Spontaneous vs Stimulated Emission
In atomic physics, electrons occupy discrete energy levels. Let E₁ be the ground state and E₂ be the excited state. When electrons transition between these levels, energy is either absorbed or emitted as photons. The emission of light occurs in two distinct ways: Spontaneous Emission and Stimulated Emission.
Spontaneous Emission
Spontaneous emission is the natural, unforced process by which an excited atom returns to its stable ground state.
- Mechanism: An electron is initially in the excited state E₂.Since higher energy states are unstable, the electron naturally drops to the lower energy state E₁ after a very short lifetime (typically 10⁻⁸ s).
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The energy difference is released as a photon according to the equation:
ΔE = E₂ − E₁ = hν
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Characteristics:
- Randomness: The exact time of emission and the direction of the emitted photon are completely random.
- Incoherence:Because millions of atoms emit photons independently at different times and in different directions, the resulting light waves are out of phase.
Stimulated Emission
Stimulated emission is the artificial, forced process of emission triggered by an external photon. This is the fundamental principle behind LASER operation.
- Mechanism: An electron is already in the excited state (E₂). An incident external photon, having an exact energy of hν=E₂−E₁, interacts with the excited atom.
- Energy Released: This interaction forces the electron to drop to the ground state (E₁) prematurely. As it drops, it releases a photon.
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Characteristics:
- Multiplication: The process starts with one incident photon and ends up with two photons (the incident one + the newly emitted one)
- Coherence: start with one incident photon and end up with two photons (the incident one + the newly emitted one)
Key Differences
| Parameter | Spontaneous Emission | Stimulated Emission |
|---|---|---|
| Trigger mechanism | No external trigger required; happens naturally. | Requires an external incident photon to trigger the drop. |
| Photon Output | One photon emitted per transition. | Two identical photons exit for every one incident photon. |
| Phase Relationship | Photons are out of phase (Incoherent). | Photons are perfectly in phase (Coherent). |
| Directionality | Multi-directional and scattered. | Highly directional and parallel. |
| Light Intensity | Low intensity (e.g., standard light bulbs, neon). | Extremely high intensity (e.g., Lasers). |
| Thermodynamic State | Dominant in thermal equilibrium. | Requires Population Inversion (more atoms in $E_2$ than $E_1$). |