$Ce^{3+}$ and $Yb^{3+}$ show strong absorption in UV region. Which of the following reason is not correct for it?

Question

Accepted Answer

Option 3 is the correct answer because the statement "Promotion of electrons in these ions is difficult than for other ions" is not a valid reason for the strong absorption of $Ce^{3+}$ and $Yb^{3+}$ in the UV region. Here's why:

1. **Explanation for Option 3:**
   - The promotion of electrons in $Ce^{3+}$ and $Yb^{3+}$ is not inherently more difficult than for other ions. In fact, these ions have partially filled 4f orbitals, which allow for electronic transitions that are relatively accessible. The strong absorption in the UV region is primarily due to the nature of the electronic transitions (4f to 5d), not because the electron promotion is difficult. Therefore, this option does not correctly explain the strong UV absorption.

2. **Explanation for why other options are incorrect:**

- **Option 1: Transition occur from 4f to 5d.**
     - This is a correct reason for the strong absorption in the UV region. The transition from 4f to 5d orbitals involves a significant change in energy, which corresponds to the UV region of the electromagnetic spectrum. This type of transition is allowed and contributes to the strong absorption observed.

- **Option 2: $\Delta l=1$.**
     - This option is correct because the transition from 4f to 5d involves a change in the orbital angular momentum quantum number ($\Delta l=1$), which is an allowed transition. This selection rule supports the occurrence of such transitions, leading to strong absorption.

- **Option 4: f-d peaks are broad.**
     - This is also a correct reason. The f-d transitions typically result in broad absorption peaks due to the nature of the 5d orbitals, which are more delocalized compared to the 4f orbitals. This broadening contributes to the strong absorption characteristics in the UV region.

In summary, Option 3 is the correct answer because it incorrectly attributes the strong UV absorption to the difficulty of electron promotion, which is not the case for $Ce^{3+}$ and $Yb^{3+}$. The other options correctly describe factors that contribute to the strong absorption in the UV region.