Modulation, polarization

The behavior of the output beam depends on the amplitude of the PZTs, since this amplitude determines the change in refractive index along the stress axis. At a certain amplitude, the maximum phase retardation for a particular wavenumber (vo) is 90° and the beam emerging from the ZnSe plate becomes circularly polarized. The handedness of the circularly polarized radiation switches sinusoidally from left to right with a frequency equal to the frequency at which the ZnSe plate is being extended and compressed. (It is this property that is used for the measurement of VCD spectra.) For most other wavenumbers in the spectrum, the radiation is elliptically polarized, with the ellipticity increasing with the difference between 

When the amplitude of the motion of the PZTs is doubled relative to the case discussed above, the phase retardation is 180° and the plane of polarization of the output beam from the PEM at A,q changes by 90° at the extreme of the modulation cycle. When there is no tension on the ZnSe plate, the plane of polarization of the output beam is the same as that of the input beam. At the extremes of the extension and compression cycles, the plane of polarization is rotated by 90°. At this amplitude, therefore, the plane of polarization is modulated between s and p at a frequency of 

---

On metals in particular, the dependence of the radiation absorption by surface species on the orientation of the electrical vector can be fiilly exploited by using one of the several polarization techniques developed over the past few decades [27, 28, 29 and 30], The idea behind all those approaches is to acquire the p-to-s polarized light intensity ratio during each single IR interferometer scan since the adsorbate only absorbs the p-polarized component, that spectral ratio provides absorbance infonnation for the surface species exclusively. Polarization-modulation mediods provide the added advantage of being able to discriminate between the signals due to adsorbates and those from gas or liquid molecules. Thanks to this, RAIRS data on species chemisorbed on metals have been successfidly acquired in situ under catalytic conditions [31], and even in electrochemical cells [32]. 

Ishida FI, Ishino Y, Bui]s FI, Tripp C and Dignam M J 1987 Polarization-modulation FT-IR refleotion speotrosoopy using a polarizing Miohelson interferometer App/. Spectrosc. 1288-94... 

Earner B J, Green M J, Saez E I and Corn R M 1991 Polarization modulation Fourier transform infrared... 

Higgins D A, Vanden Bout D A, Kerimo J and Barbara P F 1996 Polarization-modulation near-field scanning optical microscopy of mesostructured materials J. Chem. Phys. 100 13 794-803... 

RAIRS is a non-destructive infrared technique with special versatility - it does not require the vacuum conditions essential for electron spectroscopic methods and is, therefore, in principle, applicable to the study of growth processes [4.270]. By use of a polarization modulation technique surfaces in a gas phase can be investigated. Higher surface sensitivity is achieved by modulation of the polarization between s and p. This method can also be used to discriminate between anisotropic near-sur-face absorption and isotropic absorption in the gas phase [4.271]. 

Fig.4.53. Experimental and simulated PM (polarization modulated) IRRAS spectra of single monolayers of (A) PEG and (B) K(LK)7 at the air-water interface. The surface pressure was 20 mN m [4.281],...

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-... 

Recent work in our laboratory has shown that Fourier Transform Infrared Reflection Absorption Spectroscopy (FT-IRRAS) can be used routinely to measure vibrational spectra of a monolayer on a low area metal surface. To achieve sensitivity and resolution, a pseudo-double beam, polarization modulation technique was integrated into the FT-IR experiment. We have shown applicability of FT-IRRAS to spectral measurements of surface adsorbates in the presence of a surrounding infrared absorbing gas or liquid as well as measurements in the UHV. We now show progress toward situ measurement of thermal and hydration induced conformational changes of adsorbate structure. The design of the cell and some preliminary measurements will be discussed. 

Kunimatsu K, Golden WG, Seki H, Philpott MR. 1985a. Carbon monoxide adsorption on a platinum electrode studied by polarization modulated FT-IRRAS. 1. Co Adsorbed in the double-layer potential region and its oxidation in acids. Langmuir 1 245 -250. 

Kunimatsu K, Seki H, Golden WG, Gordon JG II, Philpott MR. 1985b. Electrode/electrolyte interface study using polarization modulated FTIR reflection-adsorption spectroscopy. Surf Sci 158 596-608. 

Kunimatsu K, Aramata A, Nakajima N, Kita H. 1986. Infrared spectra of carbon monoxide adsorbed on a smooth gold electrode Part II. EMIRS and polarization-modulated IRRAS study of the adsorbed CO layer in acidic and alkaline solutions. J Electroanal Chem 207 293-307. 

