The key difference between photodissociation and photoionization is that photodissociation is the breakdown of a chemical compound due to the activity of photons whereas photoionization is the interaction between photons and atoms or molecules in a sample to form ionic species.
In brief, both, photodissociation and photoionization, are physical processes that describe the interaction between photons and atoms or molecules.
CONTENTS
1. Overview and Key Difference
2. What is Photodissociation
3. What is Photoionization
4. Side by Side Comparison – Photodissociation vs Photoionization in Tabular Form
5. Summary
What is Photodissociation?
Photodissociation is a physical process in which a chemical compound is broken down due to the action of photons. We can define it as the interaction between one or more photons with a single target molecule. Furthermore, this process is not limited to visible light. This means; any photon that has sufficient energy to affect the chemical bonds of a chemical compound can undergo photodissociation process. Also, the energy of a photon is inversely proportional to the wavelength of its EMR. Therefore, EMR with high energy or small wavelength can be involved in photodissociation reactions.
A common example of photodissociation is the photolysis in photosynthesis. Photolysis is a light-dependent reaction that occurs as a part of the Hill reaction of photosynthesis. The reaction can be given as follows.
H2A + Photons ⇒ 2e + 2H+ + A
Besides, photoacids are molecules that can undergo photodissociation upon light absorption, causing the transfer a proton to form a photobase. Here, the dissociation occurs in an electronically excited state. `
What is Photoionization?
Photoionization is a physical process where an ion forms via the reaction between a photon and an atom or a molecule. However, we cannot categorize all the interactions between photons and atoms or molecules as photoionization because some interactions form non-ionized species; therefore, we have to relate an interaction to the photoionization cross-section of the chemical species. Also, this photoionization cross-section depends on the energy of the photon and the properties of chemical species undergoing the process.
Figure 1: Photoionization, which makes the once-invisible filaments in the deep space glow
Multi-photon ionization is a type of photoionization where several photons combine their energies to ionize an atom or a molecule. Here, the energy of the photons should be below the threshold of ionization energy.
In addition to the above type, tunnel ionization is another type of photoionization reaction where the laser intensity used for the photoionization process is increased, or a longer wavelength is used, allowing a multi-photon ionization to take place. The result of this process is the distortion of the atomic potential in such a way that only a relatively low and narrow barrier between a bound sate and the continuum states remain. Here, the electrons can tunnel through the barrier. These are called the tunnel ionization and over the barrier ionization respectively.
What is the Difference Between Photodissociation and Photoionization?
Photodissociation and photoionization are physical processes. The key difference between photodissociation and photoionization is that photodissociation is the breakdown of a chemical compound due to the activity of photons whereas photoionization is the interaction between photons and atoms or molecules in a sample to form ionic species.
Below infographic gives more details of the difference between photodissociation and photoionization.
Summary – Photodissociation vs Photoionization
Photodissociation and photoionization are physical processes. The key difference between photodissociation and photoionization is that photodissociation is the breakdown of a chemical compound due to the activity of photons whereas photoionization is the interaction between photons and atoms or molecules in a sample to form ionic species.
Reference:
1. “Photo-Ionization.” Encyclopædia Britannica, Encyclopædia Britannica, Inc., 10 Apr. 2008, Available here.
Image Courtesy:
1. “Hubble green filaments 20150402” By NASA, ESA, Galaxy Zoo Team and William Keel (University of Alabama, USA) – (image link) (Public Domain) via Commons Wikimedia