Polarized reflectance

The external reflection of infrared radiation can be used to characterize the thickness and orientation of adsorbates on metal surfaces. Buontempo and Rice [153-155] have recently extended this technique to molecules at dielectric surfaces, including Langmuir monolayers at the air-water interface. Analysis of the dichroic ratio, the ratio of reflectivity parallel to the plane of incidence (p-polarization) to that perpendicular to it (.r-polarization) allows evaluation of the molecular orientation in terms of a tilt angle and rotation around the backbone [153]. An example of the p-polarized reflection spectrum for stearyl alcohol is shown in Fig. IV-13. Unfortunately, quantitative analysis of the experimental measurements of the antisymmetric CH2 stretch for heneicosanol [153,155] stearly alcohol [154] and tetracosanoic [156] monolayers is made difflcult by the scatter in the IR peak heights. 

Fig. IV-13. Example of a p-polarized reflection spectrum from Ref. [154] for a stearyl alcohol monolayer on water. The dashed line is the baseline to be subtracted from the spectra. [Reprinted with permission from Joseph T. Buontempo and Stuart A. Rice, J. Chem. Phys. 98(7), 5835-5846 (April 1, 1993). Copyright 1993, American Institute of Physics.]...

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. 

It is noteworthy from Table 4.11 that omc and 7Tmc bonds differ in their polarizations, with omc polarized slightly more toward carbon and 7Tmc exhibiting either no polarization or a slight preference for the metal. The 7Tmc bond polarization reflects... 

UV data for carbazole and its substituted and reduced derivatives have been tabulated (71PMH(3)115). More recently, solution and solid-state polarized reflection spectra for carbazole have been subjected to detailed analysis and the observed states assigned with the aid of calculations by the RPA methodology (76BCJ3382). 

Light polarization Reflection Surface plasmon resonance Diode arrays... 

Figure 3. An optical arrangement for measuring polarized reflection-absorption spectra of a monolayer or thin film on the surface of a minor S at near grazing incidence. P is a polarizer, Ml, M2, M3 and M4 are mirrors.

The IR reflectivity measurements were performed on single crystals of 2 0.5 0.3 mm3 in size. A FT-IR Perkin-Elmer 1725X spectrometer equipped with microscope and a helium cryostat was used. Polarized reflectivity spectra (R(ro)) were measured from the conducting plane in two principal directions. Optical conductivity a(co) was obtained by Kramers-Kronig transformation. 

Figure 2.. Polarized reflectivity spectra of p-BEDO-TTF)5[CsHg(SCN)4]2 for E 1 L and E L at 300, 200, 100 and 10 K. (L is BEDO-TTF stack direction). The fit with Drude-Lorenz model for T=10 K is shown by thin solid line.

The behaviour of the polarized reflectivity and optical conductivity spectra of new quasi-two-dimensional organic conductor p -(BEDO-TTF)5[CsHg(SCN)4]2 versus temperature for E L and E1. L are quite different. For E . L, the temperature changes of R(ro) and ct(co) are due to the decrease of the optical relaxation constant of the free carriers as expected for a metal. For E L at temperatures below 200 K, the energy gaps in the ct(co) spectra at about 4000 cm 1 and at frequencies below 700 cm 1 appear simultaneously with the two new bands of ag vibrations of the BEDO-TTF molecule activated by EMV coupling. This suggests a dimerization of the BEDO-TTF molecules in the stacks, which leads to a metal-semiconductor transition.. In the direction perpendicular to L, the studied salt shows metallic properties due to a very favourable overlap of the BEDO-TTF molecular orbitals. 

A typical experimental ellipsometer is illustrated in Figure 3.7. Monochromatic light, typically from a continuous wave laser, e.g. a He-Ne laser, is plane polarized (the angle of polarization is given by p) and impinges on a surface. A compensator is then used to convert the elliptically polarized reflected beam to a plane polarized beam (with a being the angle of polarization). The analyzer then determines the... 

Figure 2.9. Detail of the 0-0, b-polarized reflectivity at 5 K (cf. Fig. 2.8). The arrow indicates the threshold of creation of 46-cm phonons. Part A is due to reflection from the front face alone, part B to the total reflectivity of incoherent contributions from front and back faces, and part C to the reflectivity resulting from coherent superposition of front and back faces (oscillations).

To conclude, regions B and C may show absorption-induced structures, especially thermally activated absorptions (hot bands). The diminution of this activated absorption causes the transition from region A to B in Fig. 2.9. Region B + C is a region of impurity, X-trap, or other spurious absorptions 41 it is unusable for quantitative analysis of the exciton phonon or polariton -phonon intrinsic relaxation mechanisms we investigate below. Therefore, our analysis will be concerned only with region A of the b- and a-polarized reflection spectra as the best candidates of a KK analysis. 

Basically, in general PM-IRRAS experiment on any sample with polarized reflectances Rp and Rs, the signal at the detector output can be electronically split into a first part carrying only the intensity modulation induced by the moving mirror of the Fourier transform infrared (FTIR) spectrometer ... 

