Nonlinear spectroscopy and interfacial structure and dynamics

The ubiquity and importance of surfaces and interfaces can hardly be overstated. They play a central role in processes ranging from atmospheric/environmental chemistry and biological function to electrochemistry, heterogeneous catalysis, and solar energy conversion. Surface-specific nonlinear optical techniques have a unique advantage of being able to probe buried interfaces between condensed phase media that are inaccessible otherwise. In combination with theory and computer simulations, they enable a unified approach to studying underlying physical phenomena at a wide variety of interfaces from the same standpoint, including liquid/liquid and wet/soft interfaces as well as solid surfaces. This Special Issue is a collection of papers that attempts to represent the modern state of research in this rich and diverse field.

The potential of the even-order nonlinear optical processes to be surface-selective has been noted early on, shortly after the first demonstration of second harmonic generation (SHG) in a noncentrosymmetric crystal. After the first experiments that proved the utility of the Second Harmonic Generation (SHG) and Sum Frequency Generation (SFG) for probing surfaces, the field has gone through an induction period, partly due to the technical difficulties and limitations of the nascent short-pulse laser technology. With the development of broadly accessible table-top picosecond and femtosecond tunable-wavelength laser systems, beginning in the late 1990s, the field has exploded from a few pioneer groups to a broad community of researchers exploring a variety of topics ranging from fundamental questions of molecular dynamics (MD) at liquid interfaces to more applied problems of material surfaces, polymer chemistry, environmental chemistry, electrochemistry, and life sciences. The nonlinear optical techniques have provided a set of noninvasive probes complementary to the conventional surface science, such as ultrahigh vacuum (UHV) and scattering techniques and probe microscopies, and, in fact, have been used in combination in a number of cases.

In parallel with the experimental developments, theorists have refined the methodology for modeling the molecular structure and dynamics at surfaces and interfaces, and calculating spectroscopic signals. Molecular dynamics (MD) simulation can provide a predictive description of interfacial systems ranging from simple liquids and solutions to environmentally relevant molecules, peptides, and proteins. Based on the progress of MD simulation, a new avenue of interface analysis through calculating spectroscopic signals has emerged since the 2000s. Comparison of the calculated spectra with the experimental ones provides a rigorous testing of the assumed molecular model and structure of the interfaces. Computational analysis with reliable accuracy helps us to interpret the observed spectra in terms of detailed microscopic structure of interfaces that MD simulation can reveal.

Throughout the history of the field, both technical developments and improvements have broadened the scope of applications of the surface-selective nonlinear spectroscopies and allowed deeper insight into the molecular structure and ultrafast dynamics at interfaces. The progress in this field to date in both experimental and theoretical aspects has been summarized in recent textbooks.^1,2 This special topic issue showcases some of the further recent advances in surface nonlinear spectroscopy in both experimental and theoretical aspects. The topics cover fundamental understanding of the structure and dynamics of interfaces as well as various applications of the surface nonlinear spectroscopy.

The fundamental question and the unifying theme of both experimental and theoretical studies of interfaces is what makes them different from bulk media. Perhaps the most apparent manifestation of this difference is the asymmetry of the interfacial environment, which is expressed in the preferred molecular orientation at an interface as opposed to the isotropic distribution in the bulk of a liquid, glass, or gas. Since the early demonstrations of how to obtain molecular orientation from the polarization-selected SFG and SHG measurements, these techniques have been further developed to sample three-dimensional orientation of molecules³ and characterize reorientation in response to external perturbation, such as the surface electric field.

A unique and fundamental aspect of interfaces is the existence of absolute (up vs down) orientational alignment for polar molecules, since the top and bottom phases are, by definition, dissimilar.⁴ The development of the phase-sensitive detection of SFG and SHG signals (also referred to as optical heterodyne detection),^5–8 which takes advantage of the optically coherent nature of these processes, has greatly facilitated the experimental studies of the absolute orientation of molecules. The phase-sensitive or heterodyne detection has a number of advantages over the conventional (homodyne) method to reveal phase information related to the orientation and to facilitate unambiguous comparison with MD calculations, though the precise phase calibration is challenging but indispensable.⁹ The up vs down orientation of molecules and their functional groups has obviously important chemical, biochemical, and biophysical implications in areas such as surface reactivity, heterogeneous catalysis, and biomolecular recognition.

