A fundamental understanding of the photoexcited carrier system in diamond is crucial to facilitate its application in photonic and electronic devices. Here, we report the detailed balance between free carriers and excitons in intrinsic diamond by using a deep-ultraviolet pump in combination with broadband terahertz (THz) probe spectroscopy. We investigated the transformation of photoexcited carriers to excitons by using an internal transition of excitons, which is found to occur at a frequency of 16 THz. We determined the equilibrium constant in the Saha equation from the temperature dependence of the free-carrier density measured at chemical equilibrium. The derived exciton binding energy is larger than the conventional value, which indicates an energy shift due to the fine-structure splitting of the exciton states.

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