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Owing to their unique optical and electronic properties, vertical van der Waals heterostructures (vdWHs) have attracted considerable attention in optoelectronic applications, such as photodetection, light harvesting, and light-emitting diodes. To fully harness these properties, it is crucial to understand the interfacial charge transfer (CT) and recombination dynamics across vdWHs. However, the effects of interfacial energetics and defect states on interfacial CT and recombination processes in graphene-transition metal dichalcogenide (Gr-TMD) vdWHs remain debated. Here, we investigate the interfacial CT dynamics in Gr-TMD vdWHs with different chemical compositions (W, Mo, S, and Se) and tunable interfacial energetics. We demonstrate, using ultrafast terahertz spectroscopy, that while the photo-induced electron transfer direction is universal with graphene donating electrons to TMDs, its efficiency is chalcogen-dependent: the CT efficiency of S atom-based vdWHs is 3–5 times higher than that of Se-based vdWHs thanks to the lower Schottky barrier present in S-based vdWHs. In contrast, the electron back transfer process from TMD to Gr, which defines the charge separation time, is transition metal-dependent and dominated by the mid-gap defect level of TMDs: W transition metal-based vdWHs possess extremely long charge separation, well beyond 1 ns, which is significantly longer than Mo-based vdWHs with only 10 s of ps charge separation. This difference can be traced to the much deeper mid-gap defect reported in W-based TMDs compared to Mo-based ones, resulting in modified energetics for the back electron transfer from the trapped states to graphene. Our results shed light on the role of interfacial energetics and defects by tailoring chemical compositions of TMDs on the interfacial CT and recombination dynamics in Gr-TMD vdWHs, which is pivotal for optimizing optoelectronic devices, particularly in the field of photodetection. [ABSTRACT FROM AUTHOR]

Unsaturated lipids with C=C groups in their alkyl chains are widely present in the cell membrane and food. The C=C groups alter the lipid packing density, membrane stability, and persistence against lipid oxidation. Yet, molecular-level insights into the structure of the unsaturated lipids remain scarce. Here, we probe the molecular structure and organization of monolayers of unsaturated lipids on the water surface using heterodyne-detected sum-frequency generation (HD-SFG) spectroscopy. We vary the location of the C=C in the alkyl chain and find that at high lipid density, the location of the C=C group affects neither the interfacial water organization nor the tail of the alkyl chain. Based on this observation, we use the C=C stretch HD-SFG response to depth-profile the alkyl chain conformation of the unsaturated lipid. We find that the first 1/3 of carbon atoms from the headgroup are relatively rigid, oriented perpendicular to the surface. In contrast, the remaining carbon atoms can be approximated as free rotators, introducing the disordering of the alkyl chains. [ABSTRACT FROM AUTHOR]

Two-dimensional terahertz–infrared–visible (2D TIRV) spectroscopy directly measures the coupling between quantum high-frequency vibrations and classical low-frequency modes of molecular motion. In addition to coupling strength, the signal intensity in 2D TIRV spectroscopy can also depend on the selection rules of the excited transitions. Here, we explore the selection rules in 2D TIRV spectroscopy by studying the coupling between the high-frequency CH3 stretching and low-frequency vibrations of liquid dimethyl sulfoxide (DMSO). Different excitation pathways are addressed using variations in laser pulse timing and different polarizations of exciting pulses and detected signals. The DMSO signals generated via different excitation pathways can be readily distinguished in the spectrum. The intensities of different excitation pathways vary unequally with changes in polarization. We explain how this difference stems from the intensities of polarized and depolarized Raman and hyper-Raman spectra of high-frequency modes. These results apply to various systems and will help design and interpret new 2D TIRV spectroscopy experiments. [ABSTRACT FROM AUTHOR]

Terahertz molecular motions are often probed by high-frequency molecular oscillators in different types of non-linear vibrational spectroscopy. Recently developed two-dimensional terahertz-infrared-visible spectroscopy allows direct measuring of this coupling and, thus, obtaining site-specific terahertz vibrational spectrum. However, these data are affected by the intensity and phase of the employed laser pulses. In this work, we develop a method of extracting sample response—representing solely physical properties of a material—from experimental spectra. Using dimethyl sulfoxide as a model molecule to verify this method, we measure the coupling between C–H stretch vibration of its methyl groups and terahertz intramolecular twist and wagging modes. [ABSTRACT FROM AUTHOR]