One notes that the papers in this special topic issue deal with a variety of interface systems, including various liquid and solid surfaces. The variety demonstrates the general applicability of the interfacial nonlinear spectroscopy and MD simulation. We note that many papers treat liquid interfaces, including the liquid-vapor and liquid-solid ones. The nonlinear spectroscopy is particularly powerful to study such wet or soft interfaces that most of the other surface-specific techniques are not applicable. Related topics include interfacial water in contact with charged or hydrophilic/hydrophobic monolayers,^4,5 local solvation environment at the water-silica interface,¹⁰ and adsorption of solvent molecules on the polymer surface.¹¹ Many aspects of the surface electrostatics, spatial distribution of ions, and specific ion effects remain a very active research area.¹² Besides such liquid interfaces, in-depth investigation on structure and dynamics of solid surfaces or adsorbed species on them are also a very active area of the SHG/SFG spectroscopy.^13–15 

Another set of questions that has recently seen a lot of research activity concerns charged interfaces. As was first demonstrated by Eisenthal, in the presence of a charged interface, the second-order nonlinear optical signals at the sum or second harmonic frequency will have a 3rd order contribution from the static (DC) field. This contribution can be used to characterize the surface potential and the distribution of ions near the surface, testing the conventional models of the electric double layer such as the Gouy-Chapman theory. More recent developments explored the phase relations between these χ⁽³⁾ terms and the surface contributions, including phase retardation effects due to the Debye screening length being comparable to the wavelength of light.

The development of the nonlinear optical scattering has allowed the extension of the SHG and SFG methods to study surfaces of micro- and nanoparticles, colloids, micelles and vesicles, and aerosols.^16–18 This has enabled many studies of environmentally and biologically relevant systems, ranging from colloid stability to adsorption of pollutants and to ion and drug transport across biomembranes.

Spatial heterogeneity is an attribute of many realistic systems, e.g., corrosion, lipid rafts in biomembranes, and domains in bulk heterojunctions. Several research groups have been pushing the limits of spatial SHG and SFG microscopy, enabling, e.g., rapid screening of protein crystals, and imaging of live neuron activity.

Another unique feature of the surface-selective spectroscopies is their chiral sensitivity. This has been utilized to probe the chirality of surface-adsorbed biomolecules including peptides, proteins, DNA, and even the water molecules in their hydration shells that, although achiral themselves, form a supramolecular chiral structure spatially patterned by the solute. Okuno and Ishibashi adopted the heterodyne SFG measurement to chiral systems to distinguish enantiomers as well as the origin of the chiral signals.¹⁹ 

While much of the recent activity in the field has focused on the vibrational spectroscopy, both SHG and SFG have been extensively applied to study electronic states at surfaces and interfaces,²⁰ including solvation of chromophores as probes of interfacial polarity, and excitons in organic photovoltaic materials.²¹ Double-resonance electronic-vibrational SFG has been used to elucidate the mechanism of vibronic coupling in adsorbed donor-acceptor molecules.²² 

One of the advantages of SHG and SFG spectroscopies for the interface probe lies in their high temporal resolution by using pulse lasers. A variety of time-resolved pump-probe variants of SFG and SHG have been demonstrated over the years, enabling studies of solvation dynamics, molecular reorientation, and charge transfer. Rao and co-workers pursed the ultrafast dynamics of transient surface electronic states by electronic SHG and SFG.²⁰ More recently, two-dimensional surface-selective spectroscopies allowed more detailed studies to the hydrogen-bond dynamics at aqueous interfaces. Zanni and co-workers showed that the two-dimensional IR spectroscopy can achieve selective detection of interfacial species with the help of surface plasmons.²³ On the other hand, there has been significant effort to push the SHG/SFG spectroscopy toward high frequency resolution. Ren and co-workers show their achievement using broadband SFG spectroscopy.²⁴ 

Last but not least, MD simulation has played important roles in the studies of various interfaces. MD simulation is powerful to reveal the detailed structure and dynamics of interfaces beyond experimental spatial or temporal resolution, particularly when available experimental techniques of microscopic probe are limited, such as liquid or buried interfaces. The present special topic issue showcases some of the recent advances in the theoretical studies. Gao et al. employed MD simulation to study confined water structure by graphene interfaces.²⁵ Heyden studied heterogeneous structure and dynamics of water at the protein interface,²⁶ which is of crucial importance in functions of proteins. Mundy and co-workers shed new light on the long-standing debate on the Jones-Ray effect in combination of MD simulation and Poisson-Boltzmann equation.²⁷ 

Recent development of MD analysis of computing spectroscopic signals of interfaces enabled close collaboration of SHG/SFG spectroscopy and MD simulation. The two powerful techniques of interface analysis could play quite complementary roles. The interface spectroscopy offers critical examination to the reliability of MD simulation, while MD simulation allows for detailed interpretation of the results of interface spectroscopy. The present issue includes recent advances along the MD analysis by two groups.^28,29

These techniques provide a wealth of information about the molecular structure at interfaces. They are also capable of revealing ultrafast dynamics of interfaces, such as vibrational relaxation, orientational motion, and energy and charge transfer. Recently, theoretical analysis of the surface nonlinear spectroscopy has been advanced to help extracting details from experimental measurements and provide a reliable molecular level insight into these interfaces.

The guest editors thank the authors who contributed, the journal editors, and staff who assisted this special topic.