Fundamentals of PM IRRAS and Experimental Set-up

In PM IRRAS, the incident beam polarization is modulated between two orthogonal directions (a) perpendicular (s-polarization) and (b) parallel (p-polar-ization) to the plane of incidence of the infrared beam. At the metal surface, the electric field of the s-polarized infrared beam almost vanishes, while the electric field of the p-polarized beam is enhanced (see Fig. 9.3). Thus, the first component of the modulated signal is insensitive to the presence of the film of adsorbed molecules and can be used to obtain the background spectrum, while the other component can be utilized to obtain the spectrum of the film. Modulation between s- and p-polarizations is carried out at a high frequency (50 kHz), and the reflection absorption spectrum AR/(R) is obtained as 

A diagram of an experimental set-up used for PM IRRAS experiments is shown in Fig. 9.26 and a photograph of the spectroelectrochemical cell in Fig. 9.27. The set-up is built from components purchased from Newport and custom-machined parts in an external tabletop optical mount (TOM) box. A convergent infrared beam from the spectrometer enters a port of the TOM box where it is deflected by a flat mirror and focused onto the working electrode by a parabolic mirror (f=6 in, Nicolet). Before entering the cell, the beam passes through a static polarizer (diameter 1 in, with an anti-reflective coating - 

Specac) and the photoelastic modulator (PEM) (Hinds Instruments PM-90 with II/ZS50 (ZnSe 50 kHz) optical head). The beam, exiting the cell, is refocused by a ZnSe lens (f= 1 in, diameter 1.5 in, with an anti-reflective coating - Janos Technology) onto an MCT-A detector (TRS 50 MHz bandwidth, Nicolet). A bandpass optical filter (3-4 pm pass, Specac) is placed at the entrance window of the detector when the spectra of the hydrocarbon stretching modes are acquired. 

mount 2 Rl, rail 1 RS, rotation stage OW, optical window TF, Teflon flange  

Teflon holder CL3, clamp 3 GT, glass tube PH, piston handle W, gold wire  

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