Insights into the microscopic structure of aqueous interfaces are essential for understanding the chemical and physical processes on the water surface, including chemical synthesis, atmospheric chemistry, and events in biomolecular systems. These aqueous interfaces have been probed by heterodyne-detected sum-frequency generation (HD-SFG) spectroscopy. To obtain the molecular response from the measured HD-SFG spectra, one needs to correct the measured ssp spectra for local electromagnetic field effects at the interface due to a spatially varying dielectric function. This so-called Fresnel factor correction can change the inferred response substantially, and different ways of performing this correction lead to different conclusions about the interfacial water response. Here, we compare the simulated and experimental spectra at the air/water interface. We use three previously developed models to compare the experiment with theory: an advanced approach taking into account the detailed inhomogeneous interfacial dielectric profile and the Lorentz and slab models to approximate the interfacial dielectric function. Using the advanced model, we obtain an excellent quantitative agreement between theory and experiment, in both spectral shape and amplitude. Remarkably, we find that for the Fresnel factor correction of the ssp spectra, the Lorentz model for the interfacial dielectric function is equally accurate in the hydrogen (H)-bonded region of the response, while the slab model underestimates this response significantly. The Lorentz model, thus, provides a straightforward method to obtain the molecular response from the measured spectra of aqueous interfaces in the H-bonded region. [ABSTRACT FROM AUTHOR]

Biomembrane hydration is crucial for understanding processes at biological interfaces. While the effect of the lipid headgroup has been studied extensively, the effect (if any) of the acyl chain chemical structure on lipid-bound interfacial water has remained elusive. We study model membranes composed of phosphatidylethanolamine (PE) and phosphatidylcholine (PC) lipids, the most abundant lipids in biomembranes. We explore the extent to which the lipid headgroup packing and associated water organization are affected by the lipid acyl tail unsaturation and chain length. To this end, we employ a combination of surface-sensitive techniques, including sum-frequency generation spectroscopy, surface pressure measurements, and Brewster angle microscopy imaging. Our results reveal that the acyl tail structure critically affects the headgroup phosphate orientational distribution and lipid-associated water molecules, for both PE and PC lipid monolayers at the air/water interface. These insights reveal the importance of acyl chain chemistry in determining not only membrane fluidity but also membrane hydration. [ABSTRACT FROM AUTHOR]

Many essential processes occur at soft interfaces, from chemical reactions on aqueous aerosols in the atmosphere to biochemical recognition and binding at the surface of cell membranes. The spatial arrangement of molecules specifically at these interfaces is crucial for many of such processes. The accurate determination of the interfacial molecular orientation has been challenging due to the low number of molecules at interfaces and the ambiguity of their orientational distribution. Here, we combine phase- and polarization-resolved sum-frequency generation (SFG) spectroscopy to obtain the molecular orientation at the interface. We extend an exponentially decaying orientational distribution to multiple dimensions, which, in conjunction with multiple SFG datasets obtained from the different vibrational modes, allows us to determine the molecular orientation. We apply this new approach to formic acid molecules at the air–water interface. The inferred orientation of formic acid agrees very well with ab initio molecular dynamics data. The phase-resolved SFG multimode analysis scheme using the multidimensional orientational distribution thus provides a universal approach for obtaining the interfacial molecular orientation. [ABSTRACT FROM AUTHOR]

Combining strong light absorption and outstanding electrical conductivity, hybrid nanographene–graphene (NG–Gr) van der Waals heterostructures (vdWHs) represent an emerging material platform for versatile optoelectronic devices. Interfacial charge transfer (CT), a fundamental process whose full control remains limited, plays a paramount role in determining the final device performance. Here, we demonstrate that the interlayer vdW interactions can be engineered by tuning the sizes of bottom-up synthesized NGs to control the interfacial electronic coupling strength and, thus, the CT process in NG–Gr vdWHs. By increasing the dimensions of NGs from 42 to 96 sp2 carbon atoms in the polyaromatic core to enhance the interfacial coupling strength, we find that the CT efficiency and rate in NG–Gr vdWHs display a drastic increase of one order of magnitude, despite the fact that the interfacial energy driving the CT process is unfavorably reduced. Our results shed light on the CT mechanism and provide an effective knob to tune the electronic coupling at NG–Gr interfaces by controlling the size-dependent vdW interactions. [ABSTRACT FROM AUTHOR]

In condensed molecular matter, low-frequency modes (LFMs) associated with specific molecular motions are excited at room temperature and determine essential physical and chemical properties of materials. LFMs, with typical mode energies of up to ∼500 cm−1 (62 meV), contribute significantly to thermodynamic parameters and functions (e.g., heat capacity and entropy) and constitute the basis for room temperature molecular dynamics (e.g., conformational fluctuations and change). LFMs are often analyzed indirectly by the measurement of their effect on specific high-frequency modes (HFMs); the LFM–HFM coupling is reflected in the lineshape, as well as in the spectral and angular diffusion of the HFM. Two-dimensional terahertz-infrared-visible (2D TIRV) spectroscopy allows measuring the LFM–HFM coupling directly and can thereby provide new insights into the strength and nature of the coupling and the character of LFMs. However, the interference between the different signals generated by different excitation pathways can complicate 2D TIRV spectra, preventing a straightforward analysis. Here, we develop an experimental method to distinguish different excitation pathways in 2D TIRV spectroscopy and plot them separately in different quadrants of a 2D spectrum. We validate this method by measuring the spectra of CaF2 and nitrogen gas. For CaF2, only sum-frequency mixing between infrared and terahertz fields generates the signal. In contrast, for N2, only difference-frequency mixing is observed. We then use this method to separate sum- and difference-frequency pathways in the 2D TIRV spectrum of liquid water, verifying the previous interpretation of the lineshape of the 2D TIRV spectrum of water. [ABSTRACT FROM AUTHOR]

