Photo-elastic modulator

Polarization modulation ellipsometers use a photo-elastic modulator to modulate the state of polarization of the incident beam. Polarizer and analyzer are fixed during the measurement. Eourier analysis of the time dependent signal gives the ellipse-... 

Both these concerns were addressed by the development of modified IR techniques. In the technique of Subtractively Normalised Fourier Transform IR Spectroscopy (SNIFTIRS) or Potential Difference IR (SPAIRS or PDIR) [37], the increased stability and sensitivity of Fourier Transform IR is exploited, allowing usable spectra to be obtained by simple subtraction and ratioing of spectra obtained at two potentials without the need for potential modulation or repeated stepping. A second technique which does not call for potential modulation, but actually modulates the polarisation direction of the incoming IR beam is termed Photo-elastically Modulated Infra-Red Reflectance Absorption Spectroscopy (PM-IRRAS) this was applied to the methanol chemisorption problem by Russell and co-workers [44], and Beden s assignments verified, including the potential-induced shift model for COads. 

PM-IRRAS exploits the different attenuation of s- and p-polarized light by adsorbed species at a reflective (electrode) surface to annul the unchanging contributions to the infrared signal at the detector from the solvent, window, and so on, and produces an absolute rather than difference spectrum at a particular potential. In this approach, a photo-elastic modulator is employed to modulate the polarization state of the incident infrared ray between s- and p-states. On the basis of Greenler s theory [81, 82], this polarization modulation gives rise to an AC signal at the detector, which is proportional (/p —7s)-the difference in intensity of the two polarizations. Since, in principle, /p is absorbed... 

A typical experimental configuration is shown in Figure 8.1b. The polarization of a He-Ne laser is modulated photo-elastically at 50 kHz by a quartz plate, and the laser is then directed onto the surface of the liquid in the OFC at the Brewster angle, 0b. The reflected beam is detected by a photomultiplier tube and lock-in amplifiers extract the signals at 50 and 100 kHz. is recorded every second, and an average over 50 readings is calculated. Technical details and examples of the application of ellipsometry to dynamic liquid surfaces can be found in the literature. 

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