A New Imaginary Term in the Second-Order Nonlinear Susceptibility from Charged Interfaces

• Published in: The journal of physical chemistry letters Vol. 12; no. 24; pp. 5649 - 5659
• Main Authors: Ma, Emily, Ohno, Paul E, Kim, Jeongmin, Liu, Yangdongling, Lozier, Emilie H, Miller, Thomas F, Wang, Hong-Fei, Geiger, Franz M
• Format: Journal Article
• Language: English
• Published: American Chemical Society 24.06.2021
• Subjects: Physical Insights into Chemistry, Catalysis, and Interfaces
• ISSN: 1948-7185; 1948-7185
• DOI: 10.1021/acs.jpclett.1c01103

Nonresonant second harmonic generation (SHG) phase and amplitude measurements obtained from the silica–water interface at varying pH values and an ionic strength of 0.5 M point to the existence of a nonlinear susceptibility term, which we call χX (3), that is associated with a 90° phase shift. Including this contribution in a model for the total effective second-order nonlinear susceptibility produces reasonable point estimates for interfacial potentials and second-order nonlinear susceptibilities when χX (3) ≈ 1.5χwater (3). A model without this term and containing only traditional χ(2) and χ(3) terms cannot recapitulate the experimental data. The new model also provides a demonstrated utility for distinguishing apparent differences in the second-order nonlinear susceptibility when the electrolyte is NaCl versus MgSO4, pointing to the possibility of using heterodyne-detected SHG to investigate ion specificity in interfacial processes.