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Why are atoms in high-energy states so rare in thermal equilibrium?
In physics, the importance of thermal equilibrium and the distribution of energy states to natural phenomena is self-evident. When we discuss the energy state of a system (such as an atom), we often encounter the concept of "population reversal." This is particularly important in laser science, because the operation of lasers requires a special energy distribution, that is, there must be more atoms in high-energy states than atoms in low-energy states. However, in the case of thermal equilibrium this is extremely difficult.
"In a state of thermal equilibrium, the number of high-energy atoms is almost negligible."
To understand this, you first need to consider the Boltzmann distribution. According to Boltzmann statistics, in a system in thermal equilibrium, the so-called energy level distribution is determined by the ratio of particles occupying different energy states. In a laser medium composed of atoms, these atoms can exist in two energy states: the ground state and the excited state. The energy of the ground state is lower than that of the excited state, so at room temperature, the number of atoms in the ground state is usually much higher than that of the excited state according to the Boltzmann factor.
It is known that as the temperature increases, some atoms gain energy by absorbing photons and enter an excited state. But even so, when the system reaches thermal equilibrium, the number of atoms in the excited state (N2) will never exceed the number of atoms in the ground state (N1). As you can imagine, this is a challenge in facing the laws of nature.
"Demographic reversal can only be achieved in a non-equilibrium state."
The principle of laser relies on three interactions of light: absorption, natural radiation and stimulated emission. When a beam of light passes through a group of atoms, if the frequency of the light matches a certain energy difference, the atoms in the ground state will absorb the photons and transition to the excited state. However, this process is also accompanied by the occurrence of spontaneous emission and stimulated emission, which complicates the photon exchange process. If the number of atoms in the ground state is large, the absorption process dominates, resulting in attenuation of light; whereas if the number of atoms in the excited state is large, enhancement of light and generation of laser light will occur.
This is why in the process of implementing lasers, indirect methods, such as optical pumping, are often required to achieve lasting population reversal. In three-level or four-level lasers, by selectively exciting a certain energy level, only a few high-energy state atoms are maintained, thereby achieving the advantages of the laser system.
"Three-level and four-level lasers demonstrate different pumping and amplification principles, and their efficiency differences reflect how to achieve a balance between high-energy states and ground states."
It is worth noting that in many systems selection rules limit the possibilities for energy transfer, which we must consider when making lasers. For example, different substances may respond very differently to laser emission, and certain transitions may be subject to selection rules governed by quantum mechanics, so their luminescence may be delayed by phenomena such as phosphorescence.
In summary, the state of thermal equilibrium makes the number of high-energy atoms scarce, because in this state, the number of ground state atoms is usually much greater than the number of excited states. To break this balance and achieve the majority of high-energy states, external energy needs to be used to drive the system, such as through optical pumping technology. This raises a critical question: can effective ways be found to create and maintain a state of population inversion in our daily lives to support more efficient laser technologies?
