Plane-polarized light excitation

Further work using time-resolved EPR and magnetophotoselection (MPS), using plane-polarized light to excite the triplet state, gave information on the orientation of the optical transition dipole axes relative to the principal axes of the triplet state. By this technique the transition moments of the primary donor"6, the carotenoid in the bRC"7 and the bacteriopheophytin in the inactive B branch 4>0"8 were determined. 

Use of plane polarized light. The intensity of a spectral transition is directly related to the transition dipole moment (or simply the transition moment), a vector quantity that depends upon the dipole moments of the ground and excited states. For aromatic ring systems, the transition dipole moments of the ji-n transitions lie in the plane of the ring. However, both the directions and intensities for different n-n transitions within a molecule vary. 

Anisotropy. Light emitted from excited molecules immediately after absorption is always partially polarized, whether or not the exciting beam consists of plane polarized light. When light polarized in a vertical plane is used for excitation, part of the emitted light (of intensity lv) will have its electric vector parallel to that of the exciting light. The remainder of intensity /, will be polarized in a horizontal plane. The polarization P of the emitted radiation is defined by Eq. 23-19 and the anisotropy R by Eq. 23-20. After excitation by a laser pulse both the fluorescence and its anisotropy decay with time and can be measured. The decay of R (but not of P) can usually be described as the sum... 

The plane-polarized light pulses characteristic of mode-locked lasers also provide an ideal excitation source for time-dependent fluorescence depolarization studies although conventional excitation sources can be used. If the rotational relaxation time of the excited molecule is comparable to its fluorescence decay time, then the vertical (I ) and horizontal (Ix) components of the fluorescence decay observed through suitable polarizers following excitation by polarized li t pulses, may be analysed to provide information concerning the size and motion of die molecule and Sect. 5. However, if only the true fluorescence decay characteristics are of interest it is necessary to compensate for these emission anisotropy effects Perhaps the simplest technique is to analyse only that component of fluorescence emitted at 54.7° to the direction of pdarization of the excitation source, the so-called magic-angle ... 

In time-resolved fluorescence depolarization experiments a sample of randomly oriented chromophores is excited by a short pulse of plane polarized light. The decay of the fluorescence intensities polarized parallel (I i(t)) and perpendicular (Ix(t)) to the exciting light can be written as... 

Figure 6 Schematic depiction of a fluorescence anisotropy assay. Fluorescently tagged molecules are excited by plane-polarized light only molecules in the proper orientation are excited. Emitted light is detected in the original plane and in the perpendicular plane. The quantity of fluorescence observed in the two orientations is determined by the rate of tumbling, which depends on particle size and relates to binding.

Anistropy measurements are based on the photose-lective excitation of fluorophores by plane-polarized light. In an isotropic medium, the fluorophores are randomly oriented. Upon excitation with polarized light, those fluorophores whose absorption transition dipole is aligned parallel to the electric vector of the excitation, will be preferentially excited. If the molecule rotates and tumbles out of this plane during the excited state, light is emitted in a different plane from the excitation light. The intensity of the emitted light can be monitored in vertical and horizontal planes and thus, fluorescence anisotropy (r) and polarization ( ) are defined by ... 

Kerr effect n. When plane polarized light is incident on the pole of an electromagnet, polished so as to act like a mirror, the plane of polarization of the reflected light is not the same when the magnet is on as when it is off . It was found that the direction of rotation was opposite to that of the currents exciting the pole from which the light was reflected. Weast RG (1979) CRC handbook of chemistry and physics, 59th edn. GRC Press, Boca Raton, EL. 

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