26. Luminescence. Atomic conversions in luminescent light emission. Characteristics and laws of luminescence. Diagnostic methods using luminescence.

Definition

• Luminescence is the emission of light by a substance not caused by heat (in contrast to incandescence).
• It results from atoms or molecules being excited by different processes and then emitting photons during relaxation to lower energy states.

• Characterized by:
• The excitation source (light, electric field, chemical reaction, radiation).
• The emission mechanism (radiative or non-radiative transitions).
• The delay between excitation and emission.

Types of Luminescence

1. By Excitation Source

• Photoluminescence: excitation by photons (UV, visible light).
• Electroluminescence: excitation by an electric field (used in LEDs, display panels).
• Chemiluminescence: excitation from chemical reactions.
• Radioluminescence: excitation by ionizing radiation (e.g., scintillators in nuclear physics).

2. By Delay Between Excitation & Emission

• Fluorescence:
• Very short-lived (10⁻⁹ to 10⁻⁶ s).
• Emission stops immediately when excitation ceases.
• Phosphorescence:
• Delayed emission lasting microseconds → hours.
• Due to forbidden electronic transitions (metastable states).

3. By Mechanism

• Resonant Fluorescence: only radiative transitions back to ground state.
• Indirect Luminescence: involves both radiative and non-radiative transitions.
• Recombinant Luminescence: caused by recombination of charged particles (common in semiconductors).

4. By Nature

• Spontaneous Luminescence: occurs without continuous external excitation.
• Induced Luminescence: requires constant excitation to persist.