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 km.

Short answer

(a) Photodissociation is the process where a molecule absorbs a photon and breaks apart into its constituent atoms or smaller molecules, while photoionization is a process in which an atom or molecule absorbs a photon and loses one or more electrons, becoming ionized. (b) Photodissociation of oxygen is more important than photoionization at altitudes below 90 km because the energy requirement for photodissociation (5.12 eV) is lower than that for photoionization (13.62 eV). At these lower altitudes, the concentration of oxygen molecules is higher, and the solar radiation is more likely to have the appropriate energy to cause photodissociation of molecular oxygen, rather than photoionization. Additionally, the absorption of high-energy photons by ozone and other molecules at higher altitudes filters out photons with sufficient energy for photoionization, further increasing the importance of photodissociation at lower altitudes.

Step-by-step solution

Define Photodissociation and Photoionization

Photodissociation and photoionization are both processes in which a molecule or atom interacts with a photon, leading to different outcomes. Photodissociation is the process in which a molecule absorbs a photon and subsequently breaks apart into its constituent atoms or smaller molecules. This process occurs when the absorbed photon has enough energy to break the chemical bonds that hold the molecule together. The general formula for this process can be written as: \(A_2 + h\nu \rightarrow A + A\) where \(A_2\) is the diatomic molecule, \(h\) is the Planck constant, \(\nu\) is the frequency of the photon, and \(A\) are the resulting separate atoms. Photoionization is a similar process in which an atom or molecule absorbs a photon and subsequently loses one or more electrons, thus becoming ionized. This will occur when the energy of the absorbed photon is higher than the ionization energy of the molecule or atom. The general formula for this process can be written as: \[A + h\nu \rightarrow A^+ + e^-\] where \(A\) is the atom or molecule, \(h\) is the Planck constant, \(\nu\) is the frequency of the photon, \(A^+\) is the ionized atom or molecule, and \(e^-\) is the ejected electron.

Compare the Energy Requirements

The energy requirements for photodissociation and photoionization depend on the specific molecule or atom and the energy of the photon. The photodissociation energy for molecular oxygen (\(O_2\)) is approximately 5.12 eV, while the ionization energy of atomic oxygen (\(O\)) is about 13.62 eV.

Photodissociation versus Photoionization at Lower Altitudes

At altitudes below 90 km, the solar radiation includes photons with a wide range of energies. Due to the energy requirements discussed in Step 2, it is more likely that oxygen molecules will encounter photons with enough energy to undergo photodissociation than photoionization. Moreover, at these lower altitudes, the concentration of oxygen molecules is higher, which further increases the probability of photodissociation occurring. Additionally, the absorption of high-energy photons by ozone (\(O_3\)) and other molecules at higher altitudes filters out photons with sufficient energy for photoionization. As a result, the photons that reach lower altitudes below 90 km are more likely to have the appropriate energy to cause photodissociation of molecular oxygen rather than photoionization. In conclusion, the photodissociation of oxygen is more important than photoionization at altitudes below 90 km due to the lower energy requirements for photodissociation, the higher concentration of oxygen molecules, and the selective filtering of high-energy photons by other atmospheric constituents.

Key concepts

Photoionization

Photoionization is the process where a photon strikes a molecule or atom, providing it with enough energy to eject one of its electrons, resulting in an ionized molecule or atom. This ionization process transforms the originally neutral species into a charged one. When the photon possesses energy exceeding the ionization energy required for the ejection of an electron, ionization occurs. You can visualize this interaction like a photon delivering a powerful "push" to eject the electron out of its orbit around the nucleus.
Why is this important? This process is crucial for understanding many atmospheric and cosmic phenomena. In space, photoionization is important in influencing the properties of the ionosphere and contributes to the creation of auroras.

Key Points about Photoionization:

• Involves the absorption of a photon.
• Leads to the ejection of an electron and the formation of an ion.
• Depends on the energy of the photon being higher than the ionization energy of the atom or molecule.

Oxygen Molecule

Oxygen molecules, denoted as \(O_2\), consist of two oxygen atoms bonded together. These diatomic molecules are prevalent in our atmosphere, owing to their crucial role in life processes such as respiration and combustion.
In addition, oxygen plays a significant part in atmospheric chemistry, especially in processes like the photodissociation of ozone, which shields the Earth from the sun's harmful ultraviolet radiation.

Here are some essential aspects of oxygen molecules:

• Makes up about 21% of Earth's atmosphere.
• Participates in crucial processes like photosynthesis and cellular respiration.
• Enters into chemical reactions such as oxidations.

Photon Energy

Photon energy is the energy carried by a single photon, which is the most basic unit or quantum of light. It is directly proportional to the frequency of the photon and can be calculated using the equation: \(E = h u\), where \(E\) is the energy, \(h\) is Planck's constant, and \(u\) is the frequency of the photon.
Photons with higher frequencies carry more energy. For example, ultraviolet photons have higher energy than visible light photons. This energy is pivotal in driving the processes of photodissociation and photoionization.

Understanding Photon Energy:

• Photon energy dictates the likelihood of processes like photodissociation and photoionization occurring.
• Higher energy photons can break chemical bonds or eject electrons.
• The energy is quantified in electronvolts (eV).

Chemical Bonds

Chemical bonds are the attractive forces that hold atoms together in molecules. These bonds can vary in strength, often classified into strong and weak bonds. The stronger the bond, the more energy required to break it. Chemical bonds are pivotal in the stability and formation of molecules, influencing how chemical reactions occur.
In the context of photodissociation, photon energy can provide enough force to break these bonds, resulting in the separation of a molecule into smaller units. Thus, understanding chemical bonds can help explain how molecules respond to photon interactions.

Key Characteristics of Chemical Bonds:

• Include types like covalent, ionic, and metallic bonds.
• Strong bonds require more energy (often delivered by photons) to break.
• Determine the physical properties and stability of molecules.

Ionization Energy

Ionization energy is the minimum amount of energy needed to remove an electron from an atom or molecule, thus creating a positive ion. This energy is crucial in the context of photoionization, as only photons with appropriate energy levels can accomplish this feat.
The energy is typically measured in electronvolts (eV) and varies depending upon the atom or molecule in question. For instance, oxygen atoms have a higher ionization energy compared to many simple molecules, meaning it takes more energy to ionize them.

Understanding Ionization Energy:

• Represents the energy required to remove the most loosely bound electron of an isolated gaseous atom or molecule.
• Higher ionization energies mean more energy is required for ionization to occur.
• Plays a crucial role in determining the vulnerability of different substances to photoionization.