and interfaces and have potential technological application in the area of heterogeneous enantioselective catalysis.