The contribution of covalency to satellite lines is illustrated in a calculated
spectrum for d
8 Ni(II) in Fig. 7.10. For a modest amount of covalency, new bands
grow in about 5 eV above the main L 3 and L 2 features. These represent transitions to
the higher-energy charge-transfer states. For more covalent systems such as Cu(III),
the charge-transfer energy Δ is actually slightly negative, which means that the 3d
9 L
configuration actually has a lower energy than the 3d
8 configuration.
7.7 Simulation-Free Information
It is often useful to analyze edges with quantities that do not depend on a particular
simulation procedure. For this purpose, some of the parameters that can be extracted
include (a) inflection point or centroid position, (b) branching ratio, and
(c) integrated intensity. Some examples are given below.
7.7.1 Inflection Point or Centroid Position
How do you define the position of an absorption edge, when on close inspection, it is
often a complex, structured spectrum? For K-edges, especially of metals, a useful
feature to report is the first inflection point. For L-edges, which often have rich
multiplet structure, a more useful benchmark is the centroid position. Centroid shifts
for Ni L-edges are reported in Fig. 7.11. Typical chemical shifts for K-edges of Cr
and As compounds were shown in Fig. 7.2, and examples for L-edges of Mn
compounds in different oxidation states are illustrated in Fig. 7.12.
7.7.2 Branching Ratio
We saw in the CTM4XAS simulations that the branching ratio was very sensitive to
the spin state of Ni(II). The same spin-state sensitivity is seen for other transition
metal complexes that can have high-spin or low-spin electronic structures. The
combined application of centroid position and branching ratio has been used as an
empirical approach to defining the chemical state of Ni species (Fig. 7.11).
7.7.3 Integrated Intensity
Since L 2,3 edges involve primarily p ! d transitions, it is reasonable to expect
L-edge intensity to reflect the number of d vacancies. Similarly, the number of
f vacancies should be reflected in the intensity of M 4,5 edges, which are mostly
7.7 Simulation-Free Information
179
spectrum for d
8 Ni(II) in Fig. 7.10. For a modest amount of covalency, new bands
grow in about 5 eV above the main L 3 and L 2 features. These represent transitions to
the higher-energy charge-transfer states. For more covalent systems such as Cu(III),
the charge-transfer energy Δ is actually slightly negative, which means that the 3d
9 L
configuration actually has a lower energy than the 3d
8 configuration.
7.7 Simulation-Free Information
It is often useful to analyze edges with quantities that do not depend on a particular
simulation procedure. For this purpose, some of the parameters that can be extracted
include (a) inflection point or centroid position, (b) branching ratio, and
(c) integrated intensity. Some examples are given below.
7.7.1 Inflection Point or Centroid Position
How do you define the position of an absorption edge, when on close inspection, it is
often a complex, structured spectrum? For K-edges, especially of metals, a useful
feature to report is the first inflection point. For L-edges, which often have rich
multiplet structure, a more useful benchmark is the centroid position. Centroid shifts
for Ni L-edges are reported in Fig. 7.11. Typical chemical shifts for K-edges of Cr
and As compounds were shown in Fig. 7.2, and examples for L-edges of Mn
compounds in different oxidation states are illustrated in Fig. 7.12.
7.7.2 Branching Ratio
We saw in the CTM4XAS simulations that the branching ratio was very sensitive to
the spin state of Ni(II). The same spin-state sensitivity is seen for other transition
metal complexes that can have high-spin or low-spin electronic structures. The
combined application of centroid position and branching ratio has been used as an
empirical approach to defining the chemical state of Ni species (Fig. 7.11).
7.7.3 Integrated Intensity
Since L 2,3 edges involve primarily p ! d transitions, it is reasonable to expect
L-edge intensity to reflect the number of d vacancies. Similarly, the number of
f vacancies should be reflected in the intensity of M 4,5 edges, which are mostly
7.7 Simulation-Free Information
179