Understanding the chemical and physical properties of particles is an important scientific, engineering, and medical issue that is crucial to air quality, human health, and environmental chemistry. Of special interest are aerosol particles floating in the air for both indoor virus transmission and outdoor atmospheric chemistry. The growth of bio- and organic-aerosol particles in the air is intimately correlated with chemical structures and their reactions in the gas phase at aerosol particle surfaces and in-particle phases. However, direct measurements of chemical structures at aerosol particle surfaces in the air are lacking. Here we demonstrate in situ surface-specific vibrational sum frequency scattering (VSFS) to directly identify chemical structures of molecules at aerosol particle surfaces. Furthermore, our setup allows us to simultaneously probe hyper-Raman scattering (HRS) spectra in the particle phase. We examined polarized VSFS spectra of propionic acid at aerosol particle surfaces and in particle bulk. More importantly, the surface adsorption free energy of propionic acid onto aerosol particles was found to be less negative than that at the air/water interface. These results challenge the long-standing hypothesis that molecular behaviors at the air/water interface are the same as those at aerosol particle surfaces. Our approach opens a new avenue in revealing surface compositions and chemical aging in the formation of secondary organic aerosols in the atmosphere as well as chemical analysis of indoor and outdoor viral aerosol particles.

Small-volume nanodroplets play an increasingly common role in chemistry and biology. Such nanodroplets are believed to have unique chemical and physical properties at the interface between a droplet and its surrounding medium, however, they are underexamined. In this study, we present the novel technique of vibrational sum frequency scattering (VSFS) spectroscopy as an interface-specific, high-performance method for the in situ investigation of nanodroplets with sub-micron radii; as well as the droplet bulk through simultaneous hyper-Raman scattering (HRS) spectroscopy. We use laboratory-generated nanodroplets from aqueous alcohol solutions to demonstrate this technique's ability to separate the vibrational phenomena which take place at droplet surfaces from the underlying bulk phase. In addition, we systemically examine interfacial spectra of nanodroplets containing methanol, ethanol, 1propanol, and 1-butanol through VSFS. Furthermore, we demonstrate interfacial differences between such nanodroplets and their analogous planar surfaces. The sensitivity of this technique to probe droplet surfaces with few-particle density at standard conditions validates VSFS as an analytical technique for the in situ investigation of small nanodroplets, providing breakthrough information about these species of ever-increasing relevance.

Gallium phosphide (GaP) photoelectrodes have received tremendous attention owing to their applications in photocatalysis and photoelectrocatalytic reduction of CO 2 . Surface electronic states of GaP are important in such catalysis applications. However, knowledge of surface states of GaP under ambient conditions is lacking. Here, we combined azimuth-dependent electronic sum-frequency generation (ESFG) spectroscopy with phase measurements to investigate the surface states for n-type and p-type GaP(100) semiconductors. ESFG spectroscopic studies enabled us to identify three surface states of the GaP crystals under ambient conditions. These experiments have also shown that all of the spectral features come from surface contributions for both the n-type and p-type GaP(100) crystals and that both surface dipoles and surface charges were responsible for the electronic transitions of isotropic and anisotropic components. Combined with azimuth-dependent phase measurements, surface charges were found to account for the isotropic surface ESFG components: negative for n-type and positive for p-type GaP(100). Finally, we conducted a thorough theoretical analysis of surface and bulk contributions for azimuth-dependent ESFG responses. With these spectral and phase signatures, we have further quantified surface and bulk contributions along different orientations for the n-type and p-type GaP(100) crystals.

Gallium phosphide (GaP) photoelectrodes have received remarkable focus due to their applications in photocatalysis and photoelectrocatalysis of CO 2 reduction reactions. Understanding the dynamical mechanisms of surfaces of photoelectrodes is essential in improving their working efficiencies in any application. However, knowledge of photoinduced surface dynamics of these materials is lacking. Here, we investigate surface dynamics of n-type and p-type GaP(100) semiconductors by utilizing time-resolved electronic sum frequency generation (TR-ESFG). Transient ESFG spectra showed that four surface states in both n-and p-type GaP(100) were involved in subsequent kinetics. Transient spectral signatures of the surface states showed that photoexcited electrons move toward the surface regions for p-type GaP, while photoexcited holes move to the surface regions for n-type GaP. These carriers first build up surface electric fields, resulting in fluence-dependent band flattening. The buildup rates of the surface electric fields were found to be on the order of 2.86 ± 0.30 ps −1 for n-type and 2.50 ± 0.25 ps −1 for p-type. Subsequently, a relatively slow process occurs, being attributed to population dynamics of surface states dependent upon applied fluences. We found that surface population behaves as a bimolecular process with rates of 0.020 ± 0.002 cm 2 s −1 for n-type and 0.035 ± 0.002 cm 2 s −1 for p-type GaP. The four surface states, shallow and deep for both n-and p-type GaP(100), were found to be involved in both surface electric fields and surface carrier populations, contrary to previous hypotheses. Our time-resolved surface-specific approach provides unique information on surface dynamical behaviors of photoelectrodes under ambient conditions.

Surface properties of nanodroplets and microdroplets are intertwined with their immense applicability in biology, medicine, production, catalysis, the environment, and the atmosphere. However, many means for analyzing droplets and their surfaces are destructive, non-interface-specific, not conducted under ambient conditions, require sample substrates, conducted ex situ, or a combination thereof. For these reasons, a technique for surface-selective in situ analyses under any condition is necessary. This feature article presents recent developments in second-order nonlinear optical scattering techniques for the in situ interfacial analysis of aerosol droplets in the air. First, we describe the abundant utilization of such droplets across industries and how their unique surface properties lead to their ubiquitous usage. Then, we describe the fundamental properties of droplets and their surfaces followed by common methods for their study. We next describe the fundamental principles of sum-frequency generation (SFG) spectroscopy, the Langmuir adsorption model, and how they are used together to describe adsorption processes at planar liquid and droplet surfaces. We also discuss the history of developments of second-order scattering from droplets suspended in dispersive media and introduce second-harmonic scattering (SHS) and sum-frequency scattering (SFS) spectroscopies. We then go on to outline the developments of SHS, electronic sum-frequency scattering (ESFS), and vibrational sum-frequency scattering (VSFS) from droplets in the air and discuss the fundamental insights about droplet surfaces that the techniques have provided. Finally, we describe some of the areas of nonlinear scattering from airborne droplets which need improvement as well as potential future directions and utilizations of SHS, ESFS, and VSFS throughout environmental systems, interfacial chemistry, and fundamental physics. The goal of this feature article is to spread knowledge about droplets and their unique surface properties as well as introduce second-order nonlinear scattering to a broad audience who may be unaware of recent progress and advancements in their applicability.