Computational Analysis of Vibrationally Resolved Absorption Spectra and Third-Order Nonlinear Optical Properties in Alkali Metal-Substituted Adamantane

In this study, we employ quantum chemical calculations to analyze the third-order nonlinear optical (NLO) properties of alkali metal-substituted adamantane (Ad). Specifically, we investigate the vibronic spectra and third-order NLO properties of Ad substituted with Lithium (Li–Ad), Sodium (Na-Ad), and Potassium (K-Ad). Among the studied molecules, K-Ad exhibits the highest second-order hyperpolarizability, the lower vibrational frequency resulting from the heavier mass of potassium, makes the molecule less prone to bond breakage and shows maximum intensity at a longer wavelength of 840 nm. Interestingly, substitution by alkali metals causes redshift, thus all substituted molecules are in the visible region and the maximum absorption increases from Lithium to potassium. These insights suggest that alkali metal-substituted Ad compounds are promising candidates for high-performance third-order NLO materials, offering the potential for integration into next-generation optical devices.