Transient Nutation

The simplest coherence experiment in the case of single spin EPR, is a (transient) nutation experiment [22]. In this experiment the spin is forced to precess perpendicular to the direction of the microwave magnetic field component in the rotating frame. This is equivalent to a periodic modulation of the population probability of the two T] sublevels in resonance with the microwaves. This periodic modulation leads to a change in the lifetime of the Ti state and thus to a corresponding modulation of the average fluorescence intensity. 

Due to experimental limitations only nutation traces up to a duration of 2 ps could be recorded. Within this time interval the modtilation amplitude of the nutation is hardly damped. This is in sharp contrast to EPR ensemble experiments, where a damping of the nutation usually is clearly visible within the first ps. The damping is caused by an inhomogeneity of the driving microwave field across the sample. This of course is absent in the single-molecule experiment, since the field is homogeneous over a molecular dimension. The nutation experiment can be described by [32, 33], 

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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. 

BittI R, van der Est A, Kamlowski A, Lubitz W and Stehlik D 1994 Time-resolved EPR of the radical pair bacterial reaction centers. Observation of transient nutations, quantum beats and... 

To investigate multispin systems, the so-called electron spin transient nutation (ESTN) spectroscopy is recently elaborated. This is a version of pulsed ESR. Nutation is the precessional motion of spin. The method and its applications are detailed in the paper of Itoh et al. (1997). Chapters 1 and 8 describes that the determination of spin multiplicity becomes a very important problem in organic chemistry of ion-radicals. 

In order to identify the spin multiplicity of the tris(carbene), field-swept two-dimensional electron spin transient nutation (2D-ESTN) spectroscopy was used. This technique is based on pulsed fourier transform (FT) EPR spectroscopic methods and is capable of elaborating straightforward information on electronic and environmental strucmres of high-spin species even in amorphous materials, information that conventional CW EPR cannot provide. The nutation spectra unequivocally demonstrated that the observed fine structure spectrum is due to a septet spin state. 

Section 5 deals with the theoretical and experimental analysis of the effect of time-dependent transient nutations in the effective field of multiple-pulse sequences (TNEEMS) in NQR N, which was first established in 1991, and the nature of which is still not clearly imderstood. Experimental results indicate that... 

TNEFMS— transient nutation in the effective field of multiple-pulse sequences. 

These so-called transient nutations are observed if resonance between an electron spin transition and a coherent radiation field is suddenly achieved. The phenomenon can be understood when viewing the motion of the magnetization vector, M, in a... 

Fig. 3a,b. a Transient nutation signal as probed for the zero-field transition at... 

As shown in Fig. 3 a, spin coherence is manifested in the optically detected transient nutation signal for [Rh(bpy)3] (0)04)3 the phosphorescent triplet state. In this experiment, one observes that the phosphorescence intensity becomes modulated as the pulse length of microwave pulses, resonant with the D - transition, is gradually increased. The modulation is evidence that the micro-wave excitation induces a spin coherence in the ensemble of molecules in the photoexcited triplet state [44]. Moreover, from the transient nutation experiment one obtains the information about the duration of the pulses needed in a spin echo experiment. In the case of the example, the n/2 pulse is 100 ns and the 71 pulse has a length of 200 ns. Similarly, transient nutation signals for the other zero-field spin resonances could be obtained. The optically detected spin echo decay as measured for the D - j j zero-field transition for [Rhlbpylj](004)3... 

Spectra extracted from the complete data set of a high power 200 mW) experiment at K-band are shown in Fig. 2a. The solid line is obtained from the initial amplitudes of transient nutation signals. The broken line has been obtained from the Fourier coefficients of the nutation frequency where only times longer than 900 ns after the laser pulse have been retained in the transformation of the transients. Both spectra - although totally different - belong to the same spin pair P700A1. They can be simulated quite satisfactorily (Fig. 2b) The early... 

Figure 16. Transient nutation patterns for two different microwave powers. For trace (b) the microwave power was attenuated by 20 dB with respect to the microwave power used to record trace (a). The dotted lines represent a simulation according to Eq. (16).

So far, we have considered only experiments with continuous-wave lasers under steady state conditions. With time-resolved experiments, on the other hand, energy transfer rates and transition probabilities can be obtained. Such measurements were carried out by mechanically chopping the laser beam directed into an external absorption cell together with the microwave radiation. Later, Levy et at reported time-resolved infrared-microwave experiments with an N2O laser Q-switched with a rotating mirror to produce pulses less than 1 /tsec in duration. They observed a transient nutation of the inversion levels of the molecule following the infrared laser pulse. Based on the Bloch equations, the observed phenomena could be explained quantitatively. From the decay envelope of the oscillations a value for the transverse relaxation time T2 was determined. Similar effects were produced by rapidly switching a Stark field which brings the molecules into resonance with the cw microwave radiation. 

It is of interest to describe spin systems that are completely different from nitrox-ide-labeled dendrimers, but that also bear in their organic structure a high-spin multiplicity (with S > V) High-spin polycarbenes with ji-conjugation, which are the first and second generations of the spin-mediated dendrimers based on pheny-lacetylenes, were studied by a two-dimensional electron-spin transient nutation (2D-ESTN) method based on pulsed-ESR to determine the spin mulliphcity. The contour plots of the 2D-ESTN spectra of the dendrimer at 3.5 K allowed to identify the nutation frequencies, which were attributed to 4, 4> 4, 3>, 4, 3> <-> 4, 2>, 4, 2> 4, 1>, and 4, 1> 4, 0>. The ESR allowed the detec-... 

Wilier M, Schweiger A. 1997. Determination of g values by a new electron spin transient nutation experiment the gj value of titanimn-doped sapphire. Chem Phys Lett 264 1-8. 

Astashkin AV, Schweiger A. 1990. Electron-spin transient nutation a new approach to simplify the interpretation of ESR spectra. Chem Phys Lett 174 595-602. 

Kouskov V, Sloop DJ, Liu SB, Lin TS. 1995. Pulsed transient nutation experiments on... 

Takui, T., Sato, K., Shiomi, D., Itoh, K., Kaneko, T, Tsuchida, E., and Nishide, H., FT pulsed ESR/electron spin transient nutation spectroscopy in the study of molecular based magnetism applications to high-spin polymers and ferromagnetic materials. Mol. Cryst. Liq. Cryst., 271, 191, 1995. 

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