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 exygen at altitudes below about \(90 \mathrm{~km}\).

Short answer

(a) Photodissociation is the process where a molecule absorbs a photon and breaks down into smaller molecules or atoms, while photoionization is the process where an atom or molecule absorbs a photon and loses one or more electrons, forming an ion. (b) At altitudes below 90 km, photodissociation is more important than photoionization because it requires lower energy, the available solar radiation consists of lower-energy photons that cause photodissociation, and the products of photodissociation can easily recombine due to the high concentration of oxygen in the lower atmosphere.

Step-by-step solution

Part (a): Defining Photodissociation and Photoionization

Photodissociation is the process in which a molecule absorbs a photon of light and breaks down into smaller molecules or atoms. In the case of oxygen, this process would result in the splitting of an oxygen molecule (O2) into two individual oxygen atoms (O). Photoionization, on the other hand, is the process in which an atom or molecule absorbs a photon of light and loses one or more of its electrons, resulting in the formation of an ion. For example, an oxygen molecule (O2) can undergo photoionization to form a positively charged ion (O2+) and an electron (e-).

Part (b): Energy Requirements and Altitude Effects

The energy requirements for photodissociation and photoionization are different. Photodissociation involves breaking a chemical bond, which requires lower energy than the process of photoionization, which involves removing an electron from its orbital. As a result, photodissociation can occur with lower-energy photons than photoionization. At altitudes below 90 km, the atmosphere is denser and contains a higher concentration of oxygen molecules. The incoming solar radiation at these altitudes consists of a range of wavelengths, some of which have sufficient energy to cause photodissociation, while most of the high-energy photons capable of causing photoionization have been absorbed at higher altitudes. This means that oxygen molecules in the lower atmosphere (below 90 km) are more likely to encounter photons with energies that can cause photodissociation than photons with energies that can cause photoionization. Furthermore, the products of photodissociation (oxygen atoms) can easily recombine to form oxygen molecules at these lower altitudes, where the concentration of oxygen is high. The products of photoionization, such as O2+ ions, are less likely to recombine in the lower atmosphere due to the lack of high-energy photons needed for the reverse process (i.e., ion recombination). Therefore, given the energy requirements, the range of available photon energies in the lower atmosphere, and the likelihood of recombination, photodissociation of oxygen is more important than photoionization at altitudes below 90 km.

Key concepts

Energy Requirements of Photodissociation

Photodissociation is a chemical process where a molecule absorbs a photon and breaks down into smaller components. This process is vital in understanding various atmospheric reactions. The key factor here is the energy required to break chemical bonds. Given that breaking a bond is less energy-intensive than removing an electron from an atom, photodissociation usually needs less energy compared to photoionization.

In atmospheric terms, photodissociation occurs when molecules like oxygen absorb photons of certain energies that are generally lower than those required for photoionization. Upon absorption, the molecule dissociates into separate atoms. Since this process happens with lower-energy photons, it's more common in regions with a range of photon energies, like closer to Earth's surface.

Photodissociation plays a crucial role in the stratosphere and troposphere, where it affects the concentration and formation of gases, contributing to phenomena such as ozone creation and degradation.

Energy Requirements of Photoionization

Photoionization refers to a process where an atom or molecule absorbs a photon, leading to the ejection of one or more electrons, forming ions. This process requires significantly higher photon energy compared to photodissociation. To ionize a molecule, the energy from the photon must be sufficient to overcome the attraction between electrons and the nucleus.

Photoionization is essential in upper atmospheric layers, such as the thermosphere, where ultraviolet and X-ray photons are more prevalent. These high-energy photons are necessary as they have the capability to ionize particles, creating ions and free electrons. As a result, these regions become ionized, inducting various atmospheric behaviors such as auroras.

Due to this energy requirement, photoionization is less likely to occur in the lower atmospheric altitudes, where fewer high-energy photons exist.

Effects of Altitude on Chemical Processes

The altitude in the atmosphere significantly affects the type of chemical processes that can take place, particularly photodissociation and photoionization. At higher altitudes, the presence of high-energy photons is more prominent due to the reduced interference from atmospheric layers. This makes processes like photoionization more likely, as these photons have sufficient energy to remove electrons from molecules, forming ions.

In contrast, at lower altitudes below 90 km, the atmosphere is denser and contains a higher concentration of molecules like oxygen. Most high-energy photons capable of causing photoionization have already been absorbed at higher altitudes.

Thus, the surviving lower-energy photons here are more conducive to inducing photodissociation. Oxygen molecules are split into individual atoms, influencing reactions contributing to ozone dynamics and atmospheric chemistry. - Higher altitudes: Favor photoionization with abundant high-energy photons. - Lower altitudes: Favor photodissociation due to denser molecular presence and less high-energy photon availability. - These processes play into the broader understanding of atmospheric science, helping predict and explain atmospheric conditions.