Photon Echo Technique

Ongoing studies using resonant excitation and different dyes will clarify these points. Additional studies using pump-probe as well as photon echo techniques are planned, that should increase the time resolution and give a more complete picture of solvation dynamics at water-ZrOj and water-Ti02 interfaces. 

In a 2D three-pulse spectroscopy, two of the three pulses are time-coincident and differ only by their wave vector. The system thus interacts once with a single pulse and twice with a pulse pair. In the 2D photon echo technique we set t2 = 0. We consider the heterodyne signal [Equation (10)]. This... 

Multidimentional nonlinear infrared spectroscopy is used for identification of dynamic structures in liquids and conformational dynamics of molecules, peptides and, in principle, small proteins in solution (Asplund et al., 2000 and references herein). This spectroscopy incorporates the ability to control the responses of particular vibrational transitions depending on their couplings to one another. Two and three-pulse IR photon echo techniques were used to eliminate the inhomogeneous broadening in the IR spectrum. In the third-order IR echo methods, three phase-locked IR pulses with wave vectors kb k2, and k3 are focused on the sample at time intervals. The IR photon echo eventually emitted and the complex 2D IR spectrum is obtained with the use of Fourier transformation. The method was applied to the examination of vibrational properties of N-methyl acetamid and a dipeptide, acyl-proline-NH2.in D20. The 2D IR spectrum showed peaks at 1,610 and 1, 670 cm 1, the two frequencies ofthe acyl-proline dipeptide. Geometry and time-ordering of the incoming pulse sequence in fifth-order 2D spectroscopy is shown in Fig. 1.3. 

Fast reactions (in the millisecond to second range) require special reactors with efficient mixing chambers. Faster reactions (down to the microsecond range and below) call for special techniques most of these are based on relaxation after an equilibrium state has been disturbed by an instantaneous pulse or step variation of conditions. With laser and photon-echo techniques the range has been extended down to femtoseconds. 

The dephasing time, T2, can be measured by the photon echo technique or determined from the homogeneous width of saturation spectroscopy, which is a Fourier transform of the former, as easily seen from Eq. (5.35). When a sample is irradiated with three consecutive laser pulses at times, 0, t2, and f3, an echo pulse is emitted at time, t2 + t. This is called stimulated photon echo. Several additional echo techniques have been proposed. 

It turns out that for some systems, e.g., porphin in -octane the exciting laser power must be chosen so low, to meet condition (35), that detection of the OFID signal is impossible. Also for the case of the lowest 5, transition of naphthalene in durene the factor 4 is so large, that detection of OFID is out of reach. As the photon-echo technique does not suffer from the triplet state bottleneck, this system can be studied using the photon echo, as shown by Aartsma and Wiersma. For the energetically lowest sites of pentacene the situation is particularly favorable, as A is only 9 and OFID detection is easily possible. 

This phenomenon of super-radiance is used in the photon-echo technique for high-resolution spectroscopy to measure population and phase decay times, expressed by the longitudinal and transverse relaxation times Ti and T2, see (7.1). This technique is analogous to the spin-echo method in nuclear magnetic resonance (NMR) [904]. Its basic principle may be understood in a simple model, transferred from NMR to the optical region [905]. 

This result gives the modulation in a Doppler free form. The requirement is that the line be excited uniformly. The averaging over the velocity groups has introduced an additional Gaussian degradation term which can be rewritten in the form exp- (l/2) [ ( 2 -Q)/Q] (x/ ] is of no consequence in the regime in which we work since 2Xjj 2/(Q-Q ) is of order 400 nsec. For a pulse separation of x=400 nsec the echo which appears 2x =800 nsec or 50 fluorescence lifetimes later would suffer a diminished modulation amplitude of only e . It seems then that this technique can effectively utilize the abiltiy of the photon echo technique to observe excited atoms which live many times their natural lifetime. 

Fluorescence line-narrowing and coherent photon-echo techniques (Macfarlane 1992) could give some idea about the homogeneous part of an emission line, but the statistical analysis for the whole sample should still be performed. Supposing only a sensitizer-activator interaction, an average transfer efficiency can be calculated (Dexter 1953). This was studied in some detail by Inokuti and Hirayama (1965). They considered the number of activators located at random in a sphere around a sensitizer in such a way that the activator concentration remains constant when the volume of the sphere and the number... 

Chiang et al. measured the nuclear hyperfine splittings in the excited D2 state of Lap3 Pr " with the above-mentioned photon echo technique, utilizing two independently triggered nitrogen-laser-pumped dye lasers. They found agreement with the results of Erickson based on laser saturation measurements. ... 

Photon echo techniques may be described in a related way but in these experiments two or more resonant pulses are used to create an echo and T2 is derived from the decay envelope of the echoes. The phenomena of echoes is... 

Relaxations of solvent-chromophore interactions can be studied experimentally by hole-burning spectroscopy, time-resolved pump-probe measurements, and photon-echo techniques that we discuss in the next chapter. If the temperature is low enough to freeze out pure dephasing, and a spectrally narrow laser is used to bum a hole in the absorption spectmm (Sect. 4.11), the zero-phonon hole should have the Lorentzian lineshape determined by the homogeneous lifetime of the excited state. The hole width increases with increasing temperature as the pure dephasing associated with tP comes into play [36, 37]. 

A further possibility for obtaining the EPC parameter quantitatively is given by the femtosecond photon echo technique, which is described in the next section. 

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