Depolarised light

A26. Molecular Orientation in Liquid Diphenyl Ether as Probed by Depolarised Light Scattering. 

One should keep in mind that e. m. waves are transverse vector helds and are polarized. Strictly speaking, one has to consider the polarisation characteristics of the light throughout this treatment. Polarisation plays an important practical role as instrumental depolarisation can change the coherence charac-... 

Figure 12.15 Effectx)r mechanism for a pheromone. The mechanism is similar to that shown in Figure 12.6 except cyclic AMP leads to opening of a Na ion channel which will result in depolarisation and initiation of an action potential. It is of interest that the physiology/biochemistry is opposite to that involved in the detection of light, in which the signalling system results in hyperpolarisation of a nerve ratfer that hypopolarisation (Chapter 15 see Figure 15.10).

Fluorescence depolarisation by energy transfer (rather than rotational relaxation) between donor molecules of the same type can occur. Eisenthal [174] excited solutions of rhodamine 6G (9 mmol dm-3) in glycerol with 530 nm light from a frequency-doubled neodymium laser. The polarisation... 

Fig. 1. Experimental set up (90° collection optics) for the measurement of laser Raman scattering, illustrating the definition of the depolarisation ratio. ly and Ij are the intensities of light scattered, respectively, parallel and perpendicular to the polarisation of the incident exciting beam...

Pi Ii/I for linearly polarised incident radiation. In this case, Eq. (1) must be multiplied by the factor (3/8 rr) (1 + pj)/(l + 2pi). (5a). If only the parallel component of scattered radiation is measured the factor is (3/8 tt) (1 + 2 px) . (I = Pi I ). For naturally polarised (unpolarised) or circularly polarised incident light the relevant factors are given in terms of the depolarisation ratio, p , for natural polarisation, where... 

Fig. 2. Schematic diagram for the activation of smooth muscle through phosphorylation of regulatory light chains of myosin. Upon activation of smooth muscle by membrane depolarisation or activation of a receptor (R) through an agonist (L) the cytosolic Ca " increases due to an influx of Ca through plasmalemmal Ca channels or IP3 mediated release from the sarcoplasmic reticulum (SR). Ca binds to calmodulin (CaM) and activates the myosin light chain kinase (MLCK). Myosin is dephosphorylated by specific phosphatase(s) (MLCP).

Simple systems such as that shown in Fig. 5.11 yield an excellent optical efficiency, and are almost free of pulse dispersion and wavelength-dependent pulse shift. Another benefit is that the detection path is almost free of polarisation effects compared to monochromator-based systems. The high numerical aperture of the light eollection system further reduces the influence of the rotational relaxation see Fig. 5.9. With aspheric lenses an NA of around 1 can be achieved, and at an NA this high the polariser in the detection path can often be omitted. In the setup in the left graphic of Fig. 5.11, residual depolarisation effects can be removed by slightly tilting the polarisation direetion of the laser. 

In the usual Raman experiment, the observations are made perpendicular to the direction of the incident beam, which is plane polarised. The depolarisation ratio is defined as the intensity ratio of the two polarised components of the scattered light which are parallel and perpendicular to the direction of the propagation of the (polarised) incident light. The polarisation of the incident beam is perpendicular to the plane of propagation and observation. For this geometry, the depolarisation ratio is defined as the intensity ratio ... 

Consequently, the Raman scattered light emanating from even a random sample is polarised to a greater or lesser extent. For randomly oriented systems, the polarisation properties are determined by the two tensor invariants of the polarisation tensor, i.e., the trace and the anisotropy. The depolarisation ratio is always less than or equal to 3/4. For a specific scattering geometry, this polarisation is dependent upon the symmetry of the molecular vibration giving rise to the line. 

DLS instruments typically employ vertically polarised light (Lvl Xu 2000, p. 230). Depolarised scattered light (Lh) results from anisometry or multiple scattering—signals are usually very weak. 

One characteristic of the scattered light from hqttid crystals is that it is highly depolarised. That is, if linearly polarised light is incident on the sample, the scattered light has polarisation components perpendicular to the indderrt polarisation direction. The reason for this becomes obvious if we return to orrr discussion on arrisotropy in Chapter 2. If the amplitude of the electric field in the incident light has componerrts... 

Bartoli and Litovitz [6] found values of C of about 0.2% for some typical polarised (A) bands - by making measurements on the 459 cm" band of CCl which has a well-known [18] depolarisation ratio. Calculation of I ((1d) is then straightforward. Polarisation scrambling behind the main entrance slit ensures that the monochromator is equally sensitive to transmission of and lyy scattered light. There have been reports of local heating effects caused by a relatively high powered laser beam. However, we have never found this to be a problem although, of course, it is not possible to physically monitor the microscopic temperature. Stokes/Antistokes intensity ratios which measure the Boltzman population factors (and hence the microscopic temperature) have always corresponded well to the laboratory (bath) temperature even for input powers up to 2w. 

It is often convenient to express a relation in terms of (= 1/6D ), as in Equation (3.47), although r contains no more information than (it must not be supposed to tell us anything about the motion of an individual molecule). This is because it happens that is also the relaxation time determined by measurements of various phenomena — light-scattering, fluorescence-depolarisation, and pulsed nmr — which are important in the determination of diffusion coefficients (Section S.4.2.2). 

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