Biomedical Engineering Reference
In-Depth Information
where and are coefficients, and are the matrix elements of the dipolar
electronic transition moment and dipolar magnetic transition moment, respectively, and
is the function of the line shape.
In the femtosecond liner dichroism (FLD) experiments (Önfelt et al., 2000) the
output of a femtosecond laser system is split into pump and probe pulses with
independent wavelength tunability. The pump pulse passes through an optical delay line
and a polarizer, and crosses the probe beam on the sample cell at a small angle. The
probe beam passes through a polarizer that is adjusted to produce a linear polarization at
the pump beam. After traversing the sample, the probe pulse is passed through a
polarizer for the separation of light polarized parallel and perpendicular to the pump
beam. The ratio between normalized transmitted probe energies in the presence and
absence of excitation of the sample by the pump pulse formed the basis of the
polarization-resolved transition, recorded as a function of the time delay t between
passages of the pump and probe pulses. The measured transients are analyzed by
applying them to a sum of exponential terms of amplitude A and lifetime The FLD
technique was used for the study of solvation and charge separation of {Ru(
1
,10-
pheantrolin)2dipyrido[3,2-a:2'-3'-c]phenazin} incorporated into DNA.
1.1.3. FLUORESCENCE AND PHOSPHORESCENCE
Because of their high sensitivity, fluorescence and phosphorescence techniques are
especially suited for solving many problems of structure and dynamics of the biological
molecular system. The main luminescence parameters traditionally measured, are the
frequency of maximal intensity intensity I, the quantum yield, the lifetime of the
exited state polarization and excited state energy migration (Lacovicz, 1985). The
usefulness of the fluorescence methods is greatly enhanced by the developments of new
experimental techniques such as nano-, pico- and femtosecond time-resolved
spectroscopy, single-molecule detection, cofocal microscopy and two-photon correlation
spectroscopy.
Time -resolved fluorescence spectroscopy
The excitation of a chromophore group is accompanied by a change in the electron
dipole moment of the molecule. This involves a change in the interaction energy with the
surrounding molecules, which manifests itself by a shift of the time-dependent frequency
maximum of the fluorescence spectra, (relaxation shift) (Bakhshiev, 1972):
where the indices and 0 are related to the nmax of the time -resolved emission
spectrum at a given moment, and respectively, and is the characteristic
time of reorganization of the dipoles in the medium around the fluorophore. The value of
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