Gas-phase ion luminescence spectroscopy is used to determine intrinsic electronic transition energies in the absence of a disturbing environment. Large ions produced by electrospray ionization are stored and mass-selected in a cylindrical Paul trap. Here they are irradiated by light from a 20-Hz pulsed tunable wavelelength laser from EKSPLA followed by detection of the emitted photons. The 20-Hz repetition rate of the tunable laser allows for mass selection in between every irradiation event, which implies that there is no fluorescence contribution from ion impurities. The tunability of the laser makes it possible to photo-excite a variety of dyes that absorb at different wavelengths. The figure shows spectra of oxazine dye cations that display emissions in a range from 500 nm to 750 nm. The technique can also be used to study three-dimensional structures of peptides and nucleic acids in the gas phase based on Förster Resonance Energy Transfer (FRET). The biomolecules are labeled with donor-acceptor dye pairs such as rhodamines 575 and 640. Structure information is provided as the efficiency of energy transfer depends on the distance between the donor and acceptor to the inverse power of six.
Luminescence spectroscopy of chalcogen substituted rhodamine cations in vacuo
Related products: NT342 series
Intrinsic optical properties of several rhodamine cations were probed by measuring their dispersed fluorescence spectra in vacuo. Three different rhodamine structures were investigated, each with four different chalcogen heteroatoms. Fluorescence band maxima were blue-shifted by between 0.15 and 0.20 eV (1200–1600 cm−1) relative to previous solution-phase measurements. Trends in emission wavelengths and fluorescence quantum yields previously measured in solution are generally reproduced in the gas phase, confirming the intrinsic nature of these effects. One important exception is gas-phase brightness of the Texas Red analogues, which is significantly higher than the other rhodamine structures studied, despite having similar fluorescence quantum yields in solution. These results expand the library of fluorophores for which gas-phase photophysical data is available, and will aid in the design of experiments utilizing gas-phase structural biology methods such as Förster resonance energy transfer.
Sibling rivalry: intrinsic luminescence from two xanthene dye monoanions, resorufin and fluorescein, provides evidence for excited-state proton transfer in the latter
Related products: NT342 series
While the emission spectrum of fluorescein monoanions isolated in vacuo displays a broad and featureless band, that of resorufin, also belonging to the xanthene family, has a sharp band maximum, clear vibronic structure, and experiences a small Stokes shift. Excited-state proton transfer in fluorescein can account for the differences.
Luminescence Spectroscopy of Rhodamine Homodimer Dications in Vacuo Reveals Strong Dye‐Dye Interactions
Related products: NT342 series
Being alone or together makes a difference for the photophysics of dyes but for ionic dyes it is difficult to quantify the interactions due to solvent screening and nearby counter ions. Gas‐phase luminescence experiments are desirable and now possible based on recent developments in mass spectrometry. Here we present results on tailor‐made rhodamine homodimers where two dye cations are separated by methylene linkers, (CH2)n. In solution the fluorescence is almost identical to that from the monomer whereas the emission from bare cation dimers redshifts with decreasing n. In the absence of screening, the electric field from the charge on one dye is strong enough to polarize the other dye, both in the ground state and in the excited state. An electrostatic model based on symmetric dye responses (equal induced‐dipole moments in ground state) captures the underlying physics and demonstrates interaction even at large distances. Our results have possible implications for gas‐phase Förster Resonance Energy Transfer.
