METHODS OF INVESTIGATION OF THE STRUCTURE AND ACTION MECHANISMS OF ACTIVE SITES OFENZYMES - New Trends in Enzyme Catalysis and Biomimetic Chemical Reactions - page 4

axial ligand and CO binding induces dissociation of another trans-ligand and triggers the
activation of CooA via the conformational changes.
In CARS two ultrashort pulses of laser light (from femtoseconds to picoseconds in
duration) arrive simultaneously at the sample of interest (Mukamel, 2000; Fourkas, 2001
and references herein). The difference between the frequencies matches the
frequency of a Raman active vibrational mode in the sample. A “probe” pulse
emits a signal pulse of frequency in a unique special direction. By scanning
the delay time between the pump and “probe” pulses, the delay of the vibrational
coherence can be measured. The distinct advantage of CARS is that it is a background
free technique, since the signal propagates in a unique direction.
To overcome the problem of separating homogeneous and nonhomogeneous
contributions to the line shape, the special technique, called the photon echo, has been
developed (Fourkas, 2001 and references therein). The principle idea of this method is
similar to fundamentals of spin-echo techniques in NMR and ESR (Sections 1.1.5 and
1.1.6). The photon spin echo technique generally involves five laser pulses of at least
two different colors. Two time-coincident pulses of light create a Raman coherence at
frequency that is allowed to involve for longer time after which the response for a
single vibrational frequency occurs. At this point, another pulse pair is focused upon the
sample. Each pulse in these pairs interacts with the system twice, reversing the
coherence so that it is frequency This coherence is allowed to involve for time
second delay, after which the response for a single vibrational frequency takes place.
The ability to rephase inhomogeneity in Raman-active intermolecular vibrations was
increased with the use of five-order spectroscopic technique (Tanamura and Mukamel,
1993; Mukamel, 2000; Fourkas, 2001). Five-order spectroscopy relies on the existence
of some sorts of nonlinearity, either in the coordinate dependence of polarizability or in