At λ ¼ 6.25 μm (1600 cm
À1 ) the refractive index of GC is equal to ^ n ¼ 2:9 þ i1:2
[52]. In this stratified medium, the intensity of the normal component of the ppolarized IR light (beam convergence 5%) strongly depends on the angle of incidence and reaches a maximum of 2.10 at φ i ¼ 63
. At this angle of incidence the
intensities of the parallel to the substrate surface components of the p- and spolarized light are equal to 0.05 and 0.024, respectively (Fig. 2.11a). These components are much lower than the normal component of MSEFS of the p-polarized light,
indicating that GC fulfills the surface selection rule and is applicable in IRRAS
experiments. In a cell composed of air|CaF 2 |H 2 O|GC at λ ¼ 4.34 μm (2300 cm
À1 )
the normal component of the electric field vector of the p-polarized IR light reaches
maximum at φ i ¼ 63
[51]. On the GC surface MSEFS is enhanced by a factor of
2 (Fig. 2.11a) whereas the reflection of the IR light from metallic surfaces leads to
4 to ten fold enhancement of the electric field vector of the normal component
p-polarized light (Table 2.1 and Fig. 2.5). A small enhancement of the normal
component of the electric field vector of the p-polarized light reflected from the
GC surface limits its application in IRRAS for studies of either strongly absorbing
the IR radiation thin molecular films or thick molecular films (e.g. polymer films)
[38, 48, 51]. In spectroelectrochemical experiments the electrolyte layer thickness
should range from 0.5 to 2 μm.
2.3.2 Selection of the Optical Window Material
Selection of the prism material for in situ IRRAS experiments has a large practical
importance, because it may introduce some limitations to the studied system. CaF 2 is
the most often used prism material in in situ IRRAS experiments [53, 54]. CaF 2
transmits the electromagnetic radiation in the wavelength range of 180 nm to 8 μm.
Thus, it can be used to detect IR absorption modes which absorb the IR light at e ν >
1200 cm
À1 . A large number of organic molecules have functional groups which
absorb the IR light in the 1300–1000 cm
À1 spectral region (e.g. epoxy, phosphate,
sulphate or C–O groups). The IR absorption modes of these groups have to be
analyzed using other than CaF 2 window materials. BaF 2 has similar optical properties to CaF 2 (Table 2.1), however its transmission window is larger in the IR spectral
region (up to 11 μm, e ν > 900 cm
À1 ). To prevent the dissociation of BaF 2 , the
presence of the F
À ions in the electrolyte solution is required. However, the presence
of F
À ions may affect the activity and structure of molecular films (e.g. proteins
assemblies). ZnSe provides a wide transmission spectral region (600 nm À16 μm)
and is stable in aqueous solutions. ZnSe displays the larger enhancement of MSEFS
at the mirror surface (Table 2.1 and Fig. 2.5) [3, 55]. However, at the φ i close to
30 (maximum MSEFS) the s-polarized light is strongly reflected from the ZnSe|
water interface, while the p-polarized light is transmitted into the aqueous phase.
Due to significant differences in reflectivity between the p- and s-polarized light, the
24
2 Polarization Modulation Infrared Reflection Absorption Spectroscopy: From. . .
À1 ) the refractive index of GC is equal to ^ n ¼ 2:9 þ i1:2
[52]. In this stratified medium, the intensity of the normal component of the ppolarized IR light (beam convergence 5%) strongly depends on the angle of incidence and reaches a maximum of 2.10 at φ i ¼ 63
. At this angle of incidence the
intensities of the parallel to the substrate surface components of the p- and spolarized light are equal to 0.05 and 0.024, respectively (Fig. 2.11a). These components are much lower than the normal component of MSEFS of the p-polarized light,
indicating that GC fulfills the surface selection rule and is applicable in IRRAS
experiments. In a cell composed of air|CaF 2 |H 2 O|GC at λ ¼ 4.34 μm (2300 cm
À1 )
the normal component of the electric field vector of the p-polarized IR light reaches
maximum at φ i ¼ 63
[51]. On the GC surface MSEFS is enhanced by a factor of
2 (Fig. 2.11a) whereas the reflection of the IR light from metallic surfaces leads to
4 to ten fold enhancement of the electric field vector of the normal component
p-polarized light (Table 2.1 and Fig. 2.5). A small enhancement of the normal
component of the electric field vector of the p-polarized light reflected from the
GC surface limits its application in IRRAS for studies of either strongly absorbing
the IR radiation thin molecular films or thick molecular films (e.g. polymer films)
[38, 48, 51]. In spectroelectrochemical experiments the electrolyte layer thickness
should range from 0.5 to 2 μm.
2.3.2 Selection of the Optical Window Material
Selection of the prism material for in situ IRRAS experiments has a large practical
importance, because it may introduce some limitations to the studied system. CaF 2 is
the most often used prism material in in situ IRRAS experiments [53, 54]. CaF 2
transmits the electromagnetic radiation in the wavelength range of 180 nm to 8 μm.
Thus, it can be used to detect IR absorption modes which absorb the IR light at e ν >
1200 cm
À1 . A large number of organic molecules have functional groups which
absorb the IR light in the 1300–1000 cm
À1 spectral region (e.g. epoxy, phosphate,
sulphate or C–O groups). The IR absorption modes of these groups have to be
analyzed using other than CaF 2 window materials. BaF 2 has similar optical properties to CaF 2 (Table 2.1), however its transmission window is larger in the IR spectral
region (up to 11 μm, e ν > 900 cm
À1 ). To prevent the dissociation of BaF 2 , the
presence of the F
À ions in the electrolyte solution is required. However, the presence
of F
À ions may affect the activity and structure of molecular films (e.g. proteins
assemblies). ZnSe provides a wide transmission spectral region (600 nm À16 μm)
and is stable in aqueous solutions. ZnSe displays the larger enhancement of MSEFS
at the mirror surface (Table 2.1 and Fig. 2.5) [3, 55]. However, at the φ i close to
30 (maximum MSEFS) the s-polarized light is strongly reflected from the ZnSe|
water interface, while the p-polarized light is transmitted into the aqueous phase.
Due to significant differences in reflectivity between the p- and s-polarized light, the
24
2 Polarization Modulation Infrared Reflection Absorption Spectroscopy: From. . .