Surfaces and interfaces play important roles in many processes and reactions and are therefore intensively studied, often with the aim of obtaining molecular-level information from just the interfacial layer. Generally, only the first few molecular layers next to the interface are relevant for the surface processes. In the past decades, 2nd-order nonlinear spectroscopies including sum-frequency generation and second harmonic generation have developed into powerful tools for obtaining molecularly specific insights into the interfacial region. These approaches have contributed substantially to our understanding of a wide range of physical phenomena. However, along with their wide-ranging applications, it has been realized that the implied surface-specificity of these approaches may not always be warranted. Specifically, the bulk quadrupole contribution beyond the electric dipole-approximation for a system with a weak nonlinear interface signal, as well as the diffuse layer contribution at charged interfaces, could mask the surface information. In this perspective paper, we discuss the surface-specificity of 2nd-order nonlinear spectroscopy, especially considering these two contributions. [ABSTRACT FROM AUTHOR]

Specific ion effects at interfaces are important for a variety of thermodynamic properties of electrolyte solutions, like surface tension and the phase behavior of surfactants. We report the relative surface affinity of Na+ and D3O+ at both the D2O-air and the sodium dodecyl sulfate (surfactant)-covered D2O surface by studying the alignment of interfacial D2O, using vibrational sum frequency generation spectroscopy. The surface propensity of ions is found to be a function of both the nature of the ion and the nature of the surface. Specifically, for the charged, surfactant-covered interface, Na+ has a higher affinity than D3O+. In contrast, D3O+ has a higher affinity than Na+ at the air-D2O interface. The relative surface affinity of cations thus depends on both details of the cation and the type of interface. [ABSTRACT FROM AUTHOR]

Room temperature ionic liquids (RTILs) have been shown to exhibit spatial heterogeneity or structural heterogeneity in the sense that they form hydrophobic and ionic domains. Yet studies of the relationship between this structural heterogeneity and the ∼picosecond motion of the molecular constituents remain limited. In order to obtain insight into the time scales relevant to this structural heterogeneity, we perform molecular dynamics simulations of a series of RTILs. To investigate the relationship between the structures, i.e., the presence of hydrophobic and ionic domains, and the dynamics, we gradually increase the size of the hydrophobic part of the cation from ethylammonium nitrate (EAN), via propylammonium nitrate (PAN), to butylammonium nitrate (BAN). The two ends of the organic cation, namely, the charged Nhead–H group and the hydrophobic Ctail–H group, exhibit rotational dynamics on different time scales, evidencing dynamical heterogeneity. The dynamics of the Nhead–H group is slower because of the strong coulombic interaction with the nitrate counter-ionic anions, while the dynamics of the Ctail–H group is faster because of the weaker van der Waals interaction with the surrounding atoms. In particular, the rotation of the Nhead–H group slows down with increasing cationic chain length, while the rotation of the Ctail–H group shows little dependence on the cationic chain length, manifesting that the dynamical heterogeneity is enhanced with a longer cationic chain. The slowdown of the Nhead–H group with increasing cationic chain length is associated with a lower number of nitrate anions near the Nhead–H group, which presumably results in the increase of the energy barrier for the rotation. The sensitivity of the Nhead–H rotation to the number of surrounding nitrate anions, in conjunction with the varying number of nitrate anions, gives rise to a broad distribution of Nhead–H reorientation times. Our results suggest that the asymmetry of the cations and the larger excluded volume for longer cationic chain are important for both the structural heterogeneity and the dynamical heterogeneities. The observed dynamical heterogeneities may affect the rates of chemical reactions depending on where the reactants are solvated in ionic liquids and provide an additional guideline for the design of RTILs as solvents. [ABSTRACT FROM AUTHOR]

The air-water interface has been a subject of extensive theoretical and experimental studies due to its ubiquity in nature and its importance as a model system for aqueous hydrophobic interfaces. We report on the structure and vibrational energy transfer dynamics of this interfacial water system studied with equilibrium and non-equilibrium molecular dynamics simulations employing a density functional theory-based description of the system and the kinetic energy spectral density analysis. The interfacial water molecules are found to make fewer and weaker hydrogen (H)-bonds on average compared to those in the bulk. We also find that (i) the H-bonded OH groups conjugate to the free OH exhibit rather low vibrational frequencies (3000-3500 cm-1); (ii) the presence of a significant fraction (>10%) of free and randomly oriented water molecules at the interface ("labile water"), neither of whose OH groups are strong H-bond donors; (iii) the inertial rotation of free OH groups, especially from the labile water, contribute to the population decay of excited free OH groups with comparable rate and magnitude as intramolecular energy transfer between the OH groups. These results suggest that the labile water, which might not be easily detectable by the conventional vibrational sum frequency generation method, plays an important role in the surface water dynamics. [ABSTRACT FROM AUTHOR]

