Polarization Modulation Spectroscopy

In the continuous-scan mode, a Michelson interferometer generates modulation of the radiation at each wavenumber v with a frequency F = 2Vv, where V is the mechanical velocity of the scanning mirror in centimeters per second. The frequency F is called the Fourier frequency. A typical speed of the scanning mirror is 0.1-10 cm s so that signals within the IR spectral region fall into the 10 -10 -Hz range. Since in most measurement schemes a low-pass filter is used to separate the Fourier and modulation frequencies, the modulation frequency should satisfy the sampling theorem. Specifically, the modulation rate 

The PEM is not achromatic, so modulation of the polarization can only be achieved at one wavelength, determined by the stress voltage applied to the PEM. The operating PEM frequency depends on the crystal (either 37 or 50 kHz for ZnSe), and the polarization modulation is at twice the operating frequency of 

Electronic processing of the output of the IR detector yields a PM-IRRAS signal. For an ulbathin film on a metallic substrate the PM-IRRAS signal is [568, 569] 

In the case of a nonmetal substrate, both Rs and Rp contain contributions from absorption by the film, and interpretation of the PM-IRRAS signal in the form of Eq. (4.6) is less straightforward. Moreover, a dielectric substrate has its own large, specific, and spectrally dependent PM-lRRAS signal [532], To eliminate this contribution, the PM-IRRAS of an ultrathin film at a dielectric substrate is normalized with the PM-IRRAS signal of the clean dielectric [532, 568, 569, 585]  

An important advantage of PM-IRRAS is that information about molecular orientations in ultrathin films on nonmetallic substrates can be obtained by analyzing the direction and intensity of the absorption bands (Section 3.11.4). However, in the general case, the interpretation of the PM spectrum can be rather ambiguous. To overcome this limitation, it was suggested [532] to measure PM-IRRAS only 

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Nafie LA, Buijs H, Rilling A et al (2004) Dual source Fourier transform polarization modulation spectroscopy an improved method for the measurement of circular and linear dichroism. Appl Spectrosc 58 647-654... 

Infrared reflection absorption spectroscopy (polarization modulated spectroscopy with a dispersive spectrometer) has been used in studies of the kinetics of the formation of CuSCN multilayer Aims [248]. An advantage of this method— absolute spectra showing no differential bands, which are difficult to interpret— became obvious in a study of CO adsorption on platinum [222]. With the various neutral electrolyte solutions that are used, significant differences were observed in the results obtained with acidic solutions. The mode of CO adsorbed in the bridged position grows considerably in intensity and the position of the on-top COad is slightly shifted (see Fig. 5.56). 

While ZnSe is a superior window material for potential modulation spectroscopy, this advantage is lost in photon polarization modulation spectroscopy because of significant differences between reflectivity of the s- and p-polarized light at the window/electrolyte interface at angles at which the enhancement of MSEFS for the p-polarized radiation is at maximum. One has to reduce the an-... 

Investigation of the mechanical, adhesion, and electrical behavior of ultrathin polymer films requires spatial resolution of the order of 10-100 °A. Specular reflectance spectroscopy is a most convenient technique for probing structures on either metallic or dielectric surfaces. Furthermore, techniques such as polarization modulation spectroscopy have significantly increased sensitivity and the possibility of photoacoustic spectroscopy is also available. 

Bjc(v) and B v) can be determined by the same procedures as described for doublemodulation spectrometry. However, as isclear from a comparison of Equations (11.1b) and (11.15a), polarization-modulation spectroscopy must use 2/ j as the synchronizing signal for the lock-in amplifier. By using B v) and B v) obtained as described above, the difference between A//(v) and Aj (v) is determined. The second term of Bdc(v) is negligibly... 

