Confusing term crystal field exictation
The problem is the confusing term "crystal
field excitation" and "pure electronic crystal field excitation".
Normally people call the two peaks of Fe2O3 "crystal field
excitation", which is not completely wrong, but it does not really mean
literally pure electronic excitation as often found in the text book. In
stead, the absorption peaks in Fe2O3 consist of excitations of various
natures that all have to obey selection rules. We hereby talk about some details as the following:
Type 1: Pure crystal field excitations: This is magnetic dipole
transition that does not have to obey parity selection rule. Spin orbit
coupling takes care of spin selection rule. This is purely on-site electronic
excitations. Sometimes it is also called Frenkel exciton. The intensity
is the lowest because of the magnetic dipole nature. The energy positions are expected to be at low energy end
of the spectra, if one can observe this type of excitations. Let me
emphasize that this type of excitation is does not create electric
polarization. In contrast, the other two are both electric type
excitations.
Type 2: Magnon sidebands, pure crystal field excitation + magnons. These
are many-body excitations involving Fe sites of both spin sublattice.
The total spin is conserved. Therefore the spin selection rule is
obeyed. The initial and final states have different parities, so the
parity selection rule is also obeyed. The intensity of this type is
supposed to be much higher than the type 1 but much lower than the type
3, which is why the we often can not see the peaks in spectrum of absorption
spectra directly. However, this type of excitation is sensitive to the
magnetic order, which is why they stand out when the magnetic field is
applied. Nevertheless, the intensity we see in Δα is much
smaller than α itself.
Type 3: Crystal field + odd parity phonons. This type of excitation is
mostly responsible to the total intensity we see in Fe2O3. Here
spin-orbit coupling relaxes the spin selection rules and the addition of
odd-parity phonons makes the initial and final states different parity.
If we do fitting on the absorption spectra, we are getting the profile
of this type of excitations. This type of excitations can further couple
to even parity phonons and magnons. The detail of this type of
excitation can not resolved because of the O-p Fe-d hybridization and
the complex structure of odd parity (IR) phonons. In most literatures, so-called
crystal-field excitations actually refer to this type. After all, most
intensity of the absorption comes from here.
Some references:
Srivastava V. C. et al., Solid State Comm. v11, p41(1972)
Tsuboi T., et al, PRB v45, p468 (1992)
