Physical Review A

Anomalous rate of free-induction decay

R. Boscaino, V. M. La Bella

Published May 1, 1990• 24 citations

Abstract

The steady-state free-induction decay (FID) has been investigated in samples of crystalline and amorphous quartz containing paramagnetic two-level electron spin centers (S=1/2). We report experimental data on the dependence of the FID rate on the Rabi frequency \ensuremath{\chi} induced during the preparative stage. Our results confirm that the decay of the FID signal is much slower than expected on the basis of the conventional Bloch equations. The range of \ensuremath{\chi} investigated here is wide enough to show the whole transition from Bloch-type behavior to the high-power Redfield limit, in which the FID rate is determined only by \ensuremath{\chi}. The experimental data are compared with theoretical results recently obtained by hypothesizing Gaussian-Markovian fluctuations of the resonance frequency. Even if agreement is poor, the comparison suggests by itself the possibility that more than one source of fluctuations may be effective. Other possible origins of the failure of the Bloch equations as well as of the stochastic theories are examined.

DOI: 10.1103/physreva.41.5171

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Physical Review A

Anomalous rate of free-induction decay

R. Boscaino, V. M. La Bella

Published May 1, 1990• 24 citations

Publisher

Abstract

DOI: 10.1103/physreva.41.5171

Related papers

Free-induction decay after saturation in dilute ruby

R. Boscaino, F. M. Gelardi

The after-saturation free-induction-decay (FID) rate of an electron spin transition has been measured in a dilute ruby sample, and a marked departure from the Bloch-like behavior has been found. The similarity with the results obtained recently by Szabo and Muramoto [Phys. Rev. A 39, 3992 (1989)] in the same system but for optical FID points out the close link between the non-Bloch behavior of the FID emission observed in optics and in magnetic-resonance systems. Our results are compared with the theoretical predictions of a statistical theory of the FID emission, but poor agreement is found.The after-saturation free-induction-decay (FID) rate of an electron spin transition has been measured in a dilute ruby sample, and a marked departure from the Bloch-like behavior has been found. The similarity with the results obtained recently by Szabo and Muramoto [Phys. Rev. A 39, 3992 (1989)] in the same system but for optical FID points out the close link between the non-Bloch behavior of the FID emission observed in optics and in magnetic-resonance systems. Our results are compared with the theoretical predictions of a statistical theory of the FID emission, but poor agreement is found.

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Experimental spectra of oscillatory free-induction decay

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R. N. Shakhmuratov, F. M. Gelardi, M. Cannas

Free induction decay and transient nutations are investigated in a system of two-level particles with an inhomogeneously broadened spectrum. The influence of the x (real) and y (imaginary) parts of the complex amplitude of the noise, induced by the driving field in the sample, is considered. It is shown that both noises affect ${T}_{1}$ and ${T}_{2}$ relaxation processes. When field-noise contributions to ${1/T}_{2}$ and ${1/T}_{1}$ relaxation rates are dominant and equal to each other, Bloch's saturation is changed to Redfield's saturation. This effect also accelerates transient nutation decay. Comparison with experimentally observed free induction decay and transient nutation in color centers in quartz is discussed.Free induction decay and transient nutations are investigated in a system of two-level particles with an inhomogeneously broadened spectrum. The influence of the x (real) and y (imaginary) parts of the complex amplitude of the noise, induced by the driving field in the sample, is considered. It is shown that both noises affect ${T}_{1}$ and ${T}_{2}$ relaxation processes. When field-noise contributions to ${1/T}_{2}$ and ${1/T}_{1}$ relaxation rates are dominant and equal to each other, Bloch's saturation is changed to Redfield's saturation. This effect also accelerates transient nutation decay. Comparison with experimentally observed free induction decay and transient nutation in color centers in quartz is discussed.

Physical Review LettersOct 20, 199731 citations

T

. The effect should be detectable in nuclear magnetic resonance (NMR) or in optical resonance experiments when the transition line shape is inhomogeneously broadened (${T}_{2}^{*}\ensuremath{\ll}{T}_{2}$) and the applied field is intense enough that the pulse area $\ensuremath{\chi}T\ensuremath{\ge}2\ensuremath{\pi}$, where $\ensuremath{\chi}$ is the Rabi frequency. Physically, the effect is due to Rabi oscillations which are generated in the preparative stage and are then reproduced because of atomic memory in the radiative period which follows. The atomic polarization derived from the Bloch equations assumes the form of an integral over the inhomogeneous line shape, and can be evaluated numerically for arbitrary linewidth $\ensuremath{\sigma}$ or analytically when $\ensuremath{\sigma}\ensuremath{\rightarrow}\ensuremath{\infty}$. The characteristics of the oscillation are unusual since the oscillation frequency, which is of order $\ensuremath{\chi}$, increases with time while the oscillation envelope vanishes identically for times $t\ensuremath{\ge}2T$. Coherent emission is therefore confined to one pulse width

T

immediately following the pulse. Previous NMR and infrared FID experiments with small-area pulses ($\ensuremath{\chi}T\ensuremath{\le}4\ensuremath{\pi}$), interpreted erroneously in terms of an edge echo effect, support the oscillatory FID behavior described here. For the case $\ensuremath{\chi}T>4\ensuremath{\pi}$, an experimental test would be even more decisive, as the oscillations are more pronounced.

The free-induction decay (FID) signals emitted by a two-level spin system (

{E}_{1}^{\mathcal{'}}

centers in glassy silica), coherently prepared by a short pulse of resonant radiation, have been experimentally investigated in the frequency domain. Our results show that, under the same conditions in which FID signals are oscillatory in the time domain, the spectra are characterized by a regular series of emission zeros. A vectorial model accounts for the occurrence of the zeros and for their spectral positions.

{T}_{1}

and

{T}_{2}

relaxation processes. When field-noise contributions to

{1/T}_{2}

and

{1/T}_{1}

relaxation rates are dominant and equal to each other, Bloch's saturation is changed to Redfield's saturation. This effect also accelerates transient nutation decay. Comparison with experimentally observed free induction decay and transient nutation in color centers in quartz is discussed.