Development of \u3ci\u3e in situ \u3c/i\u3e Second-Order Nonlinear Optical Scatterings for Molecular Behaviors at Aerosol Surfaces

Aerosol particles are one of the most important components of the atmosphere. During the growth of aerosol particles, they directly or indirectly affect air quality, human health, and environmental chemistry. Therefore, understanding the chemical and physical properties of such particles is an important scientific, engineering, and medical issue. The growth of aerosol particles in the atmosphere is closely related to the chemical structure at its surface, as well as the heterogeneous reactions which take place at and below the particle’s surface. However, there is a lack of suitable surface-specific analytical techniques which directly measure the chemical structure of aerosol particle surfaces in situ under ambient conditions. The focus of this research is to study the fundamental nature of aerosol particle surfaces to better understand how interfaces play a role in the growth of aerosol particles. Preliminary results in our early work demonstrated direct observations of molecules at the aerosol particle surface with the development of second harmonic scattering (SHS) spectroscopy. However, the sensitivity of the SHS system was insufficient to be an analytical tool for studying chemical compositions of aerosol surfaces. Initially, in the present work, we continued to optimize the SHS system for in situ chemical analysis of molecules at the aerosol particle surface. First, we found that femtosecond lasers with repetition rates closer to 5 MHz are more efficient for SHS. Next, we examined a more efficient detector, a charge-coupled device (CCD) detector, which greatly reduced the sampling time of the interface response. Then we combined the optimal laser system with a CCD detector, which greatly improved the detection sensitivity of interfacial molecules. These experimental results not only provided a comprehensive analysis of the SHS technique, but also laid a solid foundation for our subsequent developments of new electronic and vibrational sum frequency scattering (SFS) techniques. Next, we used our SHS technique to examine interfacial behaviors of molecules at aerosol particle surfaces under different relative humidity (RH) and salt concentrations. Both relative humidity and salt concentration can change particle size and the overall phase of aerosols. RH not only varies the concentration of solutes inside aerosol particles, but also changes interfacial hydration in local regions. It was also found that the surface and bulk of organic molecules in aerosol particles exhibited different behaviors at different RH levels. Our quantitative analysis shows that surface adsorption free energy remains constant, while surface area increases with relative humidity. Furthermore, surface tension of aerosol particles decreases with increasing RH. Our experimental results underscore the importance of interfacial water behavior for aerosol particles in the atmosphere. Later, we developed in situ surface-specific electronic sum frequency scattering (ESFS) spectroscopy to study the spectroscopic behaviors of molecules at aerosol particle surfaces. For example, we examined electronic spectra of malachite green (MG) at aerosol particle surfaces and found that the surfaces are less polar than the bulk. Our quantitative orientational analysis shows that MG is orientated with a polar angle of 25°-35° at the spherical particle surfaces of aerosols. Additionally, the adsorption free energy of MG at the aerosol surfaces was found to be much lower than that at the air/water interface. These results provide new insights into aerosol particle surfaces to further our understanding of the formation of secondary organic aerosols in the atmosphere. Lastly, we developed in situ surface-specific vibrational sum frequency scattering (VSFS) spectroscopy to directly identify chemical structures of molecules at aerosol particle surfaces. This setup also enables simultaneous probing of in-particle phases through hyper-Raman scattering (HRS) spectroscopy. In this work, we examined polarized VSFS spectra of propionic acid on the surface and in the bulk of aerosol particles, proving the technique’s ability to characterize organic constituents of aerosol particles. We also quantitatively compared the curved gas/aerosol particle interface with the planar air/liquid interface. It was shown that the surface adsorption free energy of propionic acid onto aerosol particles was 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 method opens a new avenue for uncovering surface composition and chemical reactions in secondary organic aerosol formation in the atmosphere and chemical analysis of viral aerosol particles

Development of Ultrafast Broadband Electronic Sum Frequency Generation for Charge Dynamics at Surfaces and Interfaces

Understandings of population and relaxation of charges at surfaces and interfaces are essential to improve charge collection efficiency for energy conversion, catalysis, and photosynthesis. Existing time-resolved surface and interface tools are limited to either under ultrahigh vacuum or in a narrow wavelength region with the loss of spectral information. There lacks an efficient time-resolved surface/interface-specific electronic spectroscopy under ambient conditions for the ultra fast surface/interface dynamics. Here we developed a novel technique for surface/interface-specific broadband electronic sum frequency generation (ESFG). The broadband ESFG was based on a stable two-stage BiB3O6 crystal-based optical parametric amplifier, which generates a strong broadband short-wave infrared (SWIR) from 1200 nm to 2400 nm. A resultant surface spectrum covers almost all visible light from 480 nm to 760 nm, combined a broadband electronic second harmonic generation (ESHG) with the ESFG from the SWIR laser source. We further developed the steady-state and transient broadband ESFG and ESHG techniques to investigate the structure and dynamics of charges at oxidized p-type GaAs (100) semiconductor surfaces, as an example. Both the steady-state and transient experiments have shown that two surface states exist inside the bandgap of the GaAs. The kinetic processes at the GaAs surface include both the population and recombination of the surface states after photoexcitation, in addition to the build-up of the space photo-voltage (SPV). The build-up SPV occurs with a rate of 0.56 ± 0.07 ps−1, while the population rate of the surface states exhibits a two-body behavior with a rate constant of (0.012 ± 0.002) × 1012 s−1 cm2. The photo-generated electron-hole pairs near the surface recombine with a rate of 0.002 ± 0.0002 ps−1 for the oxidized p-type GaAs (100). All the methodologies developed here are readily applied to any optically accessible interfaces and surfaces, in particular buried interfaces under ambient conditions

