In situ studies on interfacial reactions/films between metal surfaces and gas or liquid phases. The above PM-IRRAS application examples are considered ex situ studies where samples were prepared separately prior to the measurement. Another popular application of the PM-IRRAS technique is in situ studies on interfacial reactions between metal surfaces and gas or liquid phases. In this situation, a kinetic PM-IRRAS operation mode may be utilized if monitoring the entire reaction process in real time is critical. Active in situ PM-IRRAS research work has been conducted, including the specific interactions of gaseous substrates. In this study, the in situ spectra of polycarbonate bisphenol-A and diethyleneglycol adipate coatings exposed to acetonitrile, o-xylene, or dimethyl sulfoxide vapor were measured. The resulting spectra were analyzed to determine structural perturbations of the film induced by the overlying vapor and the nature of the interaction between the film and vapor. The principle advantage of PM-IRRAS is that the spectra are measured in situ. There is no contribution from the organic vapor reactant even though a fairly high concentration is flowing over the sample while the spectra are measured. Further studies were conducted on reaction kinetics that monitor the reversibility of the reaction as the organic vapor pressure changes.
6.5 Polarization Modulation Vibrational Linear Dichroism (PM-VLD) 6.5.1 Introduction The absorption of infrared radiation by an oriented sample is sensitive to the state of polarization of the incident radiation with respect to a reference direction. The reference direction can be defined by the stretching direction of a polymer, the director of a liquid crystal or a monolayer film, the polarization direction of a laser in photo-induced orientation studies, or more generally, by the direction of the external constraint that produces the anisotropy in the sample. The anisotropic optical absorption of oriented samples, so-called dichroism, can be analyzed by measuring spectra using light polarized parallel and perpendicular to the fixed reference direction. This type of experiment is called infrared linear dichroism (IRLD), or in a more general sense, vibrational linear dichroism (VLD). The dichroism ratio Rd and dichroic difference ΔA are the common parameters that are used to characterize the degree of optical anisotropy in oriented samples and they are defined by Equations (6-8) and (6-9) respectively:
where A‖ and A⊥ represent, respectively, the integrated absorbance of the investigated band measured with the radiation polarized parallel and perpendicular to the reference direction.
VLD measurement for a selected absorption band of oriented samples involves the determination of either the dichroic ratio or the dichroic difference. This measurement can be achieved using static polarization or modulated polarization. In the case of using static polarization, the direction of the electric field vector is selected by a linear polarizer. Two independent spectra are necessary to calculate the dichroic ratio or the dichroic difference.
未来三天无暇上网，先发上来。 For samples showing uniaxial symmetry with respect to a reference axis, the recording of the polarized spectra at normal incidence is the most straightforward way to measure the VLD. The two spectra can be obtained by a 90° rotation of either the polarizer or the sample. In order to avoid any environmental perturbation in the sample compartment of the spectrometer, the rotation of the polarizer or of the sample can be motorized and controlled directly by the spectrometer computer. For the first case (rotation of the polarizer), the same part of the sample is illuminated by the infrared beam to record the paralled and perpendicular spectra. This method, however, requires the recording of two reference spectra, one for each polarization direction, since the beamsplitter of FT-IR instruments is polarization sensitive. In the second case (rotation of the sample), only one reference spectrum is required but it is important to make sure that the sample is perfectly homogeneous, since different areas could be illuminated as the sample is rotated. For both case, the polarizer should be the last optical element before the sample or the first one after the sample to avoid depolarization by other elements. 6.5.2 VLD Measurement by Polarization Modulation Dichroism measurements using static polarization require the sequential recording of two different spectra to obtain the polarized absorbance paralled and perpendicular to the reference direction. However, A‖ and A⊥ are subject to instrumental and sample fluctuations occurring between the two measurements. This static method of measuring vibrational linear dichroism lacks sensitivy for the determination of small dichroic effects. Moreover, the need for the separate collection of the A‖ and A⊥ spectra increase total measurement time. To overcome these drawbacks of the static approach, the polarization modulation technique is used to measure the linear dichroism of oriented samples with high sensitivity. The sensitivity of this method comes from the fact that the dichroic difference spectrum is measured directly by modulating the polarization of the incident infrared beam at high frequency with a PEM. Thus, dichroic difference spectra can be obtained in a few second, and the corresponding kinetic measurements can be collected over a short time scale. The optical setup for VLD is shown in Figure 6.15. At the output of the interferometer, the infrared beam presents an intensity modulation at Fourier frequency fF=2vσ, typically between 0.1 to 2kHz, depending on the IR wavenumber (σ) and the velocity (v) of the interferometer moving mirror. The beam is then polarized with a linear polarizer and passes through a PEM-90. The PEM, as described in Section 6.2, consists of an infrared transparent material (zinc selenide, calcium fluoride, or silicon) coupled to piezoelastic drivers, and modulates the polarization of the infrared beam between the parallel and perpendicular polarization states with respect to the reference direction at a fixed frequency 2fPEM (100kHz). After transmission through the sample, the double-modulated infrared beam is focused with a lens on an infrared detector.
where C is a constant accounting for the transmittance of the optical setup and for the detection yield, I0(ω) is the light intensity after the polarizer, T‖ and T⊥ are the parallel and perpendicular transmittance of the sample, respectively, Jn is the Bessel function of order n, φ0 is the maximum dephasing introduced by the PEM.
The detected signal is thus the sum of DC [first two terms in Equation (6-10)] and AC [last term in Equation (6-10)] signals. The DC signal carries mainly the intensity modulation induced by the moving mirror of the interferometer [the term containing J0(φ0) is relatively small in total DC signal], whereas the AC term contains the polarization modulation induced by the PEM. The two signals are separated into two different channels by means of an electronic filter with high- and low-pass outputs. The AC signal is demodulated by a lock-in amplifier or a dedicated demodulator, at twice the frequency of the PEM (i.e. 2fPEM). Both DC and AC signals are treated as usual with high- and low-pass filtering and amplification. They are simultaneously sampled and converted by two analog-to-digital converters. Eventually, the two channel interferograms are Fourier-transformed and then ratioed to give the VLD raw spectrum (s) of the sample without calibration:
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