Quantum 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. 

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).

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). 

Auzinsh, M.P. and Ferber, R.S. (1984). Observation of a quantum-beat resonance between magnetic sublevels with AM = 4, JETP Lett., 39, 452-455. 

Watanabe, H. and Tsuchlya, S. (1983). Quantum beats in the fluorescence of jet-cooled SO2 under a weak magnetic field, J. Phys. Chem., 87, 906-908. 

Zeeman quantum beat spectroscopy was used by Gouedard and Lehmann (1979, 1981) to measure the effect of various lu perturbing states on the gj-values [Eq. (6.5.21)] of more than 150 rotational levels of the Se2 B 0+ state (see Section 6.5.2 and Fig. 6.16). In that experiment, the excitation polarization was perpendicular to the applied magnetic field so that quantum beats were observed between nominal B-state components differing in M by 2. The frequencies of these beats increase linearly from 0 MHz at 0 G until the AM — 2 splitting falls... 

Ring, et al., (1998 and 1999) have used a time-dependent magnetic field and the combination of a static magnetic field in a direction perpendicular to that of a time-dependent field to create and manipulate novel coherences and to monitor the quantum beats associated with specifiable details of the time evolution of these coherences. The frequencies and decay rates of different classes of coherence (AMj = 2 and 1 polarization beats, AMj = 0 singlet triplet population beats) may be sampled and modified selectively. 

Stark and Zeeman polarization quantum beats are discussed in Section 6.5.3. An external electric or magnetic field destroys the isotropy of space. As a result, the amplitudes for two transition sequences J", M" — J, M = M" 1 —> J ", M" interfere, and the intensity of X or Y (but not Z) polarized fluorescence is modulated at (Fj M =M"+i — Ejim =M"-i)/h. However, it is not necessary to destroy the isotropy of space in order to observe polarization quantum beats. 

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