The allowed transitions between energy levels in an atom or ion are governed by the principles of quantum mechanics. According to the Bohr model and subsequent quantum mechanical models, atoms have discrete energy levels, and transitions between these levels occur when the atom absorbs or emits electromagnetic radiation. Here are the key types of transitions:
Absorption: An atom can absorb a photon of specific energy, corresponding to the difference in energy between two energy levels. When an atom absorbs a photon, it gains energy, and an electron moves from a lower energy level to a higher energy level. This transition is known as an absorption transition.
Emission: An excited atom can return to a lower energy level by emitting a photon. The emitted photon carries away the energy difference between the two levels. This process is known as an emission transition. The emitted photon's energy corresponds precisely to the energy difference between the initial and final energy levels.
Spontaneous Emission: In some cases, an excited atom may undergo spontaneous emission without any external influence. The electron in the excited state spontaneously transitions to a lower energy level, emitting a photon. The energy of the emitted photon corresponds to the energy difference between the two levels involved.
Stimulated Emission: In addition to absorption and spontaneous emission, stimulated emission is another type of transition. If an atom in an excited state is exposed to an external photon of the correct energy, it can be stimulated to undergo an emission transition. The emitted photon has the same energy, frequency, and phase as the stimulating photon, leading to amplification and coherence in certain systems (e.g., laser technology).
These transitions are represented by spectral lines in the electromagnetic spectrum. Each atom or ion has a unique set of energy levels, resulting in a characteristic spectrum of absorption and emission lines, often used for identification and analysis purposes.
It's important to note that the specific energy levels and transitions depend on the atomic or molecular structure, including the number of protons, neutrons, and electrons, as well as their arrangement within the atom or ion.