KAHIBARO
Table of Contents
What Excitation Means
When a charged particle passes through matter, it interacts mainly with the electrons of atoms. One possible result is excitation. In excitation, the particle gives some energy to an atomic electron, but not enough to remove that electron completely from the atom. The electron is pushed into a higher energy state, and the atom becomes an excited atom.
This is different from ionization, where an electron is fully removed. Since ionization has its own chapter, the focus here is only on the case where the atom stays neutral but temporarily stores extra energy internally.
Excitation occurs when a charged particle transfers energy to an atom and raises one of its electrons to a higher allowed energy level, without ejecting the electron from the atom.
Atomic Energy Levels
Atoms do not accept arbitrary amounts of internal energy. Their electrons can occupy only specific energy levels. Because of this, excitation is a quantized process. The incoming charged particle can excite the atom only if it transfers an amount of energy equal to the difference between two allowed states.
If the atom starts in an energy level $E_1$ and is raised to a higher level $E_2$, the excitation energy is
$$
\Delta E = E_2 - E_1
$$
This energy is taken from the moving charged particle, so the particle loses part of its kinetic energy.
For excitation, the transferred energy must match an allowed energy difference,
$$
\Delta E = E_{\text{final}} - E_{\text{initial}}
$$
This is why excitation is a discrete process.
How a Charged Particle Causes Excitation
A charged particle, such as an alpha particle, proton, or electron, carries an electric field. As it moves near atoms in a material, this electric field exerts forces on the atomic electrons. In some encounters, the interaction is gentle enough that the electron is not removed, but strong enough to raise it to a higher bound state.
The probability of excitation depends on several things, including the particle's charge, speed, and the structure of the material. A more strongly charged particle generally interacts more intensely with matter. A slower particle usually spends more time near the atom during an encounter, which can also increase energy transfer.
The process can be pictured as a brief collision-like interaction, even though it is really an electromagnetic interaction between the moving charged particle and the atomic electrons.
What Happens After Excitation
An excited atom usually does not stay excited for long. It tends to return to a lower energy state. When it does, the excess energy must go somewhere. Very often, the atom emits a photon whose energy equals the difference between the two levels:
$$
E_\gamma = \Delta E
$$
Using the photon relation,
$$
E_\gamma = hf
$$
where $h$ is Planck's constant and $f$ is the photon frequency.
This emitted radiation may be visible light, ultraviolet light, or some other part of the electromagnetic spectrum, depending on the size of the energy gap.
When an excited atom returns to a lower energy state, it often emits a photon with energy
$$
E_\gamma = hf = \Delta E
$$
Excitation and Energy Loss in Matter
As a charged particle travels through a material, it undergoes many interactions. Some of its energy loss goes into excitation, and some goes into ionization. Excitation is therefore one of the important channels by which radiation deposits energy in matter.
The energy transferred in a single excitation event is often smaller than the energy needed for ionization. Even so, a very large number of excitation events can occur along the particle's path, so the total energy lost this way can be significant.
In many materials, excitation contributes to heating, light production, and chemical changes. For example, in some detector materials, excited atoms or molecules later produce visible photons, which makes the radiation detectable.
Excitation in Atoms and Molecules
In isolated atoms, excitation usually refers to an electron moving to a higher electronic state. In molecules, the situation is richer. Molecules can be excited electronically, vibrationally, or rotationally. For an introductory view, the main point is that molecules also have discrete internal energy states, so charged particles can transfer energy into these forms as well.
The spacing of molecular energy levels is often different from atomic ones. Because of this, the emitted radiation from de-excitation can have different wavelengths and may sometimes appear as bands rather than sharp spectral lines.
Comparison with Ionization
Excitation and ionization are closely related because both come from the interaction of charged particles with atomic electrons. The difference is whether the electron remains bound.
| Process | Electron removed from atom? | Atom remains neutral? | Typical result |
|---|---|---|---|
| Excitation | No | Yes | Atom in higher energy state |
| Ionization | Yes | No | Ion pair formed |
This distinction is very important in radiation physics because different materials and detectors respond differently to excitation and ionization.
Excitation means energy is transferred without removing the electron.
Ionization means enough energy is transferred to free the electron from the atom.
Why Excitation Matters
Excitation is important because it explains several observable effects when radiation passes through matter. Excited atoms and molecules can emit light, which is the basis of scintillation detectors. Excitation can also lead to chemical and biological changes, since excited states are often more reactive than ground states.
In radiation transport, excitation is one of the mechanisms that slows charged particles down. It is therefore part of the broader description of stopping power and range, which are treated in other chapters.
A Simple Energy Picture
A useful way to think about excitation is as a stepwise transfer of energy from the moving particle to the internal structure of matter. The incoming particle begins with kinetic energy. During a close interaction with an atom, a small part of that kinetic energy is converted into internal atomic energy. Later, that internal energy may appear again as light or as thermal motion after further interactions.
Key Idea to Remember
Excitation is the transfer of energy from a passing charged particle to an atom or molecule in such a way that the target is left in a higher internal energy state, but no electron is removed. It is a quantized process, it contributes to the slowing down of charged particles in matter, and it often leads to later photon emission.
Essential idea:
A charged particle can lose energy in matter by exciting atoms or molecules,
$$
\Delta E = E_{\text{excited}} - E_{\text{ground}}
$$
with no electron ejected from the target.