Polarization modulation can be done by rotating the polarizer, as is done in rotating-element ellipsometers [339], or by using photoelastic devices [337]. Data acquisition nowadays is fast, which makes real-time measurements of film and interface formation possible a full spectrum ranging from 1.5 to 5 eV can be measured in less than a second [340-342]. 

In applying RAIRS to CO adsorption, the contribution from CO molecules in the gas phase to the absorption spectrum at CO pressures above 10-3 mbar completely obscures the weak absorption signal of surface adsorbed CO. Beitel et al. found it possible to subtract out the gas phase absorption by coding the surface absorption signal by means of the polarization modulation (PM) technique applied to a conventional RAIRS spectrometer, p-polarised light produces a net surface electric field which can interact with adsorbed molecules, whereas both polarization states are equally sensitive to gas phase absorption because gas phase molecules are randomly oriented. By electronic filtering a differential spectrum is computed which does not show contributions from the gas phase and which has much higher surface sensitivity than a conventional RAIRS setup. 

A more complex but faster and more sensitive approach is polarization modulation (PM) IRLD. For such experiments, a photoelastic modulator is used to modulate the polarization state of the incident radiation at about 100 kHz. The detected signal is the sum of the low-frequency intensity modulation with a high-frequency modulation that depends on the orientation of the sample. After appropriate signal filtering, demodulation, and calibration [41], a dichroic difference spectrum can be directly obtained in a single scan. This improves the time resolution to 400 ms, prevents artifacts due to relaxation between measurements, and improves sensitivity for weakly oriented samples. However, structural information can be lost since individual polarized spectra are not recorded. Pezolet and coworkers have used this approach to study the deformation and relaxation in various homopolymers, copolymers, and polymer blends [15,42,43]. For instance, Figure 7 shows the relaxation curves determined in situ for miscible blends of PS and PVME [42]. The (P2) values were determined... 

I. Noda, A.E. Dowrey and C. Marcott, Characterization of polymers using polarization-modulation infrared techniques Dynamic infrared linear dichroism (DIRLD) spectroscopy. 

The three most commonly applied external reflectance techniques can be considered in terms of the means employed to overcome the sensitivity problem. Both electrically modulated infrared spectroscopy (EMIRS) and in situ FTIR use potential modulation while polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS) takes advantage of the surface selection rule to enhance surface sensitivity. 

In video microscopy, for instance, background is normally subtracted using differential interference contrast (DIC) [18]. This technique, which requires a number of manipulations from the user, may now be automated using a new method called polarization-modulated (PMDIC) [19,20], It requires the introduction of a liquid crystal electro-optic modulator and of a software module to handle difference images. PMDIC has been shown to bring improvements in imaging moving cells, which show a low contrast, as well as thick tissue samples. 

The combination of surface enhanced Raman scattering (SERS) and infrared reflection absorption spectroscopy (IRRAS) provides an effective in-situ approach for studying the electrode-electrolyte interface. The extreme sensitivity to surface species of SERS is well known. By using polarization modulation of the infrared beam for IRRAS, the complete band shape is obtained without modulating the electrode potential. 

Figure lb. Schematic diagram of polarized light used with the polarization modulation technique. (Reproduced with permission from Ref. 9. Copyright 1984 Elsevier.)... 

It can be readily seen from the above discussion that a simplistic version of the polarization modulation method for using Fourier Transform Infrared method would be to say that if one subtracted the message obtained from the vertical light from that obtained from the parallel light, the result will be information from the surface only. The relevant equation is shown below. 

Vibrational spectra were recorded using the polarization-modulated infrared reflection absorbance technique (PM-IRRAS). The spectrometer, the electrochemical cell, and the sample preparation and cleaning procedures are all described elsewhere (1 7) All of the measurements were performed using 0.5 M SO solutions, either with or without an added nitrile compound or SnCl,. The solutions were saturated with CO by bubbling the gas through their storage reservoirs before admitting them into the sample cell. 

A long disputed issue of the nature of strongly bound species in this reaction has been recently revived with the vibrational spectroscopy studies of Bewick et al. (30) using EMIRS technique and of Kunimatsu and Kita (31) using polarization modulation IR-reflection-absorption technique. These data indicated the only CO is a strongly bound intermediate. Heitbaum et al. (32) on the other hand advocate COH, and most recently HCO (33), as the poisoning species on the basis of differential electrochemical mass spectroscopy (DEMS). 

---