Gas-phase Ion Spectroscopy of Flexible and Nonflexible Nitrophenolates: Effect of Locking the Two Phenyl Units in 4’-nitro-[1,1’-biphenyl]-4-olate by a Bridging Atom
Related products: NT342 series
Nitrophenolates (NPs) are molecular anions that can undergo charge-transfer (CT) transitions determined by the degree of electron delocalization between the phenolate oxygen (donor group) and the nitro group (acceptor). Here we have studied four different NPs: 4’-nitro-[1,1’-biphenyl]-4-olate (1), 7-nitro-9H-carbazol-2-olate (NH linker, 2), 7-nitrodibenzo[b,d]furan-3-olate (oxygen linker, 3), and 7-nitrodibenzo[b,d]thiophen-3-olate (sulphur linker, 4), and recorded their electronic absorption spectra when isolated in vacuo to determine the effect of locking the biphenyl spacer group between the donor and acceptor on transition energies. Absorption was identified from ion dissociation (action spectroscopy) using a homebuilt setup (sector mass spectrometer combined with pulsed laser). We find that the absorption is broad in the visible region for all four NPs with significant vibronic features. The lowest energy peak is at 601 ± 4 nm, 606 ± 4 nm, 615 ± 4 nm, and 620 ± 4 nm, for 3, 4, 2, and 1, respectively. NP 1 is flexible, and its lowest energy structure is nonplanar while the other three NPs are planar according to density functional theory calculations. Hence in the case of 1 the electronic transition has a higher degree of CT than for the other three, accounting for its absorption furthest to the red. Our work demonstrates that oxygen and sulphur are best at conveying the electronic coupling between the donor and acceptor sites as 3 and 4 absorb furthest to the blue (i.e., the degree of CT is lowest for these two NPs). Based on the average spacing between the peaks in the vibrational progressions, coupling occurs to skeleton vibrational modes with frequencies of 649 ± 69 cm-1 (3), 655 ± 49 cm-1 (4), and 697 ± 52 cm-1 (2).
Sum frequency generation vibrational spectroscopy (SFG-VS) is used to characterize vibrational bonds of molecules at surfaces or interfaces. SFG spectroscopy is particularly attractive because of molecular specificity and intrinsic interfacial sensitivity. Surface sensitivity of the technique arises from the fact that within the electric dipole approximation the nonlinear generation of the sum-frequency (SF) signal from the overlapped visible and infrared beams is forbidden in the media of randomly oriented molecules or in the centrosymmetric media but is allowed at the interface where inversion symmetry is broken. Molecular specificity emerges from the ability to record vibrational spectrum.
In SFG-VS measurements, carried out by employing SFG spectromer, a pulsed tunable infrared IR (ωIR) laser beam is mixed with a visible VIS (ωVIS) beam to produce an output at the sum frequency (ωSFG = ωIR + ωVIS). SFG signal is generated in visible spectral range, so it can be efficiently measured using sensitive detectors (PMT or CCD).
Platelet-adhesion behavior synchronized with surface rearrangement in a film of poly(methyl methacrylate) terminated with elemental blocks
Related products: SFG Spectrometer
Poly(methyl methacrylate) (PMMA) terminated with elemental blocks containing polyhedral oligomeric silsesquioxane (POSS), hereafter referred to as PPMP, was synthesized by living anionic polymerization. Combining modern interfacial-sensitive spectroscopy with traditional contact angle measurements, static and dynamic structures at the surface of PPMP films in water were examined. The surface of the well-annealed PPMP films, where the POSS end groups were preferentially segregated, was flat at the sub-nanometer level. Once the PPMP film was immersed in water, the surface was reorganized, and the rate was much slower for PPMP than for the conventional PMMA. This implies that the POSS units hindered the interfacial dynamics of the polymer segments. Then, platelet-adhesion tests were performed on the PPMP films. The number of platelets adhered to the PPMP film was dependent on the pre-immersion time in phosphate-buffered saline before the platelet seeding, whereas that of the reference PMMA film was unaffected by the pre-immersion time. These results could be explained in terms of the aggregation states of water at the interface.
The complex nature of calcium cation interactions with phospholipid bilayers
Related products: SFG Spectrometer
Understanding interactions of calcium with lipid membranes at the molecular level is of great importance in light of their involvement in calcium signaling, association of proteins with cellular membranes, and membrane fusion. We quantify these interactions in detail by employing a combination of spectroscopic methods with atomistic molecular dynamics simulations. Namely, time-resolved fluorescent spectroscopy of lipid vesicles and vibrational sum frequency spectroscopy of lipid monolayers are used to characterize local binding sites of calcium in zwitterionic and anionic model lipid assemblies, while dynamic light scattering and zeta potential measurements are employed for macroscopic characterization of lipid vesicles in calcium-containing environments. To gain additional atomic-level information, the experiments are complemented by molecular simulations that utilize an accurate force field for calcium ions with scaled charges effectively accounting for electronic polarization effects. We demonstrate that lipid membranes have substantial calcium-binding capacity, with several types of binding sites present. Significantly, the binding mode depends on calcium concentration with important implications for calcium buffering, synaptic plasticity, and protein-membrane association.
