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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 · 2023 · Paper I

Q5(b). Radiative and non-radiative processes; Jablonski diagram

Question

Explain radiative and non-radiative processes in terms of the singlet and triplet electronic states of a molecule. Also explain them through a Jablonski diagram.

Answer in brief

Radiative: absorption, fluorescence (singlet → singlet) and phosphorescence (triplet → singlet); non-radiative: vibrational relaxation, internal conversion (same spin) and intersystem crossing (spin changes).

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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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Read the whole 2023 solution2023 question paper

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

CoversJablonski diagramfluorescencephosphorescenceintersystem crossinginternal conversionKasha's rule

All parts of Question 5