# A thermodynamic approach to spontaneous emission

• frankchen
In summary: When the electrons in this group are excited, they will release their energy, activating more degrees of freedom and increasing the entropy of the entire system. This is why spontaneous emission occurs. It can also be seen as a result of the progression of time, as the probability of excited atoms remaining in this state decreases over time, leading to eventual spontaneous emission. However, neither explanation fully explains why this phenomenon occurs. In summary, spontaneous emission is a natural result of the tendency towards maximum entropy in isolated macroscopic systems.
frankchen
any isolated macroscopic system, like a large group of atoms, always tends to reach a state that has the maxium entropy. if the electrons in this group of atoms are excited, they will give out their energy so that more degrees of freedom are activated, hence the entropy of the whole system increase. that's why spontaneous emission happens, is that right?

frankchen said:
any isolated macroscopic system, like a large group of atoms, always tends to reach a state that has the maxium entropy. if the electrons in this group of atoms are excited, they will give out their energy so that more degrees of freedom are activated, hence the entropy of the whole system increase. that's why spontaneous emission happens, is that right?
Maybe it is just me, but I don't find entropy to be particularly useful in explaining why things happen.

One could also look as spontaneous emission as a consequence of the progression of time. Since the probability that an excited atom will remain in the excited state declines exponentially as a function of time, the probability that excited atoms as a group will undergo spontaneous emission, eventually, approaches certainty. Spontaneous emission, being random, progresses in only one direction because there are many ways that a large system of excited atoms can undergo energy decay and only one way to remain in the original state.

Neither approach really tells us why it occurs.

AM

Yes, that is correct. In thermodynamics, entropy is a measure of the disorder or randomness in a system. The second law of thermodynamics states that the entropy of an isolated system will always tend to increase over time. In the case of a large group of atoms, the energy of the excited electrons will naturally be transferred to other degrees of freedom, such as photons, leading to an increase in the overall entropy of the system. This process is known as spontaneous emission, and it is a result of the tendency of systems to reach a state of maximum entropy. This thermodynamic approach provides a fundamental understanding of the phenomenon of spontaneous emission and its connection to the second law of thermodynamics.

## 1. What is the thermodynamic approach to spontaneous emission?

The thermodynamic approach to spontaneous emission is a theoretical framework used to understand the spontaneous emission of photons from an excited atom or molecule. It is based on principles of thermodynamics and statistical mechanics.

## 2. How does the thermodynamic approach explain spontaneous emission?

According to the thermodynamic approach, spontaneous emission occurs when an excited atom or molecule releases excess energy in the form of photons to reach a lower energy state. This process is governed by thermodynamic principles such as the increase of entropy and the decrease of free energy.

## 3. What are the key assumptions of the thermodynamic approach to spontaneous emission?

The key assumptions of the thermodynamic approach include the existence of a thermal equilibrium between the atom or molecule and its surroundings, and the validity of the Boltzmann distribution for the population of energy levels. It also assumes that the emission process is a random and irreversible one.

## 4. How is the thermodynamic approach different from other models of spontaneous emission?

The thermodynamic approach differs from other models, such as the quantum mechanical approach, in that it takes into account the effects of the surroundings on the emission process. It also provides a macroscopic description of the emission process, whereas other models focus on the microscopic behavior of individual particles.

## 5. What are the practical applications of the thermodynamic approach to spontaneous emission?

The thermodynamic approach has practical applications in fields such as laser technology, where it is used to optimize the efficiency of spontaneous emission processes. It also has applications in astrophysics, where it is used to understand the emission of radiation from stars and other celestial objects.

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