Séminaire

Mardi 17 Décembre 2024 à 11h00.

Multiscale Modeling of Dipolar Second Harmonic Generation Signatures from Liquid-Air Interfaces

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Liquid-air interfaces are crucial in many physical, chemical, and biological phenomena. For instance, the exchange of heat and particles between the atmosphere and oceans and the interactions of aerosols and water droplets are key factors in controlling global warming. Additionally, reactions at the interface of biological membranes show high selectivity and efficiency [1]. However, probing the interface structure and dynamics requires specific techniques that separate bulk and surface signal contributions, like those targeting the second-order nonlinear optical (NLO) susceptibilities, χ(2), because the usually dominant dipolar χ(2 ) contribution vanishes for media with inversion center, such as the bulk region. χ(2) gives rise to various phenomena, including the Second Harmonic Generation (SHG), which is the conversion of pairs of photons of ℏω energy into photons of 2ℏω energy.
To predict and interpret χ(2) of interfaces, sequential and multiscale simulation procedures have been elaborated and optimized. They allow decomposing the SHG responses into contributions from the successive molecular layers of the interface by examining their first hyperpolarizability, β, the molecular property associated with χ(2). In practice, classical molecular dynamics simulations of liquid slabs provide structural diversity, including the interfacial asymmetry, for the subsequent β calculations on clusters of molecules using quantum mechanics methods. Using this approach, we revealed the evolution of properties at the microscopic scale as a function of the molecular layer from the interface to the bulk for water/air and alcohol/air (from methanol to 1-pentanol) interfaces [2, 3]. The main observations include: (i) a net β response at the first molecular layer, which vanishes at the second molecular layer for water and small alcohols, indicating a strong 2-dimensional network at the interface and (ii) for alcohols with longer aliphatic chains, an orientational correlation between the first and second molecular layers that leads to a centrosymmetric bilayer structure, which quenches β and, therefore, χ(2).

[1] M. F. Ruiz-Lopez, et al. Nat. Rev. Chem. 4 (2020) 459.
[2] T. N. Ramos and B. Champagne, ChemistryOpen 12 (2023) e202200045.
[3] T. N. Ramos and B. Champagne, Phys. Chem. Chem. Phys. 26 (2024) 8658.

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