Novotny et al. [41] used p-polarized reflection and modulated polarization infrared spectroscopy to examine the conformation of 1 -1,000 nm thick liquid polyperfluoropropy-lene oxide (PPFPO) on various solid surfaces, such as gold, silver, and silica surfaces. They found that the peak frequencies and relative intensities in the vibration spectra from thin polymer films were different from those from the bulk, suggesting that the molecular arrangement in the polymer hlms deviated from the bulk conformation. A two-layer model has been proposed where the hlms are composed of interfacial and bulk layers. The interfacial layer, with a thickness of 1-2 monolayers, has the molecular chains preferentially extended along the surface while the second layer above exhibits a normal bulk polymer conformation. 

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, 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. 

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. 

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. 

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). 

In situ studies of catalytic reactions have also been a prime focus of our group. The high-pressure spectroscopic technique used in our research is polarization modulation IR reflection absorption spectroscopy (PM-IRAS). Like SFG, PM-IRAS is a highly surface-sensitive technique that yields vibrational information about adsorbed surface species. Unlike SFG, however, PM-IRAS... 

In polarization modulated ENDOR spectroscopy (PM-ENDOR)45, discussed in Sect. 4.7, the linearly polarized rf field B2 rotates in the laboratory xy-plane at a frequency fr fm, where fm denotes the modulation frequency of the rf carrier. In a PM-ENDOR experiment the same type of cavity, with two rf fields perpendicular to each other, and the same rf level and phase control units used in CP-ENDOR can be utilized. To obtain a rotating, linearly polarized rf field with a constant magnitude B2 and a constant angular velocity Q = 2 fr (fr typically 30-100 Hz), double sideband modulation with a suppressed carrier is applied to both rf signals. With this kind of modulation the phase of the carrier in each channel is switched by 180° for sinQt = 0. In addition, the phases of the two low-frequency envelopes have to be shifted by 90° with respect to each other. The coding of the two rf signals is shown in Fig. 8. 

This restilt was confimed by other groupsi2.i3, using Fourier transform infrared spectroscopy. Kunimatsu and Kita made further progress using polarization modulation to enable quantitative measurements and showed... 

PM-IRRAS Polarization-modulated infrared reflertion absorption specTroscopy... 

The measurement of vibrational optical activity requires the optimization of signal quality, since the experimental intensities are between three and six orders of magnitude smaller than the parent IR absorption or Raman scattering intensities. To date all successful measurements have employed the principles of modulation spectroscopy so as to overcome short-term instabilities and noise and thereby to measure VOA intensities accurately. In this approach, the polarization of the incident radiation is modulated between left and tight circular states and the difference intensity, averaged over many modulation cycles, is retained. In spite of this common basis, there are major differences in measurement technique and instrumentation between VCD and ROA consequently, the basic experimental methodology of these two techniques will be described separately. 

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). 

Chronocoulometry and photon polarization modulation infrared reflec-tion/absorption spectroscopy have been employed [311] to study the fusion of dimyristoylphosphatidylcholine vesicles onto an Au(lll) electrode. The fusion was controlled either by the electrode potential, or charge. Film characteristics was also potential dependent. After removing the film from the electrode surface (negative potential), phospholipid molecules remained in its close proximity, in the ad-vesicle state. Several electrochemical and nonelec-trochemical methods have been applied [312, 313] to investigate the spreading of small unilamellar vesicles onto Au(lll) electrode. Vesicles fused onto the surface at > —0.5 V (versus SSCE), to form defected bilayers in contact with the metal surface. At more negative potentials, the film was removed from the electrode surface, but it still remained in its close proximity. 

A variant of IRRAS is polarization modulation IR reflection absorption spectroscopy (PM-IRRAS). In this method, the polarization of the IR beam incident on the sample is modulated between parallel and perpendicular polarization. When the sample is metallic, only the parallel-polarized light yields signals from adsorbed molecules, because the electric field amplitude of perpendicular-polarized light vanishes at the metal surface. This statement is the basis for the metal surface selection rule 100,109). When the medium above the sample (gas or liquid phase) is isotropic, both polarizations are equivalent. The PM-IRRAS method thus enables the measurement of signals from adsorbates on a metal surface in the presence of an absorbing gas or liquid phase. 

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