Photoluminescence definition: Fluorescence occurs under which electronic transition condition?

Introduction / Context:
Fluorescence and phosphorescence are distinct emission processes. Correctly identifying the originating and final electronic states is key for interpreting lifetimes, spectra, and selection rules in photophysics and analytical fluorescence techniques.

Given Data / Assumptions:

• Fluorescence involves allowed spin-conserving transitions.
• Phosphorescence involves spin-forbidden transitions from triplet states.
• Vibrational relaxation is nonradiative (no photon emitted).

Concept / Approach:
Fluorescence: S1 → S0 + hν (spin-allowed, short lifetimes, typically ns). Phosphorescence: T1 → S0 + hν (spin-forbidden, longer lifetimes, μs to s). Recognizing these distinctions guides expectations for intensity and temporal behavior.

Step-by-Step Solution:
Identify fluorescence as singlet-to-singlet emission.Reject triplet-to-singlet as phosphorescence.Reject vibrational relaxation as nonradiative (no photon).Choose the singlet-to-singlet emission description.

Verification / Alternative check:
Time-resolved measurements show fluorescence in nanoseconds, whereas phosphorescence persists much longer due to spin-forbidden nature—confirming state assignments.

Why Other Options Are Wrong:

• Triplet → singlet photon emission: That is phosphorescence.
• Vibrational relaxation with photon: Vibrational relaxation is nonradiative.
• None of the above: Incorrect because one option is correct.

Common Pitfalls:
Confusing fluorescence with phosphorescence due to both involving emitted photons—focus on initial state multiplicity.

Final Answer:
Singlet excited state to singlet ground state with photon emission.