Adsorption and Nonlinear Optical Properties of Ascorbic Acid on Silver Clusters: Experimental and <scp>DFT</scp> Study

The interaction between ascorbic acid (AA) and a silver nanocluster (Ag₃) is investigated through a combined experimental and theoretical approach. Density Functional Theory (DFT) calculations at the B3PW91/6-311++G(d,p) and LanL2DZ levels revealed that adsorption of AA on Ag₃ induces structural reorganization accompanied by charge transfer from Ag₃ to AA. The charge transfer is confirmed by molecular electrostatic potential (MEP), natural population analysis (NPA), Fukui function analysis, and natural bond orbital (NBO) calculations. The reduced band gap of AA-Ag₃ indicates the interaction between AA and Ag₃. Experimental UV-Vis, Fourier-transform infrared (FTIR), and Fourier-transform Raman (FTR) spectra supported the computational findings, revealing changes in electronic and vibrational modes upon adsorption. The first hyperpolarizability (β) of AA-Ag₃ is calculated to be 21 times higher than that of pristine AA. Open-aperture Z-scan measurements validated the predicted nonlinear optical (NLO) response, demonstrating the potential of the AA-Ag₃ molecular system for advanced NLO device applications.