Surfaces and Sum Frequency Heather C. Allen allen@chemistry.ohio-state.edu The Ohio State University Department of Chemistry and Biochemistry

Sum frequency generation (SFG) Surface vs bulk vibrational spectroscopies Interface selectivity Instrumentation overview Experimental examples Progress Technique Silos: Collaboration for joint SFG + other Surface Technologies

Vibrational Spectroscopy and Selection Rules Infrared Dipole Raman Polarizability Stokes scattering Virtual excited state v=0 v=1 IR absorption Anti Stokes scattering vis IR sum v = 0 v = 1 Virtual electronic state Sum Frequency Generation (SFG) Raman & IR active modes Interface selective - lack of inversion symmetry SFG-Active depth depends on noncentrosymmetry

yet with low S/N and depth of hundreds of nm. Sum Frequency IR Raman Gaigeot et al., PCCP, 2018, 20, 5190 Surface-Selective SFG of Water Surface Signature is the 3700 cm-1 free OH bond of water existing in the topmost surface layer. Lui et al., JPCB, 2004, V108, 2252 Du, Superfine, Freysz, Shen In 1993 - the 1st SFG of the water surface; PRL V70, 2313 Bulk Raman of Water Raman can be used in specular reflection mode (and internal reflection), yet with low S/N and depth of hundreds of nm. Bulk Infrared on Water IR can also be used in ATR and specular reflection mode (IRRAS), and is highly useful for certain applications, yet signal depth is on the order of microns, far from being surface selective.

BBSFG: Conventional versus Heterodyne SFG General Types of Sum Frequency Generation Spectrometers Scanning SFG versus Broad bandwidth SFG (BBSFG) BBSFG: Conventional versus Heterodyne SFG Verreault et al., JPCLett, 2012, 3 3012. Hommel, Ma, Allen, Anal. Sci., 2001, 17, 1325. Hommel, Allen, Anal. Sci., 2001, 17,137. Mondal et al., JACS. 2010, 132, 10656 Typical BBSFG IR profiles Chen et al., JACS, 2010. 132, 11336

Examples of selectively identifying surface phenomena ---------------------------------Organics at Aqueous Surfaces----------------------------------------- CH2 SS CH3 SS Tang et al. 113(26), 7383 (2009)  Tang et al., JPCB 114(51), 17068 (2010) Identification of binding for Na+ vs K+ to surface-bound fatty acid Chen, Allen JPCA 2009  V113(45), 12655 Soluble DMSO is pushed out from the surface by DPPC Intensity ratio (CH3 SS/CH2 SS) indicates ordering of alkyl chains Identification of surface pKa protonation vs deprotonation

Sum Frequency Generation – Geometrical Considerations Interfacial Accessibility – Geometrical (& polarization) configurations greatly affect SFG response & interpretation Solid/Liquid Baldelli et al, JPCB 2014, 118, 5203 Vacuum/Solid Moore, Richmond 2008, V41, 739 Liquid/Liquid Zhang, App Spec 2017, V71(8) 1717 Hore, Gibbs et al., JPCC, 2017, V121, 20229 Li et al., Langmuir 2016, V32, 7086 Gas/Condensed Phase Scatena et al., Science, V292, 908 SF Scattering Vapor/Liquid Zwaschka, G., Surface Science (2018), https://doi.org/10.1016/j.susc.2018.05.009 Electrochemical Interfaces Jubb, Allen. JPCC 2012, V116, 13161 Hua et al., JPCA 2011, V115, 6233 Roke et al., Soft Matter 2011, V7(10):4959

Sum frequency generation –Progress Ostroverkhov, PRL V94, 046102 (2005) SFG Advances – last 10+ years Phase-resolved (Shen (scanning SFG)) Heterodyning for phase resolution (Tahara) SFG Scattering (Roke) Interferometry (Shultz; Shen) Nonresonant suppression (Dlott) Better than 1 cm-1 resolution BB-SFG (Wang) SFG microscopy (Baldelli) Understanding X(3) contribution to water stretching region (Eisenthal + Many!) Spectral calculations (Hynes; Morita; Skinner; Gaigeot; Paesani; + …) Yamaguchi, JCP 2015, V143, 034202 Dlott et al., JPCC, V111, 37, 2007 Paesani, Morita et al., 2018 CHEM doi.org/10.26434/chemrxiv.5743638.v1 Calculated SFG Phase resolved spectra – lipid monolayer/water interface Gaigeot et al., PCCP, 2018, 20, 5190 Calculated SFG Phase resolved spectra – quartz/water

Pros and Cons Summarized SFG Pros Can provide extraordinary sensitivity ISFG  N2 Molecules at surfaces commonly collectively orient, increasing ISFG further Surface is SELECTED via selection rules Orientational angle of molecular moieties can be determined using polarization methods Buried interfaces can be accessed SFG Cons Expensive, Large systems, and Optical alignment is nontrivial -Instrument development toward a specific application could be useful Suggestion: Rather than funding larger and more capable instruments, funding agencies could fund instrument design toward a very specific goal to minimize size, expense and difficulty SFG spectra (unless phase resolved) are complicated by the real part of the index and nonresonant contributions. SFG-active depth is variable dependent upon the depth of noncentrosymmetry

The Tale of the Tool A Tool is born (discovered or invented), over a course of many years One user becomes Many users; the Tool becomes more available and easier to implement. yet, understanding what the tool’s experimental data means takes a community, And time.

Technique Silos SFG PIs have built a strong community (100+ users currently) This has moved the field forward: interpretation, instrument development, and SFG theory Yet, the community has become a silo of sorts It is not commonplace for SFG PIs to co-publish with neutron and x-ray spectroscopists from major facilities, nor with NMR experts Exceptions are typically a result of Center funding Reasons to engage inter-community grants Synergistic Science progresses at a faster rate Breaks the community silos to then cross pollinate creative solutions

Funding Diverse Tool Communities Combining many Tools and many user communities for understanding Separation Science presents a challenge. A BIG Problem: Current funding for collaborative groups is not significant enough to break the isolation. Solution: We need “funded” linear combinations of experts: SFG expert + NMR expert + X-Ray expert + Neutron expert