Excitation by polarized light

When the incident light is vertically polarized, the vertical axis Oz is an axis of symmetry for the emission of fluorescence according to the Curie principle, i.e. Ix = Iy. The fluorescence observed in the direction of this axis is thus unpolarized. 

The components of the fluorescence intensity that are parallel and perpendicular to the electric vector of the incident beam are usually denoted as i and ij, respectively. For vertically polarized incident light, Iz = I) and Ix = Iy = I . 

The intensity component Iz corresponding to oscillations of the electric field along the Oz axis cannot be detected by the eye or by a detector placed along this axis. The fluorescence intensity observed in the direction of this axis is thus Fx T Fy 2Fj.  

In the expression of the polarization ratio, the denominator represents the fluorescence intensity in the direction of observation, whereas in the formula giving the emission anisotropy, the denominator represents the total fluorescence intensity. In a few situations (e.g. the study of radiative transfer) the polarization ratio is to be preferred, but in most cases, the use of emission anisotropy leads to simpler relations (see below). 

Ix = ly The fluorescence observed in the direction of this axis is thus unpolarized. 

On the contrary, the Ox and Oy axes are not axes of symmetry for the emission of fluorescence. When fluorescence is observed through a polarizer along the Ox axis (or the Oy axis), the intensity measured is 7z = 7 for the vertical position of the polarizer, and ly = I (or 7x = 7i) for the horizontal position. Without polarizer, the measured intensity is f + ly = 7 + I in the Ox direction and 7z + 7x — 7 + 7 in the Oy direction. 

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It is easy to show that rn = r/2. Therefore, the emission anisotropy observed upon excitation by natural light is half that upon excitation by vertically polarized light. In view of the difficulty of producing perfectly natural light (i.e. totally unpolarized), vertically polarized light is always used in practice. Consequently, only excitation by polarized light will be considered in the rest of this chapter. 

Surprisingly it was found that PS luminescence excited by polarized light emerges from the sample preferentially with the same sense of polarization. This memory effect has been observed despite the fact that the electron-hole pair loses energy in the order of 1 eV in elastic processes with lifetimes in the order of... 

The technique of fluorescence depolarization uses in principle fluorescent dyes covalently bound to proteins. The label is excited by polarized light the fluorescence is also polarized to a degree that is inversely related to the amount of Brownian motion occurring during the interval between absorption and emission of the light. The information about the amount of motion helps to form an idea on the size and shape of the protein being investigated. 

Is - intensity of scattered light from small particles excited by polarized light la incident intensity... 

In Fig. 7.38 the auto-correlation function G(t) of dye molecules in a liquid sample excited by polarized light is shown [934]. The step increase for short delay times r is due to the anti-bunching effect . When a molecule is excited at time t it can be only again excited after a is has returned into the initial ground state by spontaneous... 

Rotation rates of fluorophore molecules have been studied by measurements of rates of fluorescence depolarization. When a fluorophore molecular is excited by polarized light,... 

Fluorescence polarization has been discussed by a number of authors in somewhat different ways, which will be generalized here. The fluorescence intensity observed under an analyzer with electric vector Q from polymeric materials excited by polarized light with electric vector P is given by the product of two types of the second rank tensors of optical anisotropies, absorption and emission tensors in equation (47), where K is an instrumental constant, Pj and Qp are direction cosines of the polarization directions of the polarizer and analyzer, and A-,j and Ep, are the second rank tensors for the anisotropic absorption and emission of the bulk materials, with respect to the specimen coordinates O-X1X2X3. (In equation (47) and hereafter, an asterisk indicates the summation convention.)... 

Historical introduction. In one of the earliest investigations of resonance fluorescence. Lord Rayleigh (1922) showed that the radiation scattered from mercury vapour was polarized when the atoms were excited by polarized light. 

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