UPSC Combined Geo-Scientist, Mains · 2023 · Mains Paper II
Q1(p). Decay routes of a photoexcited molecule
Question
State the possible decay routes by which a photoexcited molecule can dissipate its excess energy. Explain all these possible processes with the help of a labelled diagram.
Answer in brief
Radiative: fluorescence, phosphorescence. Non-radiative: vibrational relaxation, internal conversion, intersystem crossing. Bimolecular or chemical: quenching, energy transfer, photoreaction.
Key points
- Vibrational relaxation passes excess energy to the solvent (10⁻¹² s); internal conversion is radiationless crossing between states of the same multiplicity.
- Fluorescence: S₁(v = 0) → S₀, spin-allowed, 10⁻⁹ to 10⁻⁷ s, at longer wavelength than the absorption (Stokes shift).
- Intersystem crossing: S₁ → T₁ with a spin flip, made possible by spin–orbit coupling.
- Phosphorescence: T₁ → S₀, spin-forbidden, slow (10⁻⁴ to 10² s); it continues after the light is switched off.
- Quenching, energy transfer and photochemical reaction; Kasha’s rule: emission comes from the lowest excited state of each multiplicity.
The full model answer
As written for the answer booklet: the diagram carries the working, the lines stay short. Read here only; it is not offered as a download.
CoversJablonski diagramfluorescencephosphorescenceintersystem crossinginternal conversionKasha's rule
All parts of Question 1
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