Discovery of Cellulose Surface Layer Conformation by Nonlinear Vibrational Spectroscopy
Related products: SFG Spectrometer
Significant questions remain in respect to cellulose’s structure and polymorphs, particularly the cellulose surface layers and the bulk crystalline core as well as the conformational differences. Total Internal Reflection Sum Frequency Generation Vibrational Spectroscopy (TIR-SFG-VS) combined with conventional SFG-VS (non-TIR) enables selectively characterizing the molecular structures of surface layers and the crystalline core of cellulose, revealing their differences for the first time. From the SFG spectra in the C-H and O-H regions, we found that the surface layers of Avicel are essentially amorphous while the surface layers of Iβ cellulose are crystalline but with different structural and spectroscopic signatures compared with its crystalline core. The differences between hydrogen bonding networks of cellulose surface and crystalline core were also shown by the SFG signal. The discovery here represents yet another instance of the importance of spectroscopic observations in transformative advances to understand the structure of the cellulosic biomass.
Vibrational fingerprint of localized excitons in a two-dimensional metal-organic crystal
Long-lived excitons formed upon visible light absorption play an essential role in photovoltaics, photocatalysis, and even in high-density information storage. Here, we describe a self-assembled two-dimensional metal-organic crystal, composed of graphene-supported macrocycles, each hosting a single FeN4 center, where a single carbon monoxide molecule can adsorb. In this heme-like biomimetic model system, excitons are generated by visible laser light upon a spin transition associated with the layer 2D crystallinity, and are simultaneously detected via the carbon monoxide ligand stretching mode at room temperature and near-ambient pressure. The proposed mechanism is supported by the results of infrared and time-resolved pump-probe spectroscopies, and by ab initio theoretical methods, opening a path towards the handling of exciton dynamics on 2D biomimetic crystals.
Reconfiguration of interfacial energy band structure for high-performance inverted structure perovskite solar cells
Related products: SFG Spectrometer
Charged defects at the surface of the organic–inorganic perovskite active layer are detrimental to solar cells due to exacerbated charge carrier recombination. Here we show that charged surface defects can be benign after passivation and further exploited for reconfiguration of interfacial energy band structure. Based on the electrostatic interaction between oppositely charged ions, Lewis-acid-featured fullerene skeleton after iodide ionization (PCBB-3N-3I) not only efficiently passivates positively charged surface defects but also assembles on top of the perovskite active layer with preferred orientation. Consequently, PCBB-3N-3I with a strong molecular electric dipole forms a dipole interlayer to reconfigure interfacial energy band structure, leading to enhanced built-in potential and charge collection. As a result, inverted structure planar heterojunction perovskite solar cells exhibit the promising power conversion efficiency of 21.1% and robust ambient stability. This work opens up a new window to boost perovskite solar cells via rational exploitation of charged defects beyond passivation.
Vibrational Relaxation Lifetime of a Physisorbed Molecule at a Metal Surface
Previous measurements of vibrational relaxation lifetimes for molecules adsorbed at metal surfaces yielded values of 1–3 ps; however, only chemisorbed molecules have been studied. We report the first measurements of the vibrational relaxation lifetime of a molecule physisorbed to a metal surface. For CO(v=1) adsorbed on Au(111) at 35 K the vibrational lifetime of the excited stretching mode is 49±3 ps. The long lifetime seen here is likely to be a general feature of physisorption, which involves weaker electronic coupling between the adsorbate and the solid due to bonding at larger distances.