Molecular Rotation in 3 Dimensions at an Air/Water Interface Using Femtosecond Time Resolved Sum Frequency Generation

This paper presents the first study of the rotations of rigid molecules in 3 dimensions at the air/water interface, using the femtosecond time resolved sum frequency generation (SFG) technique. For the purpose of this research, the aromatic dye molecule C153 was chosen as an example of a molecule having two functional groups that are SFG active, one being the hydrophilic −−C==O group and the other the hydrophobic −−CF3 group. From polarized SFG measurements, the orientations of the two chromophores with respect to the surface normal were obtained. On combining these results with the known relative orientation of the two chromophores in the molecule yields the absolute orientation of C153 at the air/water interface. It was found that the −−CF3 axis projected towards the bulk air at an angle of 59○ with respect to the interface normal and the −−C==O group projected towards the bulk water at an angle of 144○ . In order to observe the rotational motions of C153 at the air/water interface, the approach was used to perturb the ground electronic state equilibrium orientational distribution using a polarized resonant pump pulse, which preferentially excites ground state molecules that have their electronic S0 → S1 transition moment aligned closely to the electric field of the incident pump pulse. As a consequence of the photoselection perturbation, the orientational distribution of the remaining ground state molecules was not the equilibrium distribution. Similarly, the orientational distribution of the excited state molecules that were created by the polarized pump pulse was not in their final equilibrium orientational distribution. The rotational motions of the interfacial molecules towards equilibrium were obtained from time dependent measurements of the intensities of the SFG signal generated by the simultaneous incidence at the air/water interface of a visible probe pulse plus an IR probe pulse. In this way, the recovery times to achieve the orientational equilibrium of the two chromophores including the orientation of the normal of the C153 plane with respect to the interface were obtained. The photo-selection process shifts the average orientation angle of the hydrophilic −−C==O group by an increase of 4○ ± 0.6○ with a rotational recovery time constant of 130 ± 20 ps, which is the time to return to an orientational equilibrium distribution. The hydrophobic –CF3 group undergoes a shift that increases its angle by 8○ ± 1.5○ with a rotational recovery time constant of 210 ± 38 ps. We find that the orientational change of the molecular normal is 4○ ± 0.5○ and has a rotational recovery time constant of 125 ± 26 ps. The interface-specific time-dependent polarized measurements allowed us to monitor the orientational motions of molecules at interfaces, both in 3 dimensions and in real time

Vibronic Fingerprint of Singlet Fission in Hexacene

Singlet fission has the great potential to overcome the Shockley–Queisser thermodynamic limit and thus promotes solar power conversion efficiency. However, the current limited understandings of detailed singlet fission mechanisms hinder a further improved design of versatile singlet fission materials. In the present study, we combined ultrafast transient infrared spectroscopy with ab initio calculations to elucidate the roles played by the vibrational normal modes in the process of singlet fission for hexacene. Our transient infrared experiments revealed three groups of vibrational modes that are prominent in vibronic coupling upon photoexcitation. Through our computational study, those normal modes with notable Franck-Condon shifts have been classified as ring-twisting modes near 1300.0 cm−1, ring-stretching modes near 1600.0 cm−1, and ring-scissoring modes near 1700.0 cm−1. Experimentally, a ring-stretching mode near 1620.0 cm−1 exhibits a significant blue-shift of 4.0 cm−1 during singlet fission, which reaction rate turns out to be 0.59 ± 0.07 ps. More interestingly, the blue-shifted mode was also identified by our functional mode singlet fission theory as the primary driving mode for singlet fission, suggesting the importance of vibronic coupling when a correlated triplet pair of hexacene is directly converted from its first excited state singlet exciton. Our findings indicate that the ultrafast transient infrared spectroscopy, in conjunction with the nonadiabatic transition theory, is a powerful tool to probe the vibronic fingerprint of singlet fission

Relationship between birth weight and ambient temperature during pregnancy in a cross-sectional study of the residents of Suzhou, China