We propose a new force field for trimethylamine N-oxide (TMAO), which is designed to reproduce the long-lived and highly directional hydrogen bond between the TMAO oxygen (OTMAO) atom and surrounding water molecules. Based on the data obtained by ab initio molecular dynamics simulations, we introduce three dummy sites around OTMAO to mimic the OTMAO lone pairs and we migrate the negative charge on the OTMAO to the dummy sites. The force field model developed here improves both structural and dynamical properties of aqueous TMAO solutions. Moreover, it reproduces the experimentally observed dependence of viscosity upon increasing TMAO concentration quantitatively. The simple procedure of the force field construction makes it easy to implement in molecular dynamics simulation packages and makes it compatible with the existing biomolecular force fields. This paves the path for further investigation of protein-TMAO interaction in aqueous solutions. [ABSTRACT FROM AUTHOR]

We report calculations on the surface tension of the water-air interface using ab initio molecular dynamics (AIMD) simulations. We investigate the influence of the cell size on surface tension of water from force field molecular dynamics simulations. We find that the calculated surface tension increases with increasing simulation cell size, thereby illustrating that a correction for finite size effects is essential for small systems that are customary in AIMD simulations. Moreover, AIMD simulations reveal that the use of a double- basis set overestimates the experimentally measured surface tension due to the Pulay stress while more accurate triple and quadruple- basis sets give converged results. We further demonstrate that van der Waals corrections critically affect the surface tension. AIMD simulations without the van der Waals correction substantially underestimate the surface tension while the van derWaals correction with the Grimme's D2 technique results in a value for the surface tension that is too high. The Grimme's D3 van derWaals correction provides a surface tension close to the experimental value. Whereas the specific choices for the van derWaals correction and basis sets critically affect the calculated surface tension, the surface tension is remarkably insensitive to the details of the exchange and correlation functionals, which highlights the impact of long-range interactions on the surface tension. Our simulated values provide important benchmarks, both for improving van derWaals corrections and AIMD simulations of aqueous interfaces. [ABSTRACT FROM AUTHOR]

Interfacial water structures have been studied intensively by probing the O-H stretch mode of water molecules using sum-frequency generation (SFG) spectroscopy. This surface-specific technique is finding increasingly widespread use, and accordingly, computational approaches to calculate SFG spectra using molecular dynamics (MD) trajectories of interfacial water molecules have been developed and employed to correlate specific spectral signatures with distinct interfacial water structures. Such simulations typically require relatively long (several nanoseconds) MD trajectories to allow reliable calculation of the SFG response functions through the dipole moment-polarizability time correlation function. These long trajectories limit the use of computationally expensive MD techniques such as ab initio MD and centroid MD simulations. Here, we present an efficient algorithm determining the SFG response from the surface-specific velocity-velocity correlation function (ssVVCF). This ssVVCF formalism allows us to calculate SFG spectra using a MD trajectory of only ~100 ps, resulting in the substantial reduction of the computational costs, by almost an order of magnitude. We demonstrate that the O-H stretch SFG spectra at the water-air interface calculated by using the ssVVCF formalism well reproduce those calculated by using the dipole moment-polarizability time correlation function. Furthermore, we applied this ssVVCF technique for computing the SFG spectra from the ab initio MD trajectories with various density functionals. We report that the SFG responses computed from both ab initio MD simulations and MD simulations with an ab initio based force field model do not show a positive feature in its imaginary component at 3100 cm-1. [ABSTRACT FROM AUTHOR]

GALA is a 30 amino acid synthetic peptide consisting of a Glu-Ala-Leu-Ala repeat and is known to undergo a reversible structural transition from a disordered to an a-helical structure when changing the pH from basic to acidic values. In its helical state GALA can insert into and disintegrate lipid membranes. This effect has generated much interest in GALA as a candidate for pH triggered, targeted drug delivery. GALA also serves as a well-defined model system to understand cell penetration mechanisms and protein folding triggered by external stimuli. Structural transitions of GALA in solution have been studied extensively. However, cell penetration is an interfacial effect and potential biomedical applications of GALA would involve a variety of surfaces, e.g., nanoparticles, lipid membranes, tubing, and liquid-gas interfaces. Despite the apparent importance of interfaces in the functioning of GALA, the effect of surfaces on the reversible folding of GALA has not yet been studied. Here, we use sum frequency generation vibrational spectroscopy (SFG) to probe the structural response of GALA at the air-water interface and IR spectroscopy to follow GALA folding in bulk solution. We combine the SFG data with molecular dynamics simulations to obtain a molecularlevel picture of the interaction of GALA with the air-water interface. Surprisingly, while the fully reversible structural transition was observed in solution, at the water-air interface, a large fraction of the GALA population remained helical at high pH. This "stickiness" of the air-water interface can be explained by the stabilizing interactions of hydrophobic leucine and alanine side chains with the water surface. [ABSTRACT FROM AUTHOR]

