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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 · 2022 · Paper I

Q5(b). Jablonski diagram (A to E); the route to photochemistry

Question

Label each of the following processes with proper explanation in Jablonski diagram: A Allowed absorption, B Fluorescence, C Phosphorescence, D Internal Conversion (IC), E Inter System Crossing (ISC). Discuss the mode of transition which is favourable for a photochemical reaction.

Answer in brief

The transition that favours a photochemical reaction is intersystem crossing, S₁ → T₁: the triplet state is long-lived, still rich in energy and reacts like a diradical.

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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.

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

CoversJablonski diagramintersystem crossingfluorescencephosphorescenceinternal conversiontriplet state

All parts of Question 5