Based on the experimental work by Boscaino et al on the EPR transient
nutations (TNs) and free induction decay (FID) in solids, we propose the
modified Bloch equations (MBEs). In addition to the Tomita expression for
power-dependent parameter T2u, we give an original phenomenological
expression for power-dependent parameter T2v and tuning Δ. Both
analytical (in the form of a Torrey solution with these parameters) and
numerical solutions of MBE are obtained for TN and for different FID regimes
with very good agreement between theory and experiment. We also discuss the
meaning and role of the instantaneous diffusion mechanism in the transient
pulse experiments.
On the grounds of Bloch equations modified by taking into account
the power dependence of the dispersion and damping parameters, we give general
expressions for hole shapes burnt in the absorption and polarization spectra
of the two-level systems. The general expressions are used for detailed
numerical calculations of the hole shapes and hole widths in a concrete
paramagnetic system (quartz with [AlO4]0 centres). This system earlier was
studied experimentally and theoretically through the transient nutation and
free induction decay methods. The results on the hole width in our
modified-Bloch-equations model are in good qualitative agreement with the FID
data.
We propose modified Bloch equations (MBEs) with specific power-dependent
relaxation and dispersion parameters characteristic for two-pulse excitation and
when the magnetic dipole–dipole interactions in the electron spin system
control the dephasing. We discriminate between the ‘active’ (excited by
both pulses) and ‘passive’ (excited by the second pulse only) spins: it is
shown that the ‘active’ spins participate in a new effect, an active spin
frequency modulation effect giving rise to the power-dependent dispersion and
multiple electron spin echoes (ESEs); the ‘passive’ spins contribute to
the power-dependent relaxation. The MBEs are solved and a general
expression for the two-pulse ESEs is obtained. Detailed numerical analysis of
this expression gives results in good quantitative agreement with the
recent experiments on the two-pulse ESEs at conventional low applied
fields. The developed theory is applied also to high field ESEs, which
are promising for future investigations. On the basis of published results
it is deduced that the instantaneous diffusion mechanism is ineffective.
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