The electronic properties of organic conductors are discussed by physicists in terms of band structure and Fermi surface. The shape of the band structure is defined by the dispersion energy and characterizes the electronic properties of the material (semiconductor, semimetals, metals, etc.) the Fermi surface is the limit between empty and occupied electronic states, and its shape (open, closed, nested, etc.) characterizes the dimensionality of the electron gas. From band dispersion and filling one can easily deduce whether the studied material is a metal, a semiconductor, or an insulator (occurrence of a gap at the Fermi energy). The intra- and interchain band-widths can be estimated, for example, from normal-incidence polarized reflectance, and the densities of state at the Fermi level can be used in the modeling of physical observations. The Fermi surface topology is of importance to predict or explain the existence of instabilities of the electronic gas (nesting vector concept see Chapter 2 of this book). Fermi surfaces calculated from structural data can be compared to those observed by means of the Shubnikov-de Hass method in the case of two- or three-dimensional metals [152]. 

The absorption IR spectra of the organic conductors, of both ion-radical salts and charge-transfer (CT) complexes, created by a given electron acceptor with various donors, share plenty of characteristic features. In addition to rather narrow and weak bands characteristic of the donor D and acceptor A molecules, a few novel absorption bands appear. They are polarized in the plane perpendicular to that of D and A molecules, broad and very intensive. The presence of such unusually polarized bands can be accounted for by the activation of totally symmetric donor or acceptor vibrations resulting from e-mv coupling. Typical polarized reflection spectra of the triethylammonium (TEA) (TCNQ)2 salt for three light polarizations [18] are shown in Fig. 1. It is fascinating that the reflectivity for... 

Measurements of the polarized reflectance in the NIR have frequently been used to obtain estimates for the transfer integrals. The method consists in fitting a reflectance model based on the Drude expression [Eq. (1)] to the experimental data. The Drude expression should be considered as a tool in estimating the plasmon frequency, ftp the background dielectric constants, e0 plasma frequency, (op and so on. The validity of the Drude analysis is limited to the conducting organic materials, with the electrical conductivity not less than a few S cm-1. 

TTF type. The optical anisotropy of such two-dimensional conductors and their electron parameters may also be deduced from reflectance studies. As an example, from the (TMTSF)2X family we present the polarized reflectance of (TMTSF)2PF6 at three temperatures (Fig. 7). It is evident that optical anisotropy decreases at low temperature, and a reasonably well-defined plasma edge appears in the b direction at 25 K. The transverse reflectance edge appears at the frequency about 10 times lower than that of the stacking axis edge (tb< = 22 meV, about 10 times smaller than ta) [46]. Drude parameters for typical (TMTSF)2X salt are eq = 3.5, 1500 cm-1 < cop < 2000 cm-1, 250 cm-1 < y < 500 cm-1, and tb = 0.02 eV. 

The BEDT-TTF trihalides and the related salts attract much attention because of a relatively high superconducting transition temperature. Figure 8 shows the polarized reflectance of a- and (3-(BEDT-TTF)2I3 crystals for two light polarizations. For both phases the electronic reflection bands with a Drude-like edge are observed in two perpendicular polarizations [47]. Drude parameters and transfer integrals of typical (BEDT-TTF)2X salts are 5000 cm-1 < top < 9600 cm-1, 500 cm-1 < y < 2000 cm-1, and 0.08 eV < t < 0.20 eV. Near isotropy of the optical properties of typical BEDT-TTF salts is confirmed by electrical transport studies. Rather small values of t are consistent with relatively low room-temperature conductivity. 

Figure 7 Polarized reflectance of (TMTSF)2PF6 at 300, 100, and 25 K. The solid lines are Drude fits. (From Ref. 46.)...

Figure 8 Polarized reflectance of (BEDT-TTF)2I3 single crystals (a) a-phase —, Elb —E b (b) (5-phase —, E d —Eld —, calculated. (From Ref. 48.)...

Recently, the trimer theory has been used for the interpretation of the optical properties of the. Ymethylthiouronium salt [(MT)2(TCNQ)3 2H2Oj [67,68], The dominant feature of the polarized reflection spectrum cf (MT)2(TCNQ)3 2H20 (Fig. 13) is a broad intensive band of electronic reflection, with a sharp edge and low minimum at the frequency co = 9090 cm The intensive structure observed in the middle IR range (lines lto 8) is attributed to the e-mv coupling. Lines 2 and 4 have a fine structure which could be understood if one takes into account the equilibrium charge density shift pb = 0.25e and 3g = 0.15< in the two halves of TCNQ ( 3). 

Figure 13 Polarized reflectance of (MT)2(TCNQ)3 2HzO for two polarizations (a) and calculated spectrum for if c (b). (From Ref. 67.)...

A detailed study between 80 and 300 K of the temperature dependence of the polarized reflection spectra of TEA(TCNQ)2 single crystals has been performed quite recently by Olejniczak and Graja [52]. The observed thermal evolution of the spectra is consistent with a wide semiconductor-to-semiconductor transition centered at about 220 K. The transition is... 

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