to, e.g., Jahn–Teller effect, or the degree of covalence of bonds formed by the ion
with surrounding atoms or ligands.
With the increase of radiation frequency, the energy of photons increases and
hence the possibility appears of studying phenomena in which energy changes are
higher and correspond to radiation with frequencies of about 10
11
–10
13 Hz, e.g.,
molecular rotations and atomic vibrations in molecules and solids (the domain of IR
and Raman spectroscopy). While rotational spectra can only be obtained for
molecules (those with a permanent dipole moment) and thus provide the information on the structure of such molecules (lengths of bonds and angles between
bonds), atomic vibrations are present in both, molecules and solids (amorphous and
crystalline), hence the vibrational spectroscopy (IR and Raman) [6] is a universal
method of studying the local structure of materials (force constants, bonds lengths,
angles, etc.) by analyzing normal modes of vibrations and changes in absorption
(IR) or scattering (Raman) of infrared radiation associated with structural changes
resulting, for example, from phase transitions or changes in chemical composition
(defects, admixtures, etc.).
The next group of spectroscopic methods is UV-Vis [7, 8] techniques related to
electronic transitions between different energy levels and the corresponding radiation range, from near infrared, through visible region, to ultraviolet (about 10
14
–
10
15 Hz). Various types of electronic transitions are possible, both between the low
lying localized levels of the internal shells of individual atoms and the levels of the
outermost valence shells (either localized on atoms or delocalized, forming bands).
Generally, there are four basic types of electron transitions:
• Excitation of an electron from a localized orbital to another localized orbital on
the same atom but with higher energy: The examples are d-d or f-f transitions in
compounds containing transition metals, transitions on outer shells in heavy
metals compounds (e.g., 6s–6p in lead(II) compounds), or transitions related to
defects (trapped electrons or holes, color centers);
• Excitation of an electron from a localized orbital on one atom to a higher energy
orbital localized on an adjacent atom (charge transfer processes, e.g., from
oxygen to chromium in tetrahedral [CrO 4 ]
2− anion);
• Excitation of an electron from a localized orbital on an atom to a delocalized
conduction band (e.g., photoconductive materials);
• Electron transitions from valence band to conduction band (e.g., in
semiconductors).
UV-Vis spectroscopy has a very wide range of applications related to the study
of the local structure of materials, because the positions of absorption bands are
very sensitive to the immediate surroundings and the nature of chemical bonding.
For example, by adding a small amount of spectroscopically active elements, such
as transition metals or p-block heavy metals (e.g., Pb
2+ ), a number of information
on the local structure of amorphous materials can be obtained. Materials for lasers
often contain transition metal ions as active species, e.g., Al 2 O 3 doped with Cr
3+
(ruby laser) or Nd
3+ -doped glass (neodymium laser), and UV-Vis spectroscopy
1 Computational Methods in Spectroscopy
3
with surrounding atoms or ligands.
With the increase of radiation frequency, the energy of photons increases and
hence the possibility appears of studying phenomena in which energy changes are
higher and correspond to radiation with frequencies of about 10
11
–10
13 Hz, e.g.,
molecular rotations and atomic vibrations in molecules and solids (the domain of IR
and Raman spectroscopy). While rotational spectra can only be obtained for
molecules (those with a permanent dipole moment) and thus provide the information on the structure of such molecules (lengths of bonds and angles between
bonds), atomic vibrations are present in both, molecules and solids (amorphous and
crystalline), hence the vibrational spectroscopy (IR and Raman) [6] is a universal
method of studying the local structure of materials (force constants, bonds lengths,
angles, etc.) by analyzing normal modes of vibrations and changes in absorption
(IR) or scattering (Raman) of infrared radiation associated with structural changes
resulting, for example, from phase transitions or changes in chemical composition
(defects, admixtures, etc.).
The next group of spectroscopic methods is UV-Vis [7, 8] techniques related to
electronic transitions between different energy levels and the corresponding radiation range, from near infrared, through visible region, to ultraviolet (about 10
14
–
10
15 Hz). Various types of electronic transitions are possible, both between the low
lying localized levels of the internal shells of individual atoms and the levels of the
outermost valence shells (either localized on atoms or delocalized, forming bands).
Generally, there are four basic types of electron transitions:
• Excitation of an electron from a localized orbital to another localized orbital on
the same atom but with higher energy: The examples are d-d or f-f transitions in
compounds containing transition metals, transitions on outer shells in heavy
metals compounds (e.g., 6s–6p in lead(II) compounds), or transitions related to
defects (trapped electrons or holes, color centers);
• Excitation of an electron from a localized orbital on one atom to a higher energy
orbital localized on an adjacent atom (charge transfer processes, e.g., from
oxygen to chromium in tetrahedral [CrO 4 ]
2− anion);
• Excitation of an electron from a localized orbital on an atom to a delocalized
conduction band (e.g., photoconductive materials);
• Electron transitions from valence band to conduction band (e.g., in
semiconductors).
UV-Vis spectroscopy has a very wide range of applications related to the study
of the local structure of materials, because the positions of absorption bands are
very sensitive to the immediate surroundings and the nature of chemical bonding.
For example, by adding a small amount of spectroscopically active elements, such
as transition metals or p-block heavy metals (e.g., Pb
2+ ), a number of information
on the local structure of amorphous materials can be obtained. Materials for lasers
often contain transition metal ions as active species, e.g., Al 2 O 3 doped with Cr
3+
(ruby laser) or Nd
3+ -doped glass (neodymium laser), and UV-Vis spectroscopy
1 Computational Methods in Spectroscopy
3