Spontaneous emission

Tiny bits of matter can hold extra energy. This is called an excited state.

Sometimes, these tiny bits want to rest. They move to a lower energy state. When they do this, they let go of energy. This energy comes out as a tiny bit of light.

This light can happen in many ways. It can happen when we use heat. It can even happen with light. Some things glow for a long time. This is called phosphorescence.

This process helps make lasers work. It is a very special way for light to start. The tiny bits of light move in different directions.

Tiny bits of matter, like atoms, can hold extra power. We call this an excited state.

Sometimes, these atoms want to rest. They move to a lower level called a ground state. As they move down, they must let go of their extra power. They give this off as a tiny bit of light called a photon. This way of moving is called spontaneous emission.

Albert Einstein first predicted this in 1916. Later, Paul Dirac helped explain it using new math. This process is a quantum process. That means it follows the rules of the very small.

There are many ways atoms can glow. If light makes them glow, it is called fluorescence. If they glow for a long time, it is called phosphorescence. This is very useful for making lasers. Lasers start with this process.

When an atom lets out a photon, it can go any way. The direction of the light is random. This is different from other types of light. The light can also come out as heat. We call this nonradiative decay. This happens when the energy is released as tiny shakes called phonons.

Spontaneous emission is a very special way that tiny things work. It happens when a small system like an atom or a molecule is in an excited state. An excited state means the system has extra energy. To reach a lower energy state, like its ground state, it must let that energy go. It releases this energy as a tiny packet of light called a photon.

This process is a quantum process. This means it follows the rules of quantum mechanics, which govern the very small. It is different from how light works in our everyday world.

How does this happen step by step? First, the atom starts with a lot of energy in its excited state. Next, the atom moves down to a lower energy level. As it moves, it releases the difference in energy as a single photon. The energy of that photon depends on the gap between the two levels. The direction the photon travels is completely random. The phase of the photon is also random. This randomness is a key part of how spontaneous emission works.

Scientists have studied this for a long time. Albert Einstein first predicted this idea in 1916. He wrote a series of papers about it. His work led to what we now call the Einstein A Coefficient. Later, in 1926, Paul Dirac used quantum theory to describe the rate of this process. In 1930, Victor Weisskopf and Eugene Wigner wrote a famous paper on the topic. Their work is still a standard way to study how atoms and molecules emit light.

There are many interesting facts about these light bursts. If an atom is excited by something other than heat, we call it luminescence. If light causes the glow, it is called fluorescence. Some things, called phosphorescence, glow for a long time even after the light is gone. This happens because they have a metastable level. This is a state where the energy stays for a while. Lasers actually start using spontaneous emission before they switch to a different way of working.

We can see these ideas in many places. For example, quantum dots can be tuned by changing their size. This changes how fast they emit light. We also see this in how energy can turn into heat. This is called nonradiative decay. Instead of light, the energy is released as tiny vibrations called phonons.

Understanding these tiny movements helps us build better technology like lasers and lights. It helps us see how the smallest parts of our world interact with light.

Spontaneous emission is a fundamental quantum mechanical process. It occurs when a system, such as an atom, molecule, or subatomic particle, moves from an excited energy state to a lower energy state. This transition is often a move to the ground state, which is the lowest possible energy level. During this jump, the system releases a specific, quantized amount of energy. This energy is emitted in the form of a particle of light called a photon.

This process is essential to how light is produced in the universe. It is a purely quantum phenomenon that cannot be explained by classical electromagnetic theory.

The mechanism follows a specific sequence of energy changes. First, the system must be in an excited state, meaning it possesses more energy than its ground state. When the system decays to the lower level, it must account for the energy difference between these two states. The energy of the emitted photon is exactly equal to this difference. This energy is calculated using the formula E = hf, where h is the reduced Planck constant and f is the angular frequency. A key feature of this process is randomness. Unlike stimulated emission, the direction in which the photon travels and its phase are both completely random.

Scientists categorize light emission based on how the initial excitation occurs. If a system is excited by something other than heat, the process is called luminescence. There are several sub-types of luminescence. Electroluminescence and chemiluminescence are two examples, distinguished by how the atoms are excited. If the excitation happens because the system absorbs radiation, it is called fluorescence. Some systems possess a metastable level, which is a state where energy is held for a longer period. This leads to phosphorescence, where the system continues to fluoresce long after the initial radiation is turned off.

The history of this discovery involves several major scientific milestones. Albert Einstein first predicted spontaneous emission in a series of papers starting in 1916. His work eventually led to the development of the Einstein A Coefficient. This coefficient represents the rate of spontaneous decay for a specific transition. Later, after the formal discovery of quantum mechanics in 1926, Paul Dirac provided an accurate description of the emission rate. In 1930, Victor Weisskopf and Eugene Wigner published a landmark paper. Their Weisskopf-Wigner calculation remains the standard approach for studying radiation in atomic and molecular physics today.

Modern physics explains this process through the lens of quantum electrodynamics (QED). In QED, the electromagnetic field is quantized at every point in space. Even in a vacuum, there is a ground state known as the QED vacuum. This vacuum contains fluctuations, often called zero-point energy. These fluctuations interact with the excited state of the atom. This interaction causes the atom's state to mix with the vacuum field. This mixing makes the transition from an excited state to a ground state possible. Because the electromagnetic field has infinitely many ways a photon can be emitted, the decay is practically irreversible.

The rate of this emission, known as the radiative rate, depends on specific factors. According to Fermi's golden rule, the rate is determined by an atomic part and a field part. The atomic part relates to the internal structure and the transition dipole moment of the source. The field part describes the density of the electromagnetic modes in the surrounding environment. In free space, the rate of spontaneous emission is proportional to the cube of the frequency. This relationship has been tested using quantum dots, which are tiny particles that can be tuned by changing their size. By changing the size, scientists can observe how the frequency affects the emission rate.

Not all energy transitions result in the emission of light. There is also a process called nonradiative decay. In this type of relaxation, the energy is released as phonons, which are essentially heat. The total decay rate of a system is the sum of the radiative and nonradiative rates. Scientists use the term quantum efficiency to describe the fraction of decays that result in light. If every decay produces a photon, the quantum efficiency is 100 percent. In many semiconductors, electrons move quickly through small nonradiative transitions to reach a metastable level. From there, they often make a final move via an optical transition across the bandgap.

Spontaneous emission also connects to advanced studies in light and matter. For example, the Jaynes-Cummings model was developed in 1963. This model describes a two-level atom interacting with a quantized field mode within an optical cavity. This research showed that the rate of spontaneous emission can be controlled by the boundary conditions of the surrounding vacuum field. This discovery led to the field of cavity quantum electrodynamics (CQED). This field studies how mirrors and cavities can affect radiative corrections and light behavior.