High-performance graphdiyne-based electrochemical actuators
Related products: SFG Spectrometer
Electrochemical actuators directly converting electrical energy to mechanical energy are critically important for artificial intelligence. However, their energy transduction efficiency is always lower than 1.0% because electrode materials lack active units in microstructure, and their assembly systems can hardly express the intrinsic properties. Here, we report a molecular-scale active graphdiyne-based electrochemical actuator with a high electro-mechanical transduction efficiency of up to 6.03%, exceeding that of the best-known piezoelectric ceramic, shape memory alloy and electroactive polymer reported before, and its energy density (11.5 kJ m−3) is comparable to that of mammalian skeletal muscle (~8 kJ m−3). Meanwhile, the actuator remains responsive at frequencies from 0.1 to 30 Hz with excellent cycling stability over 100,000 cycles. Furthermore, we verify the alkene–alkyne complex transition effect responsible for the high performance through in situ sum frequency generation spectroscopy. This discovery sheds light on our understanding of actuation mechanisms and will accelerate development of smart actuators.
Segregation of an amine component in a model epoxy resin at a copper interface
Related products: SFG Spectrometer
We have studied the chemical composition of the epoxy and amine components, HDGEBA and CBMA, of an epoxy resin in close proximity to a copper interface by using ADXPS in conjunction with SFG vibrational spectroscopy. A bilayer sample of epoxy resin and copper was first prepared on a solid substrate before etching the copper layer just before the interface with Ar+ beams. Using ADXPS, in which an incident X-ray was guided from the copper surface, it was found that the CBMA component was preferentially segregated at the copper interface, with the segregation extending over ~10 nm. SFG spectroscopy was used to confirm the above observation. Postulating that copper ions diffused from the metal copper into the internal phase during the curing process and reacted with amine groups to form copper complexes, the interfacial segregation of CBMA can be understood. This knowledge should be useful for understanding and controlling the adhesive properties of epoxy resins.
Aggregation States of Poly(4-methylpentene-1) at a Solid Interface
A thin film of poly(4-methylpentene-1) (P4MP1) was prepared on a quartz substrate, which was a model system of an interface in filler-reinforced semicrystalline polymer composites. Grazing-incidence wide-angle X-ray diffraction measurements revealed that P4MP1 in the thin film after isothermal crystallization formed a Form I crystal polymorph composed of a tetragonal unit cell with a 72 helix, in which the chain axis was oriented along the direction parallel to the quartz interface. Combining sum-frequency generation vibrational spectroscopy with molecular dynamics simulation enabled us to gain access to the local conformation of P4MP1 chains at the quartz interface and the changes that occurred with isothermal crystallization. Finally, the way in which the initial chain orientation at the substrate interface impacted the crystalline structure in the thin film was discussed.
Alkanethiols as Inhibitors for the Atmospheric Corrosion of Copper Induced by Formic Acid: Effect of Chain Length
Related products: SFG Spectrometer
Self assembled monolayers (SAMs) of n-alkanethiols of different chain length (4, 6, 8, 12, and 18 carbons in the chain) have been explored as corrosion inhibitors for copper exposed to humidified air containing formic acid, an environment used to mimic accelerated indoor atmospheric corrosion. Near-surface sensitive in-situ infrared reflection/absorption spectroscopy combined with interface sensitive vibrational sum frequency spectroscopy revealed unique molecular information on the role of each SAM during ongoing corrosion. All SAMs protect copper against corrosion, and this ability increases continuously with chain length. Their structural order is high prior to exposure, but an increased disorder is observed as a result of the corrosion process. The protection ability of the SAMs is attributed to a selective hindrance of the corrosion stimulators water, oxygen gas, and formic acid to reach the copper-SAM interface through each SAM, which results in different corrosion mechanisms on SAM protected copper and unprotected copper. This significantly retards the formation of the corrosion products copper hydroxide and copper formate, and results in essentially no formation of cuprite.