Superatom solids are promising for optoelectronic applications, owing to their tunable structural and electronic properties; yet, the electronic transport properties of these materials have been largely unexplored. Here, we report the Auger recombination dynamics of free carriers in two representative two-dimensional superatomic semiconductors, Re6Se8Cl2 and Mo6S3Br6, studied using ultrafast terahertz photoconductivity measurements. The fast Auger recombination dynamics are characterized by a cubic dependence of the Auger rate on carrier density in Re6Se8Cl2 and a quadratic dependence in Mo6S3Br6. The effective lifetimes of Mo6S3Br6 (∼ 0.5 ps) are over an order of magnitude shorter than those (∼ 20 ps) of Re6Se8Cl2. These results highlight the variability of the optoelectronic properties of different superatom solids. [ABSTRACT FROM AUTHOR]

We investigate the structure and electronic properties of phosphatidylcholine (PC) under different degrees of hydration at the single-molecule and monolayer type level by linear scaling ab initio calculations. Upon hydration, the phospholipid undergoes drastic long-range conformational rearrangements which lead to a sickle-like ground-state shape. The structural unit of the tilted gel-phase PC appears to be a water-bridged PC dimer. We find that hydration dramatically alters the surface potential, dipole and quadrupole moments of the lipids and consequently guides the interactions of the lipids with other molecules and the communication between cells. [ABSTRACT FROM AUTHOR]

We report the vibrational and orientational dynamics of water molecules in isotopically diluted NaOH and NaOD solutions using polarization-resolved femtosecond vibrational spectroscopy and terahertz time-domain dielectric relaxation measurements. We observe a speed-up of the vibrational relaxation of the O-D stretching vibration of HDO molecules outside the first hydration shell of OH- from 1.7 ± 0.2 ps for neat water to 1.0 ± 0.2 ps for a solution of 5 M NaOH in HDO:H2O. For the O-H vibration of HDO molecules outside the first hydration shell of OD-, we observe a similar speed-up from 750 ± 50 fs to 600 ± 50 fs for a solution of 6 M NaOD in HDO:D2O. The acceleration of the decay is assigned to fluctuations in the energy levels of the HDO molecules due to charge transfer events and charge fluctuations. The reorientation dynamics of water molecules outside the first hydration shell are observed to show the same time constant of 2.5 ± 0.2 ps as in bulk liquid water, indicating that there is no long range effect of the hydroxide ion on the hydrogen-bond structure of liquid water. The terahertz dielectric relaxation experiments show that the transfer of the hydroxide ion through liquid water involves the simultaneous motion of ∼7 surrounding water molecules, considerably less than previously reported for the proton. [ABSTRACT FROM AUTHOR]

We investigate the structure and orientation of water molecules at the water-lipid interface, using vibrational sum-frequency generation in conjunction with a maximum entropy phase retrieval method. We find that interfacial water molecules have an orientation opposite to that predicted by electrostatics and thus are likely localized between the lipid headgroup and its apolar alkyl chain. This type of water molecule is observed for phospholipids but not for structurally simpler surfactants. [ABSTRACT FROM AUTHOR]

The deposition and the isothermal crystallization kinetics of thin amorphous solid water (ASW) films on both Ru(0001) and CO-precovered Ru(0001) have been investigated in real time by simultaneously employing helium atom scattering, infrared reflection absorption spectroscopy, and isothermal temperature-programmed desorption. During ASW deposition, the interaction between water and the substrate depends critically on the amount of preadsorbed CO. However, the mechanism and kinetics of the crystallization of ∼50 layers thick ASW film were found to be independent of the amount of preadsorbed CO. We demonstrate that crystallization occurs through random nucleation events in the bulk of the material, followed by homogeneous growth, for solid water on both substrates. The morphological change involving the formation of three-dimensional grains of crystalline ice results in the exposure of the water monolayer just above the substrate to the vacuum during the crystallization process on both substrates. [ABSTRACT FROM AUTHOR]

The isothermal crystallization process of thin amorphous solid water (ASW) films on Ru(0001) has been investigated in real time by simultaneously employing helium atom scattering, infrared reflection absorption spectroscopy, and isothermal temperature-programmed desorption. The measurements reveal that the crystallization mechanism consists of random nucleation events in the bulk of the ASW films, followed by homogeneous growth. Morphological changes of the solid water film during crystallization expose the water monolayer just above the substrate to the vacuum during the crystallization process. [ABSTRACT FROM AUTHOR]

