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UPSC Civil Services (Mains)

Chemistry optional

  • 2 papers
  • 250 marks each
  • 3 h per paper
  • 5 of 8 questions, Q1 and Q5 compulsory

Paper I covers physical and inorganic chemistry.

UPSC Civil Services (Mains), Chemistry optional · 2024 · Paper I

Q5(a). Fluorescence and phosphorescence on a Jablonski diagram

Question

Indicating 'Fluorescence' and 'Phosphorescence' processes in a Jablonski diagram, distinguish between both processes.

Answer in brief

Fluorescence is spin-allowed emission from the lowest excited singlet, S₁ → S₀, over within about 10⁻⁸ s; phosphorescence is spin-forbidden emission from the lowest triplet, T₁ → S₀, so it is slow and continues after the light is cut off.

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Topic notes: photochemistry (quantum yield, Jablonski diagram, H₂–halogen reactions), with every Civil Services, IFoS and Geo-Scientist PYQ →

CoversJablonski diagramfluorescencephosphorescenceintersystem crossingsinglet and triplet statesinternal conversion

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