Hydrogen bonding interactions of H2O and SiOH on a boroaluminosilicate glass corroded in aqueous solution
Related products: SFG Spectrometer
Hydrogen bonding interactions play an important role in many chemical and physical processes occurring in bulk liquids and at interfaces. In this study, hydrous species (H2O and Si-OH) on nano-porous alteration layers (gels) formed on a boroaluminosilicate glass called International Simple Glass corroded in aqueous solutions at pH 7 and pH 9, and initially saturated with soluble silicon-containing species were analyzed using linear and non-linear vibrational spectroscopy in combination with molecular dynamics simulations. The simulation results revealed various possible types of hydrogen bonds among these hydrous species in nanoconfinement environments with their populations depending on pore-size distribution. The nano-porous gels formed on corroded glass surfaces enhance hydrogen bond strength between hydrous species as revealed by attenuated total reflectance infrared spectroscopy. Sum frequency generation spectroscopy showed some significant differences in hydrogen bonding interactions on alteration layers formed at pH 7 and pH 9. The glass dissolution under the leaching conditions used in this study has been known to be ten times faster at pH 7 in comparison to that at pH 9 due to unknown reasons. The simulation and experimental results obtained in this study indicate that the water mobility in the gel formed at pH 9 could be slower than that in the gel formed at pH 7, and as a result, the leaching rate at pH 9 is slower than that at pH 7.
Study of self-assembled triethoxysilane thin films made by casting neat reagents in ambient atmosphere
We studied four trialkoxysilane thin films, fabricated via self-assembly by casting neat silane reagents onto hydrophilic SiOx/Si substrates in the ambient. This drop-casting method is simple, yet rarely studied for the production of silane self-assembled monolayers (SAMs). Various ex-situ techniques were utilized to systematically characterize the growth process: Ellipsometry measurements can monitor the evolution of film thickness with silanization time; water droplet contact angle measurements reveal the wettability; the change of surface morphology was followed by Atomic Force Microscopy; the chemical identity of the films was verified by Infrared–Visible Sum Frequency Generation spectroscopy. We show that the shorter carbon chain (propyl-) or branched (2-(diphenylphosphino)ethyl-) silane SAMs exhibit poor ordering. In contrast, longer carbon chain (octadecyl and decyl) silanes form relatively ordered monolayers. The growth of the latter two cases shows Langmuir-like kinetics and a transition process from lying-down to standing-up geometry with increasing coverage.
Sum frequency generation vibrational spectroscopy (SFG-VS) for complex molecular surfaces and interfaces: Spectral lineshape measurement and analysis plus some controversial issues
Related products: SFG Spectrometer
Sum-frequency generation vibrational spectroscopy (SFG-VS) was first developed in the 1980s and it has been proven a uniquely sensitive and surface/interface selective spectroscopic probe for characterization of the structure, conformation and dynamics of molecular surfaces and interfaces. In recent years, there have been many progresses in the development of methodology and instrumentation in the SFG-VS toolbox that have significantly broadened the application to complex molecular surfaces and interfaces. In this review, after presenting a unified view on the theory and methodology focusing on the SFG-VS spectral lineshape, as well as the new opportunities in SFG-VS applications with such developments, some of the controversial issues that have been puzzling the community are discussed. The aim of this review is to present to the researchers and students interested in molecular surfaces and interfacial sciences up-to-date perspectives complementary to the existing textbooks and reviews on SFG-VS.
Sum Frequency Generation Vibrational Spectroscopy for Characterization of Buried Polymer Interfaces
Related products: SFG Spectrometer
Sum frequency generation vibrational spectroscopy (SFG-VS) has become one of the most appealing technologies to characterize molecular structures at interfaces. In this focal point review, we focus on SFG-VS studies at buried polymer interfaces and review many of the recent publications in the field. We also cover the essential theoretical background of SFG-VS and discuss the experimental implementation of SFG-VS.
Phase measurement in nondegenerate three-wave mixing spectroscopy
Related products: SFG Spectrometer
A detailed model is presented that describes the temporal and spectral interference patterns resulting from phase-recovery infrared–visible sum-frequency spectroscopy. Included in this model are the effects of dispersive elements other than the phase shifting unit placed between the sample and local oscillator signals. This inclusion is critical when considering the interference patterns arising from studies of buried interfaces. Furthermore, in the midinfrared where it is difficult to have high visibility of the fringes, it is demonstrated that local field corrections have a significant effect on the shape of the interference pattern. By collecting and subsequently fitting a two-dimensional interference pattern displaying both temporal and spectral fringes, a complete characterization of all these effects is possible.
