Which prevents spontaneous emission from excited atomic orbitals?

💡 Explanation

Selection rules dictate possible transitions; spontaneous emission requires allowed transitions. The quantum mechanical prohibition of certain momentum changes due to selection rules removes the mechanism of spontaneous de-excitation, because only particular reductions in orbital spin or angular momentum are allowed rather than random shifts.

• Which risk increases when a polymer's cross-linking density decreases?
• Which consequence result during liquid crystal display production when coating alignment layers incorrectly?
• Which risk significantly increases when a semiconductor crystal contains dislocations?
• Which consequence results when excessive regenerative feedback overwhelms a Rankine cycle?
• Which outcome occurs when excess charge accumulates non-uniformly on an ionic crystal surface?
• Which consequence results from gravitational influence on electromagnetic wave travel through spacetime?