Q11 · CSIR-NET Chemistry, December 2011
Paper: CSIR-NET December 2011 · Subject: Physical Chemistry · Chapter: Atomic and Molecular Term Symbol · Topic: Selection Rules · Marks: 2 · Difficulty: Easy
Observe the following electronic transition of a diatomic molecule. \[ \begin{array}{l} \text{(A)}\ { }^{1}\Sigma_{g}^{+} \rightarrow { }^{3}\Sigma_{g}^{+} \\ \text{(B)}\ { }^{1}\Sigma_{u}^{+} \rightarrow { }^{1}\Sigma_{g}^{+} \\ \text{(C)}\ { }^{1}\Delta_{u} \rightarrow { }^{1}\Sigma_{g}^{+} \\ \text{(D)}\ { }^{1}\Pi_{g} \rightarrow { }^{1}\Sigma_{u}^{+} \end{array} \]
The allowed transitions are (a)(A) and (C) only
(b)(B) and (D) only
(c)(A), (B) and (C) only
(d)(A), (C) and (D) only
Answer
Answer: B ✓ checked by 4AB · confidence high
Explanation
Selection rules: ΔS = 0, g ↔ u, ΔΛ = 0, ±1, Σ⁺ ↔ Σ⁺. (A) changes spin and (C) has ΔΛ = 2, so both are forbidden. (B) and (D) are allowed (b).
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