Structure Determination of Hen Egg-White Lysozyme Aggregates Adsorbed to Lipid/Water and Air/Water Interfaces
We use vibrational sum-frequency generation (VSFG) spectroscopy to study the structure of hen egg-white lysozyme (HEWL) aggregates adsorbed to DOPG/D2O and air/D2O interfaces. We find that aggregates with a parallel and antiparallel β-sheet structure together with smaller unordered aggregates and a denaturated protein are adsorbed to both interfaces. We demonstrate that to retrieve this information, fitting of the VSFG spectra is essential. The number of bands contributing to the VSFG spectrum might be misinterpreted, due to interference between peaks with opposite orientation and a nonresonant background. Our study identified hydrophobicity as the main driving force for adsorption to the air/D2O interface. Adsorption to the DOPG/D2O interface is also influenced by hydrophobic interaction; however, electrostatic interaction between the charged protein’s groups and the lipid’s headgroups has the most significant effect on the adsorption. We find that the intensity of the VSFG spectrum at the DOPG/D2O interface is strongly enhanced by varying the pH of the solution. We show that this change is not due to a change of lysozyme’s and its aggregates’ charge but due to dipole reorientation at the DOPG/D2O interface. This finding suggests that extra care must be taken when interpreting the VSFG spectrum of proteins adsorbed at the lipid/water interface.
Heavy Anionic Complex Creates a Unique Water Structure at a Soft Charged Interface
Ion hydration and interfacial water play crucial roles in numerous phenomena ranging from biological to industrial systems. Although biologically relevant (and mostly smaller) ions have been studied extensively in this context, very little experimental data exist about molecular-scale behavior of heavy ions and their complexes at interfaces, especially under technologically significant conditions. It has recently been shown that PtCl6 2− complexes adsorb at positively charged interfaces in a two-step process that cannot fit into well-known empirical trends, such as Hofmeister series. Here, a combined vibrational sum frequency generation and molecular dynamics study reveals that a unique interfacial water structure is connected to this peculiar adsorption behavior. A novel subensemble analysis of molecular dynamics simulation results shows that after adsorption PtCl6 2− complexes partially retain their first and second hydration spheres and that it is possible to identify three different types of water molecules around them on the basis of their orientational structures and hydrogen-bonding strengths. These results have important implications for relating interfacial water structure and hydration enthalpy to the general understanding of specific ion effects.
This in turn influences interpretation of heavy metal ion distribution across, and reactivity within, liquid interfaces.
Simultaneous measurement of magnitude and phase in interferometric sum-frequency vibrational spectroscopy
Related products: SFG Spectrometer
We present a visible-infrared sum-frequency spectroscopic technique that is capable of simultaneously determining the magnitude and phase of the sample response from a single set of experimental conditions. This is especially valuable in cases where the phase stability is high, as in collinear beam geometries, as it enables multiple experiments to be performed without re-measuring the local oscillator phase or the reference phase. After illustrating the phase stability achievable with such a geometry, we provide a technique for quantitatively determining the magnitude and phase from a single set of two-dimensional spectral-temporal interference fringes. A complete demonstration is provided for the C–H stretching frequency region at the surface of an octadecyltricholosilane film.
A structural and temporal study of the surfactants behenyltrimethylammonium methosulfate and behenyltrimethylammonium chloride adsorbed at air/water and air/glass interfaces using sum frequency generation spectroscopy
Molecular scale information about the structure of surfactants at interfaces underlies their application in consumer products. In this study the non-linear optical technique of Sum Frequency Generation (SFG) vibrational spectroscopy has been used to investigate the structure and temporal behaviour of two cationic surfactants used frequently in hair conditioners. SFG spectra of films of behenyltrimethylammonium methosulfate (BTMS) and behenyltrimethylammonium chloride (BTAC) were recorded at the air/water interface and on glass slides following Langmuir Blodgett (LB) deposition. The assignment of the BTMS and BTAC spectral features (resonances) to the C--H stretching modes of the surfactants was consolidated by comparison with the SFG spectrum of deuterated cetyltrimethylammonium bromide (d-CTAB) and by recording spectra on D2O as well as on water. The C--H resonances arise from the methylene and methyl groups of the tail and head-groups of the surfactants. A slow collapse mechanism was observed following film compression of both BTAC and BTMS. The change in molecular structure of the films undergoing this slow collapse was followed by recording sequential SFG spectra in the C--H region, and by monitoring the SFG intensity at specific wavenumbers over time. Additionally, LB deposition onto glass was used to capture the state of the film during the slow collapse, and these SFG spectra showed close similarity to the corresponding spectra on water. Complementary Atomic Force Microscopy (AFM) was used to elucidate the layering of the compressed and relaxed films deposited onto mica by LB deposition.
