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

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. Inclu...

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Bibliographic Details
Published in:The journal of physical chemistry letters 2021-06, Vol.12 (24), p.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: Article
Language:English
Subjects:
Physical Insights into Chemistry, Catalysis, and Interfaces
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Summary: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.
ISSN:1948-7185
1948-7185
DOI:10.1021/acs.jpclett.1c01103