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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 · 2018 · Paper I

Q6(b)(i). Fluorescence and phosphorescence

  • 5 marks
  • Answered
Question

Distinguish fluorescence from phosphorescence.

Answer in brief

Fluorescence is emission from the excited singlet S₁ to S₀: spin-allowed, fast (ns), stops when the light is cut off. Phosphorescence is emission from the triplet T₁ to S₀ after intersystem crossing: spin-forbidden, slow (ms to s), persists after the light is removed.

The full model answer

Written out in full, as it should go in the answer booklet. The solution page with its diagrams follows it.

FeatureFluorescencePhosphorescence
TransitionS₁→ S₀ (singlet to singlet)T₁→ S₀ (triplet to singlet)
Spinallowed, no spin changeforbidden, spin flips; made weakly allowed by spin–orbit coupling
Lifetime10⁻⁹ to 10⁻⁷ s10⁻³ s to seconds or longer
After the light is cut offstops at onceglow continues (afterglow)
Wavelengthlonger than the absorbed lightlonger still (T₁ lies below S₁)
Conditionssolution at room temperaturefavoured in rigid media (frozen glass, 77 K); quenched by O₂ and collisions
Examplesfluorescein, quinine sulfate, anthraceneZnS:Cu, benzophenone at 77 K, glow-in-the-dark paints

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Model answer to Q6(b)(i), 2018 Paper I: solution page 37
Solution page 37 of 55
Read the whole 2018 solution2018 question paper

All parts of Question 6