In the realm of photochemistry and molecular spectroscopy, when a molecule absorbs energy, such as from a photon, it often transitions to an excited electronic state.
Once in this excited state, the molecule will undergo processes to return to its ground state.
Internal and external conversions are two of these processes that lead to the non-radiative dissipation of the absorbed energy.
Internal Conversion (IC):
Internal Conversion is a process where a molecule in an excited electronic state transitions to a lower electronic state without the emission of a photon, but rather by converting the energy into vibrational or rotational energy of the same molecule.
Mechanism:
This process is often facilitated by molecular vibrations that are similar in energy to the energy gap between the two electronic states.
The excess energy is then redistributed among the molecule's vibrational and rotational modes.
Outcome:
The molecule ends up in a higher vibrational level of a lower electronic state (often the ground electronic state).
Since vibrational energy is typically rapidly converted to heat through collisions with surrounding molecules, internal conversion often results in the heating of the molecular environment.
External Conversion (EC):
External Conversion is a process in which the energy from an excited molecule is transferred to its surroundings without the emission of radiation.
This is particularly common when the molecule is in a condensed phase (like in a solid or liquid), where neighbouring molecules or lattice structures can directly interact with the excited molecule.
Mechanism:
The vibrational or rotational energy of the excited molecule is directly transferred to the surrounding molecules or lattice, often through collisions or close molecular interactions.
Outcome:
The excited molecule returns to its ground state (or a lower excited state), while the surrounding medium gains energy and may increase in temperature.