Despite extensive study the phase behavior of phospholipid monolayers at an air–water interface is still not fully understood. In particular recent vibrational sum-frequency generation (VSFG) spectra of DPPC monolayers as a function of area density show a sharp transition in the order of the lipid chains at 1.10 nm2/molecule. This is in a region where the lateral pressure as a function of area is effectively constant. We have investigated the nature of this transition by studying the phase behavior of DPPC monolayers as a function of area density using molecular-dynamics simulations. The changes in order within the monolayer as a function of area density correlate well with the experimental signal. At 0.58 nm2/molecule we observe the onset of lateral separation of highly ordered and disordered lipids, indicating the coexistence of a gel-like liquid condensed and a fluidlike liquid expanded phase. At 0.97 nm2/molecule the monolayer ruptures, marking the onset of the liquid–gas (G) coexistence region. This is much earlier than suggested by fluorescence microscopy results and implies that at the point of rupture, the initial pores have an equilibrium size smaller than ∼500 nm in diameter. The rupture of the monolayer leads to a sharp increase in the overall lipid order that explains the sharp transition observed in the VSFG measurements. VSFG measurements thus may represent a sensitive means to determine the onset of the liquid–gas (G) coexistence region for such systems. © 2005 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We present an experimental and theoretical investigation of the adsorption, desorption, and dissociation of NO on the stepped Pt (533) surface. By combining temperature programmed desorption and reflection absorption infrared spectroscopy, information about the adsorption sites at different temperatures is obtained. Surprisingly, metastable adsorption structures of NO can be produced through variation of the dosing temperature. We also show that part of the NO molecules adsorbed on the step sites dissociates around 450 K. After dissociation the N atoms can desorb either by combining with an O fragment, or with another N atom, resulting in NO and N2. The N2 production can be enhanced by coadsorbing CO on the surface: CO scavenges the oxygen atom, thereby suppressing associative recombinative desorption of N and O atoms. Density functional theory calculations are used to reveal the adsorption energies and vibrational frequencies of adsorbed NO as well as barriers for dissociation of NO and for diffusion of N atoms. The combined experimental results and theoretical calculations reveal that dissociation of NO is the rate limiting step in the formation of N2.© 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We report a theoretical study of phase transition kinetics in confined two-dimensional systems, motivated by recent experimental results on the amorphous-to-crystalline transition in supported, thin amorphous water films [E.H.G. Backus, M.L. Grecea, A.W. Kleyn, and M. Bonn, Phys. Rev. Lett. (to be published)]. We generalize and extend existing theories to simultaneously describe the converted (crystalline) fractions in the bulk, at the sample-vacuum surface, and at the sample-support interface as a function of time. The general approach presented here results in expressions for the time-dependent converted bulk, surface, and interface fractions, for arbitrary desorption rate from the thin film, nucleation and growth rates and also includes finite nucleation grain size. The converted bulk, surface, and interface fractions are calculated for nucleation of the new phase occurring (i) in the bulk, (ii) at the support-sample interface, and (iii) at the sample surface (sample-vacuum interface), resulting in nine expressions. The results demonstrate the advantage of monitoring bulk, surface and interface fractions simultaneously to make definite statements regarding the location of the nucleation, and to reliably determine the values of the relevant crystallization parameters. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We present investigations of the transient photoconductivity and recombination dynamics of quasifree electrons in liquid n-hexane and cyclohexane performed using terahertz time-domain spectroscopy (THz-TDS). Quasifree electrons are generated by two-photon photoionization of the liquid using a femtosecond ultraviolet pulse, and the resulting changes in the complex conductivity are probed by a THz electromagnetic pulse at a variable delay. The detection of time-domain wave forms of the THz electric field permits the direct determination of both the real and the imaginary part of the conductivity of the electrons over a wide frequency range. The change in conductivity can be described by the Drude model, thus yielding the quasifree electron density and scattering time. The electron density is found to decay on a time scale of a few hundred picoseconds, which becomes shorter with increasing excitation density. The dynamics can be described by a model that assumes nongeminate recombination between electrons and positive ions. In addition, a strong dependence of the quasifree electron density on temperature is observed, in agreement with a two-state model in which the electron may exist in either a quasifree or a bound state. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We present an experimental and theoretical study of vibrational excitation of the C–O stretch vibration of carbon monoxide adsorbed on a ruthenium Ru(001) surface with ultrashort femtosecond infrared laser pulses. After broadband excitation leading to transfer of a significant fraction of the CO molecules to their first (∼15%) and second (∼5%) vibrationally excited states, we observe a competition between vibrational energy relaxation and energy delocalization through dipole–dipole coupling. We reproduce the observed excited state spectra by solving the three-level Bloch equations and accounting for intermolecular vibrational energy transfer on a picosecond time scale. The rate of vibrational energy transfer, and its coverage-dependence, can be described by a Fo¨rster energy transfer mechanism. We discuss possibilities to optimize the degree of localized vibrational excitation of a specific bond of molecules at surfaces through chirped pulse IR excitation. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

