Effect of H₂O Molecule Adsorption on the Electronic Structure and Optical Properties of the CsI(Na) Crystal

We investigated H₂O molecule adsorption that had an effect on the luminescence properties of the CsI(Na) crystal using experiments and first-principle calculations. We measured the emission spectra of the CsI(Na) crystal at different exposure times under gamma ray excitation. The experimental results showed that the energy resolution of the CsI(Na) crystal was worse when the crystal surface adsorbed more H₂O molecules, and the crystal surface deliquescence decreased the luminescence efficiency of the CsI(Na) crystal. We studied the band structure, density of states, and optical properties changes caused by H₂O molecule adsorption on the CsI(Na) (010) surface. The generalized gradient approximation (GGA) was used to describe the exchange and correlation potential between the electrons. Our calculation results showed that the band gap width of the CsI(Na) (010) surface decreased after adsorbing H₂O molecules, while three new peaks appeared in the valence band, and the absorption coefficient decreased from 90,000 cm^-1 to 65,000 cm^-1, and the reflection coefficient decreased from 0.195 to 0.105. Further, the absorption coefficient was reduced by at least 25% because of H₂O molecule adsorption, which led to the luminescence degradation of the CsI(Na) crystal.