The new in situ optical technique of electrochemically modulated surface plasmon resonance is described and applied to the measurement of the electrostatic fields inside noncentrosymmetric zirconium phosphate films. In situ EM-SPR measurements on noncentrosymmetric ZP films yield a value for the change in electric field strength of 4 X 103 V/cm for a change in electrode potential of +/- 25 mV. This electric field strength value indicates that there is substantial ion penetration into the film in the electrochemical environment. Both the phase and magnitude of the surface optical response in the EM-SPR measurements are used to distinguish the molecular and metal electrode contributions to the overall optical signal. These two EM- SPR contributions are identified and separated in a quantitative fashion through a series of theoretical Fresnel calculations.
The technique of polarization modulation Fourier transform infrared (PM-FTIR) spectroscopy is used to obtain grazing incident angle differential reflectance spectra of molecular monolayers and multilayers adsorbed onto gold substrates. These spectra are obtained by the photoelastic modulation of the FTIR beam polarization and a novel real-time sampling methodology that generates the average and differential FTIR interferograms from measurements of the infrared signal during each modulation cycle. In comparison with conventional electronics which utilize a lock-in amplifier, the real-time electronics permit the operation of the FTIR spectrometer at normal mirror velocities, and without the need for a background spectrum. We have applied PM-FTIR spectroscopy to study of the modification of alkanethiol monolayers via amide and ester formation reactions.
The technique of optical second harmonic generation (SHG) is applied to the measurement of molecular adsorption at the interface between two immiscible electrolyte solutions (ITIES). The resonant second harmonic response from 2-(N-octadecyl)aminonaphthalene-6-sulfonate (ONS) is used in conjunction with interfacial tension measurements to optically determine the relatively surface coverage of the anionic surfactant molecule at a charged water- dichloroethane interface. At a pH of 9, ONS adsorption occurs at all potentials positive of the potential of zero charge. The potential dependent adsorption of ONS can be described by a Frumkin isotherm with a free energy of adsorption that varies linearly with applied potential. The potential dependence of the SHG from the interface provides important information on the position of the adsorbed ONS molecules with respect to the ITIES. At a pH of 3, both the anionic form of ONS and the protonated zwitterionic form of ONS are present at the liquid- liquid interface.
The nonlinear optical process of second harmonic generation (SHG) is an inherently surface sensitive technique for studying the interface of two centrosymmetric media. This surface selectivity has led to its application as an in situ probe of chemisorption, molecular orientation, and adsorbate organization at interfaces. This paper describes the use of resonant SHG to measure molecular adsorption and orientation at solid-air, solid-liquid, and liquid-liquid surfaces. The polarization and phase dependence of the resonant SHG from molecules at these surfaces can be related to the average molecular orientation at the interface. Perturbation theory calculations using pi-electron wavefunctions are used to identify the molecular nonlinear polarizability tensor elements required in the orientation calculation.
The sensitivity of an FT-IR spectrometer for measuring monolayers and other thin films can be enhanced if one takes advantage of the surface selection rule to reduce the dynamic range of the experiment. This is accomplished in polarization modulation infrared reflection-absorption (PM-IRRAS) by rapid modulation (74 KHz) of the linear polarization of the incident light , and subsequent demodulation of the detector signal. PM-IRRAS yields a differential signal which contains only the surface information. This differential signal is usually ratioed against the total reflectance signal to yield the spectrum of interest. Normally, two sequential measurements are required. First, the differential signal is acquired, then the total reflectance measurement is made. However, a better method is to use the dual channel approach first described by Buffeteau, et. al.1 The principal advantage of this method is that all isotropic (i.e. atmospheric and nonsurface) absorptions are perfectly ratioed.
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