  Multidimensional coherent optical spectroscopy is one of the most powerful
tools for investigating complex quantum mechanical systems. While it was
conceived decades ago in magnetic resonance spectroscopy using micro- and
radio-waves, it has recently been extended into the visible and UV spectral
range. However, resolving MHz energy splittings with ultrashort laser pulses
has still remained a challenge. Here, we analyze two-dimensional Fourier
spectra for resonant optical excitation of resident electrons to localized
trions or donor-bound excitons in semiconductor nanostructures subject to a
transverse magnetic field. Particular attention is devoted to Raman coherence
spectra which allow one to accurately evaluate tiny splittings of the electron
ground state and to determine the relaxation times in the electron spin
ensemble. A stimulated step-like Raman process induced by a sequence of two
laser pulses creates a coherent superposition of the ground state doublet which
can be retrieved only optically due to selective excitation of the same
sub-ensemble with a third pulse. This provides the unique opportunity to
distinguish between different complexes that are closely spaced in energy in an
ensemble. The related experimental demonstration is based on photon echo
measurements in an n-type CdTe/(Cd,Mg)Te quantum well structure detected by a
heterodyne technique. The difference in the sub-$\mu$eV range between the
Zeeman splittings of donor-bound electrons and electrons localized at potential
fluctuations can be resolved even though the homogeneous linewidth of the
optical transitions is larger by two orders of magnitude.
