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I'm studying silicate photosensitive glasses that contain $\ce{Ce}$ ions, and such glasses have an absorption band at 305 nm due to $\ce{Ce^3+}$. After that, the $\ce{Ce^3+}$ ion becomes $\ce{Ce^4+}$, which absorbs at about 270 nm.

Colleagues say that under irradiation by photons with wavelength near 305 nm, the process of photoionization take place. I find this surprising because another rare earth ion ($\ce{Nd^3+}$) doesn't ionize under excitation, but only via transfer to an excited state.

So why does the absorption of a photon by $\ce{Ce^3+}$ ions result in ionization and the appearance of a free electron, while only excitation and relaxation happens with the electrons of $\ce{Nd^3+}$ ions?

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