Single polarization modulation

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]. 

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],...

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... 

Polarization modulation reflection absorption infrared spectroscopy (PM-RAIRS) was employed to follow the reaction of CO, C2H4 and CO/C2H4 with microcrystalline ]Pd(Me)(OTf)(dppp)] deposited onto a gold coated wafer. Single insertion steps were observed by alternately exposing the catalyst precursor to low CO (500-333 mbar) and ethene (333 mbar) pressures (Figure 7.12). 

Bell (1981) (see also Bell and Bickel, 1981) measured all matrix elements for fused quartz fibers of a few micrometers in diameter with a photoelastic polarization modulator similar to that of Hunt and Huffman (1973) the HeCd (441.6 nm) laser beam was normal to the fiber axes. Advantages of fibers as single-particle scattering samples are their orientation is readily fixed and they can easily be manipulated and stored. Two of the four elements for a 0.96-jtim-radius fiber are shown in Fig. 13.16 dots represent measurements and solid lines were calculated using an earlier version of the computer program in Appendix C. Bell was able to determine the fiber radius to within a few tenths of a percent by varying the radius in calculations, assuming a refractive index of 1.446 + iO.O, until an overall best fit to the measured matrix elements was obtained. 

Equations (16) and (17) describe second-harmonic generation (SHG) and third-harmonic generation (THG) of one laser beam with a single polarization. Self-phase modulation (SPM) of a single laser beam is described in Eq.(18) as e.g. employed in z-scan experiments [6]. Equation (19) is the cross-phase modulation (XPM) process between two laser beams and Eq.(20) describes the four wave mixing with degenerate frequencies (DFWM). 

Ha T, Enderle T, Chemla D S, Selvin P R and Weiss S 1996 Single molecule dynamics studied by polarization modulation Phys. Rev. Lett. 77 3979-82... 

Ha, T, Enderle, T, Chemla, DS, Selvin, PR, and Weiss, S, Single molecule dynamics studied by polarization modulation.P/iysicn/Revien Ietrers77(1996) 3979-3982. 

In combination with polarization modulation, synchrotron IR microspectroscopy of single spruce fibers Picea abies L.) provided information on hydrogen bonding, orientation, and accessibility of structural units to heavy water and dimethylacetamide applied in a custom-built microfluidic cuvette [106]. A tabulated overview about the assignments of hydrogen bonds and their orientation is also provided by the authors. 

Lasers ROA requires laser powers of 100 to 1000 mW at the sample, with the upper limit determined by what a sample can stand without serious degradation rather than by what would be desirable for the experiment. In view of the light losses by polarizers, modulators, beam splitters, lenses and mirrors, about 1.5 W of single line output power from the laser is desirable. 

---