Resonantly enhanced sum frequency spectroscopy of adsorption on hydrophilic mica substrates

Change log
Lambert, Alexander Geoffrey 

Sum frequency (SF) spectroscopy is a non-linear optical technique that provides vibrational spectra of molecules adsorbed at an interface. The spectra may be analysed to provide the polar orientation, molecular conformation and average tilt angle of the adsorbates to the surface normal. In this work a novel technique is developed to obtain resonantly enhanced SF spectra of molecules adsorbed on a hydrophilic mica surface backed with gold. The enhancement phenomenon is based on coherent interference between the resonant SF signal from adsorbates at the mica surface and the non-resonant SF signal generated from the displaced gold surface. The first stage of this work considers the experimental aspects of the technique. Methods for freshly cleaving, gold coating and mounting micron thick mica samples are presented. Whilst, the technique is shown to result in substantial resonant signal enhancement and superior signal to noise ratios in comparison to spectra from non-gold backed samples, the resonant lineshapes are found to be dependent on the thickness of the mica sheet. To investigate this thickness dependent phenomenon a model monolayer at the mica/air interface was required and a protocol for reproducibly forming well-ordered octadecylsiloxane monolayers is therefore presented. The essential pre-requisites for reproducibility were found to be the accurate control of the reaction temperature and surface hydration level, a clean reaction environment, fresh octadecyltrichlorosilane and strict control of the degree of bulk hydration. A detailed experimental and theoretical investigation of the effect of mica thickness on the form of the SF spectra is presented. A periodic variation in the resonant lineshapes as a function of mica thickness was determined empirically and concurred with the modelling predictions. These combined studies provide a calibration curve that for a given mica thickness predicts spectral lineshapes for given molecular orientations. This thesis presents a complete and characterised technique which may be used to obtain orientational and conformation information of species adsorbed at hydrophilic surfaces using nanosecond SF spectroscopy.

Doctor of Philosophy (PhD)
Awarding Institution
University of Cambridge