Question

It is given that the electronic ground state of a diatomic molecule $X_2$ has even parity and the nuclear spin of $X$ is 0. Which one of the following is the CORRECT statement with regard to the rotational Raman spectrum ($J$ is the rotational quantum number) of this molecule?

• A. Lines of all $J$ values are present
• B. Lines have alternating intensity in the ratio of $3:1$
• C. Lines of only even $J$ values are present
• D. Lines of only odd $J$ values are present

Hint:

In Raman spectroscopy, the parity of the electronic state and selection rules ($\Delta J = \pm 2$) govern the allowed transitions, leading to either even or odd $J$ values.

Answer 1

The Correct Option is C

- In rotational Raman spectroscopy, the selection rule for the rotational quantum number $J$ is that the allowed transitions are between $J$ and $J+2$ (i.e., $\Delta J = \pm 2$).
- Since the molecule has even parity and the nuclear spin of $X$ is 0, the transitions in the rotational Raman spectrum will be restricted to only even values of $J$ for the ground state.
Thus, only lines with even values of $J$ will be present. Therefore, the correct answer is (C).