With infrared transient hole-burning spectroscopy we have investigated the influence of OD···X hydrogen bonds on the vibrational line shape of O–D stretch vibrations in acid zeolites. The effect of hydrogen bonding on the line shape depends critically on the type of hydrogen bond. For hydrogen bonding in a rigid structure, the hydrogen bond determines the inhomogeneous linewidth, but the homogeneous linewidth is determined by coupling to a ∼200 cm-1 lattice mode as concluded from the temperature dependence of the dephasing rate. When the hydrogen bond is formed with an adsorbing molecule, the coupling between the high-frequency O–D stretch vibration and the low-frequency OD···X hydrogen-bond stretching mode does determine the homogeneous linewidth. The difference between the two systems can be explained by the different hydrogen-bond potentials. Variation of the adsorbate provides a means of obtaining conclusive information on the coupling mechanism between the high-frequency O–D stretching mode and the low-frequency OD···X hydrogen-bond stretching mode. © 1996 American Institute of Physics. [ABSTRACT FROM AUTHOR]

In this time-resolved study of vibrational dynamics of deuterated surface hydroxyls at acid sites in the zeolite Mordenite, we investigate the O–D T1 vibrational lifetime and transient band shifts. It is shown that after infrared excitation of the stretching mode of a surface hydroxyl, the excess energy is rapidly distributed over delocalized low-energy lattice modes upon de-excitation. This is asserted from the observation that nonexcited hydroxyls are perturbed by the relaxation of their excited counterparts immediately after this relaxation. This observation can be made owing to better resolution in transient transmission spectroscopy obtained by deuteration of the surface hydroxyls. This assignment allows for accurate estimates of lattice temperatures after relaxation of the vibration. Further, from the observation that the vibrational lifetime is dependent on frequency (increasing from 25 to 70 ps with increasing frequency), it is concluded that the deuterated acidic protons are hydrogen bonded to lattice oxygen atoms in the zeolite. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Knowledge of the interfacial water structure is essential for a basic understanding of the many environmental, technological, and biophysical systems in which aqueous interfaces appear. Using ultrafast two-dimensional surface-specific vibrational spectroscopy we show that the structure of heavy water at the water-air interface displays short-lived heterogeneity and is very different from that at the water-lipid interface. [ABSTRACT FROM AUTHOR]

The buildup of photoconductivity in an archetypal polar semiconductor GaAs was monitored on the timescale of carrier-carrier and carrier-lattice correlations, with sub-50 fs temporal resolution provided by ultra-broadband transient terahertz spectroscopy. Two relevant timescales are identified: the transition in photoconductivity from a regime of extremely weak screening to a screened Drude plasma was observed on the ~280 fs timescale, followed by the gradual increase and saturation of electron mobility on a timescale of ~160 fs. The Drude photoconductivity picture following carrier photoinjection was found to stabilize only on the timescale corresponding to several lattice and plasma oscillations. [ABSTRACT FROM AUTHOR]

Biophysical studies of the interaction of peptides with model membranes provide a simple yet effective approach to understand the transport of peptides and peptide based drug carriers across the cell membrane. Herein, the authors discuss the use of self-assembled monolayers fabricated from the full membrane-spanning thiol (FMST) 3-((14-((40-((5-methyl-1-phenyl-35-(phytanyl)oxy-6,9,12,15,18,21,24,27,30,33,37-undecaoxa-2,3-dithiahenpentacontan-51-yl)oxy)-[1,10-biphenyl]-4-yl)oxy)tetradecyl)oxy)-2-(phytanyl)oxy glycerol for ultrahigh vacuum (UHV) based experiments. UHV-based methods such as electron spectroscopy and mass spectrometry can provide important information about how peptides bind and interact with membranes, especially with the hydrophobic core of a lipid bilayer. Near-edge x-ray absorption fine structure spectra and x-ray photoelectron spectroscopy (XPS) data showed that FMST forms UHV-stable and ordered films on gold. XPS and time of flight secondary ion mass spectrometry depth profiles indicated that a proline-rich amphipathic cell-penetrating peptide, known as sweet arrow peptide is located at the outer perimeter of the model membrane. [ABSTRACT FROM AUTHOR]

The surface vibrational spectrum of water at biological interfaces is often interpreted as having 'ice-like' and 'liquid-like' components. Here we show that the vibrational spectrum of water at both water-lipid and water-protein interfaces greatly simplifies upon H/D isotopic dilution, which is inconsistent with the presence of 'ice-like' structures. The changes in the spectra as a function of isotope content can be explained by intramolecular coupling between bend and stretch vibrations of the water molecules. [ABSTRACT FROM AUTHOR]

We present a novel setup to elucidate the dynamics of interfacial molecules specifically, using surface-selective femtosecond vibrational spectroscopy. The approach relies on a fourth-order nonlinear optical interaction at the interface. In the experiments, interfacial molecules are vibrationally excited by an intense, tunable femtosecond midinfrared (2500–3800 cm-1) pump pulse, resonant with the molecular vibrations. The effect of the excitation and the subsequent relaxation to the equilibrium state are probed using broadband infrared+visible sum frequency generation (SFG) light, which provides the transient vibrational spectrum of interfacial molecules specifically. This IR pump-SFG probe setup has the ability to measure both vibrational population lifetimes as well as the vibrational coupling between different chemical moieties at interfaces. Vibrational lifetimes of interfacial molecules are determined in one-dimensional pump-SFG probe experiments, in which the response is monitored as a function of the delay between the pump and probe pulses. Vibrational coupling between molecular groups is determined in two-dimensional pump-SFG probe experiments, which monitor the response as a function of pump and probe frequencies at a fixed delay time. To allow for one setup to perform these multifaceted experiments, we have implemented several instrumentation techniques described here. The detection of the spectrally resolved differential SFG signal using a combination of a charge-coupled device camera and a piezocontrolled optical scanner, computer-controlled Fabry–Pérot etalons to shape and scan the IR pump pulse and the automated sample dispenser and sample trough height corrector are some of the novelties in this setup. [ABSTRACT FROM AUTHOR]

