Beats magnetic

The low MW power levels conuuonly employed in TREPR spectroscopy do not require any precautions to avoid detector overload and, therefore, the fiill time development of the transient magnetization is obtained undiminished by any MW detection deadtime. (3) Standard CW EPR equipment can be used for TREPR requiring only moderate efforts to adapt the MW detection part of the spectrometer for the observation of the transient response to a pulsed light excitation with high time resolution. (4) TREPR spectroscopy proved to be a suitable teclmique for observing a variety of spin coherence phenomena, such as transient nutations [16], quantum beats [17] and nuclear modulations [18], that have been usefi.il to interpret EPR data on light-mduced spm-correlated radical pairs. 

Fig. 7.17 Time evolution of the nuclear forward scattering for metallic Ni foil. All measurements except for the upper curve were performed with external magnetic field B = 4 T. The solid lines show the fit. The arrows emphasize stretching of the dynamical beat structure by the applied magnetic field. The data at times below 14.6 ns had to be rescaled (from [34])...

When, however, phonons of appropriate energy are available, transitions between the various electronic states are induced (spin-lattice relaxation). If the relaxation rate is of the same order of magnitude as the magnetic hyperfine frequency, dephasing of the original coherently forward-scattered waves occurs and a breakdown of the quantum-beat pattern is observed in the NFS spectrum. 

Fig. 23. The fluorescence decay of Cd vapor in a magnetic field, (a) Experimental data exhibiting the phenomenon of quantum beats, (b) The exponentially decaying component, (c) The decaying modulated component. This figure is reproduced from the work of Dodd, Kaul, and Warrington (158).

Fig. 16.10 Quantum beat signals of high lying 2D states of Na obtained by time resolved selective field ionization. The variation of the beat frequency with principal quantum number is shown. Several quantum beat frequencies appear due to a Zeeman splitting of the fine structure levels in the earth s magnetic field (from ref. 43).

Shubnikov - de Haas (SdH) and de Haas - van Alphen (dHvA) quantum oscillations were observed in the crystals studied at different magnetic field directions and temperatures. Fig. 6 (inset) shows an example of these SdH oscillations. It should be noted that no beating node occurs in these oscillations, suggesting again a strong 2D... 

It is this term that describes magnetic ( Zeeman ) quantum beats in the form of intensity modulation with frequency ojmm -... 

The problem of determining the set u>mfmf, from the observed quantum beat signal is solved with the aid of Fourier transformation. It is more difficult to interpret the signals of magnetic quantum beats in the case of polyatomic molecules, such as in [389] with SO2 and in [90] with N02(i2 2), where, as a general rule, one observes many perturbations between different states. 

So far we have been discussing magnetic (Zeeman) quantum beats taking place at frequencies of coherent sublevel splitting in an external magnetic field. 

For a description of the ground state magnetic quantum beats one might conveniently use the solution of Eq. (4.10) for multipole moments aPq-Assuming that the excitation takes place by a 6-pulse at time t = 0, one may write its solution for t > 0 in the form ... 

Magnetic quantum beats in the transient process after pulsed depopulation of the ground state may be observed not only in fluorescence, but also in a more direct way, namely in absorption. In connection with what was discussed in Section 3.5, one must expect maximum sensitivity if the experiment is conducted according to the laser interrogated dichroism method see Fig. 3.17. To this end it is convenient to direct the external magnetic field B along the 2-axis as shown in Fig. 4.21 where the probe beam E-vector can be either in the xy plane (Em) or in the yz plane (Epr2). 

Among other methods of creating and observing optical manifestations of coherent superpositions of non-degenerate magnetic sublevels (an elegant analysis may be found in [4]), the most widely used method is that of resonance of beats. The effect appears when the frequency fli of harmonic modulation of the excitation rate... 

The method of beat resonance has been used episodically to study the magnetism of excited states of molecules. One of the first papers of this kind appears to have been that by Lehmann et al. [89] on iodine I2(53n0+). Subsequently the same group [172, 173] used beat resonance for determining Lande factors for a number of rovibronic levels of... 

Fig. 4.23. Non-linear beat resonance signal of K2(X1E+,w" = 1, J" = 73) in the form of the dependence of the degree of polarization of radiation on modulation frequency fii in a constant magnetic field B = 0.589 T. The arrow indicates...

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