Photon Absorption

As solar photons penetrate into the atmosphere some fraction of them will be absorbed. The size of this fraction depends on both the wavelength of the incident photon and the interacting molecule. The primary quantity used to described a molecule's tendency to absorb light is the absorption cross-section, $\sigma$, which has units of area (e.g.: cm²). Absorption cross-sections are measured in the laboratory where atmospheric conditions are simulated. They are generally functions of temperature and pressure. The depth to which direct solar radiation can penetrate depends on wavelength. Radiation between 120 and 300 nm is absorbed mainly in the mesosphere and stratosphere by O₂ and O₃. Radiation between 300 and 800 nm can penetrate into the troposphere and through to the surface. Radiation shortward of 120 nm is fully attenuated before reaching the mesosphere.

 

Table 2.1: Possible energy pathways following photon absorption ${\rm AB}+{\rm h\nu }\longrightarrow {\rm AB^*}$.

 

$\rm AB^*$	$\longrightarrow$	$\rm AB^+ + e^-$	Photoionization
$\rm AB^*$	$\longrightarrow$	$\rm A + B$	Photodissociation
$\rm AB^*$	$\longrightarrow$	$\rm AB + h\nu'$	Fluorescence (or radiative decay)
$\rm AB^* + M$	$\longrightarrow$	$\rm AB + M^*$	Quenching
$\rm AB^* + C$	$\longrightarrow$	$\rm A + BC$	Chemical Reaction

The photon absorption process can be represented by,

$$\rm AB + h\nu \longrightarrow \rm AB^*$$ (3.17)

where h$\nu$ represents the photon (sometimes represented as $\gamma$) and ^* identifies molecule AB as being in an excited state. The excited states, or energy levels, are at discrete intervals (i.e. quantized) so that the energy of the photon must coincide with the energy level of one of the excited states to within the width of its absorption line. If the energy levels are densely spaced or possess relatively short lifetimes (and hence wide absorption lines), a pseudo-continuous absorption spectrum will result. For widely spaced excited states with long lifetimes, the absorption spectrum will be made up of individual lines. Five different pathways, listed in Table 2.1, describe the possible fate of the absorbed photon energy.

The first, photoionization, is the process where the energy level of a valence electron is raised into the continuum. Generally, kinetic energy is imparted to both the election and the ion. Wavelengths which are capable of ionizing an atom or molecule do not penetrate below the mesosphere. The second, photodissociation, or photolysis (described in section 2.3.2) results in the splitting of a molecule through the channeling of the absorbed photon energy into overcoming the binding force of a chemical bond. The third, fluorescence, is the process whereby the excited molecule releases its energy by radiating another photon. This new photon need not have the same energy, polarization, or direction as the incident photon. This is, in fact, a form of inelastic scattering but is not important for this work. The fourth, quenching, is the transfer of energy and momentum as the result of a collision with another molecule (M is N₂ or O₂). This energy generally ends up as kinetic energy. The fifth describes a chemical reaction. The absorbed photon energy is required to overcome the reaction activation energy.