Page 322 - Instant notes

Physical Chemistry 308

Emission, absorption and scattering spectroscopy

The different types of spectroscopy are illustrated schematically in Fig. 2. In emission
spectroscopy, a molecule (or atom) already in an excited state undergoes a transition
from a state of high energy to a state of low energy, and emits the excess energy as a
photon. The distribution in frequency of the emitted photons is the emission spectrum.
In absorption spectroscopy, a photon of specific frequency is absorbed by the
molecule to promote it from a low energy state to a high energy state. The absorption
spectrum is obtained by observing the intensity of the transmitted radiation relative to the
incident radiation over a range of frequencies of incident light.
In the Raman variant of scattering spectroscopy, a monochromatic (single
frequency) beam is directed at the sample and the frequency of the light that is scattered
away from the direction of the incident beam is analyzed. A proportion of the scattered
photons have different frequency to the incident photons because the molecule gains or
loses energy in the collision. Photons that lose energy in the interaction travel away at
lower frequency to the incident light (Stokes scattering); photons that acquire energy
from the interaction travel away at higher frequency (anti-Stokes scattering).
Emission, absorption and Raman spectroscopy all provide essentially the same
information about energy level separations, but practical considerations and selection
rules generally dictate which technique is most appropriate. Absorption spectroscopy is
usually the most straightforward to apply.