CHAP. 12: BASIC TERMS OF CHEMICAL PHYSICS [CONTENTS] 434
Photons of visible or ultraviolet light interact readily with rotational and vibrational states of
molecules. The spectrum of scattered light is then called theRaman spectrum.
If the photon loses part of its energy during the collision, and its frequency after the collision
is thus lower, we speak about theStokes radiation. If, on the contrary, the photon gains
further energy on colliding with a previously excited molecule, we speak about theanti-Stokes
radiation.
The condition for a peak in a Raman spectrum is either a change of the molecule polariz-
ability due a change of the vibrational state [see12.1.4], or anisotropic polarizability, i.e. the
change of polarizability (in selected direction) due to rotation. The Raman spectra thus add
to information gained using the absorption or emission rotation-vibration spectra.
12.3.7 Magnetic resonance spectra
In the presence of a magnetic field the energy of electrons and nuclei may change in dependence
on the spin quantum number [see (12.40) and (12.41)]. Consequently, photons with appropri-
ately chosen frequency may be absorbed, and subsequently again emitted, in transition between
the spin quantum states. The name “resonance” comes from a classical picture of the spin as ro-
tation of electrons giving rise to a magnetic moment. Interaction of this magnetic moment with
a magnetic field creates a torque causing a precession of the rotating object (flywheel). When
the frequency of the incident radiation matches the frequency of the precession, a resonance
occurs and a signal is detected.
If a molecule with an odd number of electrons transits between the states given by the
quantum numbersms=−^12 andms= +^12 , we speak aboutelectron spin resonance(ESR),
and the resonance frequency is given by
ν=gemBB
h. (12.52)
The resulting frequency depends on the magnitude of the magnetic field, and it is usually in
the range of microwave radiation. The ESR is primarily employed in detecting free radicals in
chemical kinetics.
If it is the nucleus spin that interacts with the magnetic field, we speak aboutnuclear
magnetic resonance(NMR). The resonance frequency is given by the relation
ν=gNmNB
h