25-08-2017, 09:32 PM
PHOTOCHEMISTRY
[attachment=59826]
Absorption and emission of
electromagnetic (EM) radiation
by atoms and molecules
Photochemical processes (e.g. photodissociations) are
initiated by the absorption of EM radiation (i.e. light).
This absorption is induced by the electric ( E ) field of
the EM radiation [in the case of an electric dipole (or
electric quadrupole) interaction], or the magnetic ( B )
field [magnetic dipole interaction].
Electric dipole transitions are typically ~105 times
stronger than those carried by a magnetic dipole
interaction, and ~107 times more intense than electric
quadrupole transitions. Thus we shall concentrate on
electric dipole transitions.
Widths of Transitions
Absorption may be spread over a range of frequencies
for many reasons:
• Spectral congestion − due to the overlap of many
rotational or vibrational transitions.
• Finite spectrometer resolution.
• Doppler broadening; the spread of velocities in a
sample of molecules typically introduces a width of
~10–6 of the transition frequency.
The Doppler broadened width scales as T M .
• If either state involved in a transition has a short
lifetime, τ, the transition will exhibit a homogeneous
linewidth.
• This linewidth can be estimated from the energy-
time form of the uncertainty principle
[attachment=59826]
Absorption and emission of
electromagnetic (EM) radiation
by atoms and molecules
Photochemical processes (e.g. photodissociations) are
initiated by the absorption of EM radiation (i.e. light).
This absorption is induced by the electric ( E ) field of
the EM radiation [in the case of an electric dipole (or
electric quadrupole) interaction], or the magnetic ( B )
field [magnetic dipole interaction].
Electric dipole transitions are typically ~105 times
stronger than those carried by a magnetic dipole
interaction, and ~107 times more intense than electric
quadrupole transitions. Thus we shall concentrate on
electric dipole transitions.
Widths of Transitions
Absorption may be spread over a range of frequencies
for many reasons:
• Spectral congestion − due to the overlap of many
rotational or vibrational transitions.
• Finite spectrometer resolution.
• Doppler broadening; the spread of velocities in a
sample of molecules typically introduces a width of
~10–6 of the transition frequency.
The Doppler broadened width scales as T M .
• If either state involved in a transition has a short
lifetime, τ, the transition will exhibit a homogeneous
linewidth.
• This linewidth can be estimated from the energy-
time form of the uncertainty principle