Question

The O atom has several emission features. Which will have the longest lifetime?  

(A) 2p^{4 1}D --> 2p^{4 3}P

(B) 2p^{4 1}S --> 2p^{4 3}P

(C) 2p³ 3p ³P --> 2p^{4 3}P

(D) 2p³ 3s  ³P --> 2p⁴³P

From the question set I found this from, I noticed a problem involving emission with a d to p transition giving the angular quantum number value change as -1(allowed selection rules). I would like to know if this is an iron rule trend for emission (i.e. d -->p =emission) . I know the answer from the study guide but I don't know why? Any help would be greatly appreciated.

Answer 1

The given options can be analyzed explicitly as shown below.

午 L11 C's > 虹1] → (3p)

The highest L-value possible for the configuration of the oxygen atom is +2, the corresponding term is 'D'.

The L-value for the configuration of the ground state of the oxygen atom is +1, the corresponding term is 'P'.

(The allowed transition rules are: $\Delta$S = 0 and $\Delta$L = ±1)

Hence, the transition for the oxygen atom involving the transition between the D and P terms (i.e. option A) has the longest lifetime, although it is spin forbidden.