What term describes the transition of energy within a molecule or atom from a higher to a lower state without accompanying photon emission?

The transition of energy within a molecule or atom from a higher to a lower state without the emission of a photon is referred to as internal conversion. This process occurs when an excited electron transfers its energy to another electron or to vibrational modes within the molecule rather than releasing it in the form of electromagnetic radiation, such as light.

In internal conversion, the excited state of a molecule can interact with its surrounding environment and redistribute energy internally, which can result in various energy relaxation pathways, including vibrational, rotational, or electronic transitions. This process is significant in the context of molecular photophysics and plays a crucial role in understanding how excited states evolve in molecular systems.

The other concepts mentioned are related but distinct phenomena. For example, the photoelectric effect involves the emission of electrons from a material after absorbing photons, while pair production refers to the creation of a particle-antiparticle pair from a high-energy photon. Transitional emission does not correspond to a standard term used in this context. Therefore, internal conversion is the accurate term describing energy transition without photon emission.