theoretical consideration of Raman spectroscopy
I.1 Introduction
Matter is made from charged building blocks (electrons, nuclei, positive and negative ions). A
specific kind of matter, such as a specific molecule, a solid, or an atom has a specific charge
distribution. These charges are not only isolated charges (monopoles), there are also positive
and negative charges forming electric dipoles (and higher multipoles). There is also a way that
the electric field of the light wave itself can shift charges and in this way, to induce a dipole
moment. The electric (and, in a much weaker extend, also the magnetic) field of a light wave
can interact with the permanent existing and induced dipole moments. In a light wave the
fields change about 1015 times per second, that means the charged “pieces of matter” move
(oscillate) very fast. On the other hand, changes (oscillations) of charge distributions, such as
an oscillating dipole can result in the generation of electromagnetic fields. These processes
between light and matter are, in a “classical view”, the basic processes of each spectroscopy.
Since the charge distributions are very specific for a specific kind of matter, such as for a specific
molecule, the exchange of energy between the electromagnetic wave and the matter is
also very specific and provides unique information on the matter. In the quantum picture,
these processes are described by the annihilation of a photon, which is accompanied by the
transition of the system in a higher energy state or, vice versa, by generating a photon when
the system goes from a higher energy state to a lower one. There is a specific probability that
photons (or light waves) interact with matter. This can be described by the so-called “cross
section” of a spectroscopic effect.
Raman and infrared (IR) spectroscopies1,2 provide information about the vibrational and vibrational-
rotational modes of molecules. Vibrational-rotational bands are generally observed
when the samples are in gaseous state, where the molecules are able to rotate freely. In condensed
(liquid or solid) phases, only vibrational frequencies of the sample can be observed. IR
and Raman spectroscopies are complementary techniques, since transitions allowed in Raman
may be forbidden in IR or vice-versa. This depends on symmetry considerations. An IR-active
mode is one in which a particular vibration causes a change in the dipole moment of the
molecule, while only those vibrations which change the molecular polarizability lead to Raman
scattering. Therefore, the activity of a certain vibrational mode depends highly on its
symmetry and the symmetry of the molecule. One simple symmetry rule is the so-called mutual
exclusion rule for molecules with a center of symmetry. In such molecules no normal
mode may be active in both, the infrared and the Raman spectrum.
请点击此处下载
收藏
