Polarized absorption spectra oriented films

Figure 12 (a) Optical absorption of trans-PA and a related oligomer. Solid lines polarized absorptions of an oriented PA film (precursor route), calculated from the reflection spectrum (from Ref. 113). The polymer chains are parallel to c. Dashed line absorption of the hexamer all-trans dodecahexaene, in solution in hexane (from Ref. 114). Dotted line hexamer emission in hexane, (b) Optical absorption and reflectivity of unoriented cis- and trans- A films (Shirakawa type) (from Ref. 116). 

Figure 16 shows a change in the absorption spectrum of the LB film of APT(8-12) with UV (365 nm) and visible (436 nm) photoirradiation. The strong band around 360 nm is due to the trans isomer of azobenzene. The absorption due to the local excitation of TCNQ polarized along the long axis is located at about 315 nm but is indiscernible in this spectrum since the transition moment of this band is oriented almost perpendicular to the film surface and the electric field of the light is parallel to the film surface [149]. 

The decrease of the optical absorption spectrum is due to the incident light polarization in the film plane, whereas molecules orient, under poling, in the direction perpendicular to the thin film surface. From this variation one can determine the order parameter < 2> using the following equation ... 

As described above, the /3-carotene molecules are randomly oriented in the SC film, but the above discussion suggests that the short-range order similar to that in of the single crystal exists in the SC film. Therefore, we can now conclude definitely that the SC film is an amorphous film, and this conclusion supports the assumption by Babaev and Al perovich (1973). As for the LB film, the optical absorption spectrum measured on normal irradiation with light polarized parallel to the x-axis ofthe substrate showed exactly the same spectral pattern with that parallel to they-axis, although the intensifies of these spectra were apparently different. This shows that there is only one transition moment in the mixed LB film. Saito et al. (1991 a,b) reported the optical absorption spectra of various molecular aggregates in the... 

The thin films or coatings can be studied nondestructively, with no sample preparation other than deposition on a polished metal surface if necessary. Specular reflectance has been used to study lubricant films on computer disks, oxide layers on metal surfaces, paint curing as a function of time, and molecules adsorbed on surfaces. For example, the IR absorption spectrum of proteins adsorbed onto a polished gold surface can be studied. This spectrum from an adsorbed layer can form the basis of sensors for compounds that will bind to the proteins and change the spectrum. Use of a polarizer in conjunction with grazing angle analysis can provide information about the orientation of molecules adsorbed onto surfaces. 

Bradbury et al. (1967) have studied partial nucleoproteins produced by removal of histone fractions from nucleohistone. Figure 12.6 shows a polarized infrared spectrum of one of the nucleoproteins oriented by shearing. The spectrum is that of a film deuterated by D2O vapor. The bands of the in-plane vibrations of cytosine and guanine moieties are highly polarized when the electric vector is placed perpendicular to the fiber axis. The 1452 cm absorption is that of the amide IF band (Miyazawa et al., 1956 Miyazawa, 1962) for the readily deuterated fraction of the partial nucleoprotein. As seen in Fig. 12.6, the 1452 cm band is polarized considerably in the direction parallel to the fiber axis, giving evidence that the extended polypeptide chain is situated so that its axis lies between the angle of the groove in the DNA double helix and the axis of the DNA helices (Bradbury et al., 1967). 

Figure 3-9. Infrared absorption spectrum of isotactic polypropylene in the various physical states, (a) Melt (—) and normal solid at room temperature (—) New bands appear associated to regularity bands originating from k = 0 phonons of the chain as ID crystal (b) stretch-oriented film in polarized light showing which of the bands are of A( j (------i or E(X i—) species (from [74]).

One published application of time-resolved asynchronous FT-IR spectrometry involves the reorientation dynamics of ferroelectric liquid crystals induced by the reversal of the external electric field [26,27]. For the study to be described here, the liquid crystals were held in a 2-pm CaFa cell whose windows had been coated with indium tin oxide, which served as the electrode, and with a polyimide film that had been oriented by stroking it in one direction. After the sample had been heated to 90° C to ensure that it was all in the isotropic liquid phase, it was injected into the cell, which had been heated to the same temperature. The cell was then allowed to cool to 40°C at a rate of 1°C min to obtain a homogeneously oriented liquid crystal. An IR polarizer was set at an angle of 45° to the orientation direction of the liquid crystal to obtain the maximum change in the absorption spectrum. Rectangular electric pulses with 20-V peak voltages and a 20-ps period were applied to the cell. 

In ATR-FTIR excitation occurs only in the immediate vicinity of the surface ol the reflection element, in an evanescent wave resulting from total internal reflection. The intensity of the evanescent field decays exponentially in the direction normal to the interface with a penetration depth given by (1.7.10.121, which for IR radiation is of the order of a few hundreds of nm. Absorption leads to an attenuation of the totally reflected beam. The ATR spectrum is similar to the IR transmission spectrum. Only for films with a thickness comparable to, or larger than, the penetration depth of the evanescent field, do the band intensities depend on the film thickness. Information on the orientation of defined structural units can be obtained by measuring the dichroic ratio defined as R = A IA, where A and A are the band absorbances for radiation polarized parallel and perpendicular with respect to the plane of incidence, respectively. From this ratio the second-order parameter of the orientation distribution (eq. [3.7.13]) can be derived ). ATR-FTIR has been extensively used to study the conformation and ordering in LB monolayers, bilayers and multilayers of fatty acids and lipids. Examples of various studies can be found... 

Crystal orientation can have some striking effects on a spectrum. In an oriented crystalline film, for example, a vibrating group may be fixed in such a position that it will not interact with the radiation from the source. Another difficulty is that the incident radiation in a spectrometer is partially polarized, and the relative intensities of absorption bands may change as a crystalline sample is rotated. 

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