Charged aqueous interfaces studied by Second Harmonic and Sum Frequency Generations

Envoyé par laetitia.dalstein 
Charged aqueous interfaces studied by Second Harmonic and Sum Frequency Generations
vendredi 30 septembre 2022 18:10:51
Charged aqueous interfaces studied by Second Harmonic and Sum Frequency Generations

Interfaces of water and aqueous solutions play a prominent role in many technological and natural processes. The liquid/solid interface is the main driver for many electrochemical reactions. Water being present everywhere, the fields of applications are numerous.
Water interfaces are the most commonly used platforms for chemistry and biological processes, but the current understanding at the molecular level and ultrashort timescales is uncharted territory.
The charges repartition near the surface, forming an electrical double layer (EDL), can be described thanks to the Gouy-Chapman-Stern (GCS) model 3.
The inner element, known as the ‘‘Stern layer’’, has an opposite charge compared to the surface and its width dStern is of tens of nanometers. This so-called Stern layer is at the immediate neighborhood of the interface, where the interfacial bonding network frames.
Underneath, the second layer is the diffuse layer, in which particles follow the Poisson-Boltzmann distribution.
The main idea is then to structure and charge water interfaces, without adding any chemical compound, by intense terahertz (THz) pulses. This original idea is based on the fact that the coupling between the permanent dipole moment of the water molecules and the THz field should result in the re-orientation of the dipole moment which in turn should affect the arrangement of water molecules in the bulk and at the interface. This re-arrangement will be then revealed by performing time-resolved THz pump/SHG or SFG probe experiments
Our goal is to develop a hybridized Second Harmonic generation (SHG)/ Sum frequency generation (SFG) spectroscopies to study vibrational modes for aqueous interfaces. More precisely, we intend to develop a method based on terahertz field-induced surface charging and study its consequent interface dynamics through SFG/SHG spectroscopies.

To prove that we can structure a charged interface with THz, we shall start this project by measuring, as mentioned previously, the surface nonlinear signal for a charged interface and control the surface charge thanks to an intense THz field focused at the interface. We already built an SHG optical setup and implemented the THz beam. During the project, you will first study the water structure dynamics through SHG measurements. The second step will be the SFG setup implementation, i.e. adding an IR beam light. You will then be able to study the microscopic water structure, deduce the vibrational spectra of the Stern layer at the air/water interface as a function of electric field for the first time without adding any chemical, providing microscopic insight into the interfacial bonding structure at the air/water interface.

Wen, Y.-C.; Zha, S.; Liu, X.; Yang, S.; Guo, P.; Shi, G.; Fang, H.; Shen, Y. R.; Tian, C., Unveiling Microscopic Structures of Charged Water Interfaces by Surface-Specific Vibrational Spectroscopy. Physical Review Letters 2016, 116, 016101.
Dalstein, L.; Chiang, K.-Y.; Wen, Y.-C., Direct Quantification of Water Surface Charge by Phase-Sensitive Second Harmonic Spectroscopy. The Journal of Physical Chemistry Letters 2019, 10,5200-5205.

Research profile:
The PhD/postdoc candidate will be in charge of all the research activity regarding the analysis of the water molecules dynamics at the air/water interfaces (setting and calibrating the setup and instruments, measurements, numerical analysis…). Ideally, the candidate has a training in (nonlinear) optics. A strong taste for multidisciplinary and experimental research is required (nonlinear optics, optics, physical chemistry). Previous expertise in one or many of the above-mentioned fields would be a strong value for PhD candidate and is required for postdoc candidate.

The Laboratoire Ondes et Matière d’Aquitaine is composed of three research teams tackling fundamental and applied subjects. Topics include materials and photonics, condensed matter physics, soft matter, and biophysics. The research carried out is both theoretical and experimental. Within the photonics and materials team comprising 19 researchers, you will work in the “PULS” group and you will be in direct interaction with several doctoral students, postdocs and researchers.
The candidate will have a full optical table at his/her disposal for carrying out the experiments.
All components are already available to start the project.

Interested candidates should send their application by October 16th, 2022.
Please feel free to contact Laetitia Dalstein for any complementary information.

The following documents are required for applying:
- a full resume with academic results
- a motivation letter
- one or two letters of recommendation
- a copy of the last degree obtained.
- a copy of academic records for 1st and 2nd Master years.