ObjectiveThe association between birth weight and ambient temperature during pregnancy remains inconclusive, and data from Chinese populations are scarce. We conducted a cross-sectional study to investigate the association between birth weight and ambient temperature during pregnancy among the residents of Suzhou Industrial Park, Suzhou, China.MethodsInformation regarding 10,903 infants born between January 2018 and December 2018 who were born at the hospitals in Suzhou Industrial Park, Jiangsu province was obtained via public birth records.ResultsThis study found that the ambient temperature during the first trimester of pregnancy was negatively correlated with birth weight, suggesting that elevated temperature may be related to lower birth weight. However, the ambient temperatures during the second and third trimesters of pregnancy were positively correlated with birth weight. Moreover, when the ambient temperature was below 15°C during the second trimester of pregnancy, the birth weight increased with temperature. However, when the temperature was higher than 15°C, the birth weight decreased with temperature. The relationship between ambient temperature in the third trimester and birth weight presented an inverted “U” curve. When the ambient temperature was lower than 20°C, the birth weight increased with ambient temperature, but when the ambient temperature was higher than 20°C, the increase of ambient temperature showed no significant relationship with the increase of birth weight.ConclusionThe ambient temperature was correlated with birth weight. The ambient temperature during the first trimester of pregnancy was negatively correlated with birth weight. The relationship between ambient temperature in the third trimester and birth weight presented an inverted “U” curve

Anisotropic Singlet Fission in Single Crystalline Hexacene

Singlet fission is known to improve solar energy utilization by circumventing the Shockley-Queisser limit. The two essential steps of singlet fission are the formation of a correlated triplet pair and its subsequent quantum decoherence. However, the mechanisms of the triplet pair formation and decoherence still remain elusive. Here we examined both essential steps in single crystalline hexacene and discovered remarkable anisotropy of the overall singlet fission rate along different crystal axes. Since the triplet pair formation emerges on the same timescale along both crystal axes, the quantum decoherence is likely responsible for the directional anisotropy. The distinct quantum decoherence rates are ascribed to the notable difference on their associated energy loss according to the Redfield quantum dissipation theory. Our hybrid experimental/theoretical framework will not only further our understanding of singlet fission, but also shed light on the systematic design of new materials for the third-generation solar cells

Translocase of the Outer Mitochondrial Membrane 40 Is Required for Mitochondrial Biogenesis and Embryo Development in Arabidopsis

In eukaryotes, mitochondrion is an essential organelle which is surrounded by a double membrane system, including the outer membrane, intermembrane space and the inner membrane. The translocase of the outer mitochondrial membrane (TOM) complex has attracted enormous interest for its role in importing the preprotein from the cytoplasm into the mitochondrion. However, little is understood about the potential biological function of the TOM complex in Arabidopsis. The aim of the present study was to investigate how AtTOM40, a gene encoding the core subunit of the TOM complex, works in Arabidopsis. As a result, we found that lack of AtTOM40 disturbed embryo development and its pattern formation after the globular embryo stage, and finally caused albino ovules and seed abortion at the ratio of a quarter in the homozygous tom40 plants. Further investigation demonstrated that AtTOM40 is wildly expressed in different tissues, especially in cotyledons primordium during Arabidopsis embryogenesis. Moreover, we confirmed that the encoded protein AtTOM40 is localized in mitochondrion, and the observation of the ultrastructure revealed that mitochondrion biogenesis was impaired in tom40-1 embryo cells. Quantitative real-time PCR was utilized to determine the expression of genes encoding outer mitochondrial membrane proteins in the homozygous tom40-1 mutant embryos, including the genes known to be involved in import, assembly and transport of mitochondrial proteins, and the results demonstrated that most of the gene expressions were abnormal. Similarly, the expression of genes relevant to embryo development and pattern formation, such as SAM (shoot apical meristem), cotyledon, vascular primordium and hypophysis, was also affected in homozygous tom40-1 mutant embryos. Taken together, we draw the conclusion that the AtTOM40 gene is essential for the normal structure of the mitochondrion, and participates in early embryo development and pattern formation through maintaining the biogenesis of mitochondria. The findings of this study may provide new insight into the biological function of the TOM40 subunit in higher plants

Waste glass used in ambient cured alkali-activated materials

Alkali-activated binder offers a large reduction in CO2 emissions (up to 80%) compared to ordinary Portland cement (OPC), promoting alkali-activated binder as a component of sustainable concrete constructions. Alkali-activated mortar and concrete exhibit better engineering properties and durability than OPC mortar and concrete, respectively. In addition, alkali-activated mortar and concrete can effectively include by-products and waste materials. Waste glass can be reused as raw material in the construction industry to reduce the negative impacts on the environment and preserve natural resources. However, the use of waste glass as construction material still remains under-utilised and inadequately explored, and there is a significant opportunity for the expansion of the value-added use of waste glass. This thesis presents a research study on the use of waste glass powder and waste glass sand in ambient cured alkali-activated mortars (AAMs) and concrete (AAC)