1. Surfaces and Sum Frequency
Heather C. Allen
The Ohio State University
Department of Chemistry and Biochemistry

2. 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

3. 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

4. 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 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.

5. 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,
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

6. 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), (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

7. 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),
Electrochemical Interfaces
Jubb, Allen.
JPCC 2012,
V116, 13161
Hua et al., JPCA 2011, V115, 6233
Roke et al., Soft Matter 2011, V7(10):4959

8. Sum frequency generation –Progress
Ostroverkhov, PRL V94,
(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,
Dlott et al., JPCC, V111, 37, 2007
Paesani, Morita et al., 2018 CHEM
doi.org/ /chemrxiv v1
Calculated SFG Phase
resolved spectra –
lipid monolayer/water
interface
Gaigeot et al., PCCP,
2018, 20, 5190
Calculated SFG Phase
resolved spectra –
quartz/water

9. 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

10. 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.

11. 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

12. 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