Question

(a) Distinguish between photodissociation and photoionization. (b) Use the energy requirements of these two processes to explain why photodissociation of oxygen is more important than photoionization of oxygen at altitudes below about \(90 \mathrm{~km}\).

Short answer

Photodissociation is the process where a molecule absorbs a photon of light and breaks apart into smaller fragments, while photoionization is the process where an atom or molecule absorbs a photon and loses an electron. Photodissociation of oxygen requires less energy (around \(5.12 \mathrm{~eV}\) per photon) than photoionization (approximately \(12.08 \mathrm{~eV}\) per photon), making it more likely to occur at lower altitudes where the energy of incoming photons is lower. Additionally, less ozone at altitudes below \(90 \mathrm{~km}\) means a higher level of ultraviolet light available for photodissociation. Therefore, photodissociation is more important than photoionization at altitudes below about \(90 \mathrm{~km}\).

Step-by-step solution

(a) Definitions of photodissociation and photoionization)

Photodissociation is the process where a molecule absorbs a photon of light and breaks apart into smaller fragments. This can be represented as: \[AB + h\nu \rightarrow A + B\] Where \(AB\) is the molecule, \(h\nu\) represents the energy of the photon, and \(A\) and \(B\) are the resulting molecular fragments. On the other hand, photoionization is the process where an atom or molecule absorbs a photon and loses an electron. The general equation for this process can be represented as: \[X + h\nu \rightarrow X^{+} + e^-\] Here, \(X\) is the atom or molecule, \(h\nu\) represents the energy of the photon, \(X^{+}\) is the resulting ion, and \(e^-\) represents the released electron.

(b) Energy requirements of photodissociation and photoionization)

To explain the importance of photodissociation of oxygen at altitudes below \(90 \mathrm{~km}\), we need to compare the energy requirements of photodissociation and photoionization. For oxygen, the photodissociation process can be represented as: \[O_2 + h\nu \rightarrow O + O\] The energy required for this process is around \(5.12 \mathrm{~eV}\) per photon. On the other hand, the photoionization of oxygen involves: \[O_2 + h\nu \rightarrow O_2^{+} + e^-\] The energy required for oxygen photoionization is more significant at approximately \(12.08 \mathrm{~eV}\) per photon.

(b) Comparing the importance of photodissociation and photoionization at low altitudes)

Since photodissociation of oxygen (\(O_2\)) requires less energy than photoionization (\(5.12 \mathrm{~eV}\) versus \(12.08 \mathrm{~eV}\)), it is more likely to occur at low altitudes where the energy of incoming photons is lower. Furthermore, when considering the Earth's atmosphere's composition, there is less ozone (O3) at altitudes below \(90 \mathrm{~km}\), which means there is less absorption of ultraviolet light by ozone. Consequently, there is a higher level of ultraviolet light available to cause photodissociation of oxygen molecules. This is why photodissociation is more important than photoionization at altitudes below about \(90 \mathrm{~km}\). Photodissociation leads to more effective dissociation of oxygen molecules at lower altitudes due to lower energy requirements and a greater availability of ultraviolet light to drive this process.

Key concepts

Molecular Physics

Molecular physics plays a key role in understanding the processes of photodissociation and photoionization. These processes are fundamental interactions between light and matter at the molecular level.
Photodissociation involves the breaking of a molecular bond due to the absorption of a photon, a packet of light energy, which results in the division of the molecule into two or more fragments. This process can be symbolized as \(AB + hu \rightarrow A + B\), where \(AB\) is the original molecule, and \(A\) and \(B\) are the resulting fragments.
On the other hand, photoionization occurs when the energy of a photon is sufficient to remove an electron from a molecule, creating a positively charged ion. This can be described as \(X + hu \rightarrow X^{+} + e^-\), where \(X\) becomes \(X^{+}\) upon losing an electron \(e^-\).
Both processes involve the absorption of ultraviolet or higher energy photons, but they differ in their energy requirements and the resulting chemical products.

Ultraviolet Radiation

Ultraviolet (UV) radiation is a form of electromagnetic energy with a wavelength shorter than that of visible light. It is crucial in driving photochemical reactions in the atmosphere, such as photodissociation and photoionization.
UV radiation is divided into three categories: UVA, UVB, and UVC, based on their wavelengths. UVC has the highest energy and can cause the breakdown of oxygen molecules into individual oxygen atoms through photodissociation. For instance, the energy required to split an \(O_2\) molecule is about \(5.12 \mathrm{~eV}\), which can be provided by UVC radiation.
This same UV energy can also ionize an \(O_2\) molecule, but since this requires significantly more energy, approximately \(12.08 \mathrm{~eV}\), it is less prevalent. At lower altitudes, where UV energy is diminished by atmospheric filters, photodissociation becomes the more dominant process due to its lower energy requirement.

Atmospheric Chemistry

Atmospheric chemistry involves the study of chemical reactions in the Earth's atmosphere, shaped largely by processes such as photodissociation and photoionization. These reactions are influenced by the availability of sunlight and the composition of atmospheric gases.
In the lower atmosphere, particularly below \(90 \mathrm{~km}\), photodissociation of oxygen is significant. This is due to the higher relative availability of photons with just enough energy to break the \(O_2\) bond, compared to those needed for photoionization.
The breakdown of \(O_2\) into individual oxygen atoms via photodissociation is a critical step in forming ozone \((O_3)\), a vital layer that absorbs harmful UV radiation.
The balance between photodissociation and photoionization impacts the concentration of reactive species and ultimately influences atmospheric phenomena such as the distribution of ozone and the occurrence of various chemical cycles. Mastery of these processes helps us understand broader environmental issues, including climate change and air quality.