The key difference between excitation and absorption is that excitation is the process of absorbing a photon and moving into a higher energy level whereas absorption is the process of transferring energy from a photon to a certain object.
The terms absorption and excitation are useful in the fields of quantum mechanics, analytical chemistry, relativity, and many others. You need a good understanding of these terms to understand the content of these fields properly. The concepts of absorption and excitation are also the foundation concepts in the field of spectroscopy and spectrometry.
CONTENTS
1. Overview and Key Difference
2. What is Excitation
3. What is Absorption
4. Side by Side Comparison – Excitation vs Absorption in Tabular Form
5. Summary
What is Excitation?
Excitation is the transfer of a system that is at a low energy state to a state of high energy. Thus, this term can be discussed regarding an electron bound to the nucleus at the ground state. Quantum mechanics suggests that an electron can only take specific energy states. Furthermore, the probability of finding an electron in between these stationary states is zero. Therefore, the energy differences between the two stages are discrete values. That means; an electron can absorb or emit energies corresponding to any difference between stationary states, but not in between.
Figure 01: Excitation by Irradiation
Excitation is the process of absorbing such a photon to go up to a higher energy level. The opposite process of excitation is emitting a photon to come down to a lower energy level. If the energy of the incident photon is sufficiently large enough, the electron will move to a very large energy state, thus removing itself from the atom. We call it “ionization”.
What is Absorption?
Absorption is a term we generally use to identify some quantity becoming a part of another quantity. In chemistry, we mainly use the term absorption in the sense of electromagnetic waves. The absorption of electromagnetic waves refers to the transferring process of the energy of the photon to the system in which the photon has been absorbed. In the process of absorption, the incident photon is lost.
Let us take a system with a single electron bound to the nucleus. For example, assume that the electron is in the ground state. If a photon collides with the electron, the electron can absorb the photon depending on the energy of the photon. Moreover, if the energy of the photon is equal to the energy difference between the ground state and some other state, the electron can absorb the photon. However, if the energy of the photon is not equal to an energy gap, the photon will not be absorbed. The photon has initial momentum due to the mass of the photon. It causes a momentum change of the electron when the photon is absorbed. Absorption is the major principle of the absorption and emission spectra.
Figure 02: Absorption Spectra for Carotenoids
What is the Difference Between Excitation and Absorption?
Excitation is the change of the state of a system to a state of higher energy while absorption is the energy transfer from a photon to a system. Hence, the key difference between excitation and absorption is that excitation is the process of absorbing a photon and moving into a higher energy level whereas absorption is the process of transferring energy from a photon to a certain object.
Moreover, for excitation to occur, absorption must happen, and for absorption to occur, the system must be excited. Therefore, absorption and excitation are mutual processes.
Summary – Excitation vs Absorption
Excitation and absorption are closely related terms. The key difference between excitation and absorption is that excitation is the process of absorbing a photon and moving into a higher energy level whereas absorption is the process of transferring energy from a photon to a certain object.
Reference:
1. “Excitation.” Encyclopædia Britannica, Encyclopædia Britannica, Inc., 17 Aug. 2006, Available here.
Image Courtesy:
1. “Excitation of energy levels by irradiation (diagram)” By Jordan Levine – Own work (CC BY-SA 4.0) via Commons Wikimedia
2. “Carotenoids Absorption Spectrum” By byr7 (CC BY 2.0) via Flickr