In a series of systematic optical pump-terahertz probe experiments, we study the density-dependent electron scattering rate in photoexcited GaAs in the regime of strong carrier diffusion. The terahertz frequency-resolved transient sheet conductivity spectra are perfectly described by the Drude model, directly yielding the electron scattering rates. A diffusion model is applied to determine the spatial extent of the photoexcited electron-hole gas at each moment after photoexcitation, yielding the time-dependent electron density, and hence the density-dependent electron scattering time. We find that the electron scattering time decreases from 320 to 60 fs, as the electron density changes from 1015 to 1019 cm-3. [ABSTRACT FROM AUTHOR]

The air-water interface has been a subject of extensive theoretical and experimental studies due to its ubiquity in nature and its importance as a model system for aqueous hydrophobic interfaces. We report on the structure and vibrational energy transfer dynamics of this interfacial water system studied with equilibrium and non-equilibrium molecular dynamics simulations employing a density functional theory -based description of the system and the kinetic energy spectral density analysis. The interfacial water molecules are found to make fewer and weaker hydrogen (H)-bonds on average compared to those in the bulk. We also find that (i) the H-bonded OH groups conjugate to the free OH exhibit rather low vibrational frequencies (3000-3500 cm -1 ); (ii) the presence of a significant fraction (>10%) of free and randomly oriented water molecules at the interface ("labile water"), neither of whose OH groups are strong H-bond donors; (iii) the inertial rotation of free OH groups, especially from the labile water, contribute to the population decay of excited free OH groups with comparable rate and magnitude as intramolecular energy transfer between the OH groups. These results suggest that the labile water, which might not be easily detectable by the conventional vibrational sum frequency generation method, plays an important role in the surface water dynamics.

We report calculations on the surface tension of the water-air interface using ab initio molecular dynamics (AIMD) simulations. We investigate the influence of the cell size on surface tension of water from force field molecular dynamics simulations. We find that the calculated surface tension increases with increasing simulation cell size, thereby illustrating that a correction for finite size effects is essential for small systems that are customary in AIMD simulations. Moreover, AIMD simulations reveal that the use of a double-ζ basis set overestimates the experimentally measured surface tension due to the Pulay stress while more accurate triple and quadruple-ζ basis sets give converged results. We further demonstrate that van der Waals corrections critically affect the surface tension. AIMD simulations without the van der Waals correction substantially underestimate the surface tension while the van der Waals correction with the Grimme's D2 technique results in a value for the surface tension that is too high. The Grimme's D3 van der Waals correction provides a surface tension close to the experimental value. Whereas the specific choices for the van der Waals correction and basis sets critically affect the calculated surface tension, the surface tension is remarkably insensitive to the details of the exchange and correlation functionals, which highlights the impact of long-range interactions on the surface tension. Our simulated values provide important benchmarks, both for improving van der Waals corrections and AIMD simulations of aqueous interfaces.

Interfacial water structures have been studied intensively by probing the O-H stretch mode of water molecules using sum-frequency generation (SFG) spectroscopy. This surface-specific technique is finding increasingly widespread use, and accordingly, computational approaches to calculate SFG spectra using molecular dynamics (MD) trajectories of interfacial water molecules have been developed and employed to correlate specific spectral signatures with distinct interfacial water structures. Such simulations typically require relatively long (several nanoseconds) MD trajectories to allow reliable calculation of the SFG response functions through the dipole moment-polarizability time correlation function. These long trajectories limit the use of computationally expensive MD techniques such as ab initio MD and centroid MD simulations. Here, we present an efficient algorithm determining the SFG response from the surface-specific velocity-velocity correlation function (ssVVCF). This ssVVCF formalism allows us to calculate SFG spectra using a MD trajectory of only ∼100 ps, resulting in the substantial reduction of the computational costs, by almost an order of magnitude. We demonstrate that the O-H stretch SFG spectra at the water-air interface calculated by using the ssVVCF formalism well reproduce those calculated by using the dipole moment-polarizability time correlation function. Furthermore, we applied this ssVVCF technique for computing the SFG spectra from the ab initio MD trajectories with various density functionals. We report that the SFG responses computed from both ab initio MD simulations and MD simulations with an ab initio based force field model do not show a positive feature in its imaginary component at 3100 cm(-1).