Chapter: Modern Analytical Chemistry: Spectroscopic Methods of Analysis
Spectroscopy Based on Emission
Spectroscopy Based
on Emission
An analyte in an excited
state possesses an energy, E2, that
is greater than
that when it is in a lower energy
state, E1. When the
analyte returns, or relaxes to a lower
en- ergy state the excess energy,
∆E,
∆E = E2
– E1
must be released. Figure 10.5 shows
a simplified picture
of this process.
The lifetime
of an analyte
in the excited state, A*, is short; typically 10–5–10–9 s for electronic excited states and 10–15 s
for vibrational excited
states. Relaxation oc- curs through
collisions between A* and other species in the sample,
by photochemi- cal reactions, and by the emission of photons. In the first
process, which is called vi- brational deactivation, or nonradiative relaxation, the excess
energy is released
as heat; thus
A* → A + heat
Relaxation by a photochemical reaction
may involve a decomposition reaction
in which A* splits apart
A* → X+Y
or a reaction between A* and another
species
A* +Z → X+Y
In either case the excess
energy is used up in the chemical
reaction or released
as heat.
In the third
mechanism excess energy
is released as a photon
of electromagnetic radiation.
A* → A + hv
The release of a photon following thermal
excitation is called emission, and that fol- lowing the absorption of a photon is
called photoluminescence. In chemilumines-
cence and bioluminescence, excitation results
from a chemical or biochemical reac- tion,
respectively.
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