Retrieval of complex χ(2) parts for quantitative analysis of sum-frequency generation intensity spectra
Vibrational sum-frequency generation (SFG) spectroscopy has become an established technique for in situ surface analysis. While spectral recording procedures and hardware have been optimized, unique data analysis routines have yet to be established. The SFG intensity is related to probing geometries and properties of the system under investigation such as the absolute square of the second-order susceptibility |χ(2)|2 . A conventional SFG intensity measurement does not grant access to the complex parts of χ(2) unless further assumptions have been made. It is therefore difficult, sometimes impossible, to establish a unique fitting solution for SFG intensity spectra. Recently, interferometric phase-sensitive SFG or heterodyne detection methods have been introduced to measure real and imaginary parts of χ(2) experimentally. Here, we demonstrate that iterative phase-matching between complex spectra retrieved from maximum entropy method analysis and fitting of intensity SFG spectra (iMEMfit) leads to a unique solution for the complex parts of χ(2) and enables quantitative analysis of SFG intensity spectra. A comparison between complex parts retrieved by iMEMfit applied to intensity spectra and phase sensitive experimental data shows excellent agreement between the two methods.
Unified treatment and measurement of the spectral resolution and temporal effects in frequency-resolved sum-frequency generation vibrational spectroscopy (SFG-VS)
The lack of understanding of the temporal effects and the restricted ability to control experimental conditions in order to obtain intrinsic spectral lineshapes in surface sum-frequency generation vibrational spectroscopy (SFG-VS) have limited its applications in surface and interfacial studies. The emergence of high-resolution broadband sum-frequency generation vibrational spectroscopy (HR-BB-SFG-VS) with sub-wavenumber resolution [Velarde et al., J. Chem. Phys., 2011, 135, 241102] offers new opportunities for obtaining and understanding the spectral lineshapes and temporal effects in SFG-VS. Particularly, the high accuracy of the HR-BB-SFG-VS experimental lineshape provides detailed information on the complex coherent vibrational dynamics through direct spectral measurements. Here we present a unified formalism for the theoretical and experimental routes for obtaining an accurate lineshape of the SFG response. Then, we present a detailed analysis of a cholesterol monolayer at the air/water interface with higher and lower resolution SFG spectra along with their temporal response. With higher spectral resolution and accurate vibrational spectral lineshapes, it is shown that the parameters of the experimental SFG spectra can be used both to understand and to quantitatively reproduce the temporal effects in lower resolution SFG measurements. This perspective provides not only a unified picture but also a novel experimental approach to measuring and understanding the frequency-domain and time-domain SFG response of a complex molecular interface.
Investigating buried polymer interfaces using sum frequency generation vibrational spectroscopy
This paper reviews recent progress in the studies of buried polymer interfaces using sum frequency generation (SFG) vibrational spectroscopy. Both buried solid/liquid and solid/solid interfaces involving polymeric materials are discussed. SFG studies of polymer/water interfaces show that different polymers exhibit varied surface restructuring behavior in water, indicating the importance of probing polymer/water interfaces in situ. SFG has also been applied to the investigation of interfaces between polymers and other liquids. It has been found that molecular interactions at such polymer/liquid interfaces dictate interfacial polymer structures. The molecular structures of silane molecules, which are widely used as adhesion promoters, have been investigated using SFG at buried polymer/silane and polymer/polymer interfaces, providing molecularlevel understanding of polymer adhesion promotion. The molecular structures of polymer/solid interfaces have been examined using SFG with several different experimental geometries. These results have provided molecularlevel information about polymer friction, adhesion, interfacial chemical reactions, interfacial electronic properties, and the structure of layerbylayer deposited polymers. Such research has demonstrated that SFG is a powerful tool to probe buried interfaces involving polymeric materials, which are difficult to study by conventional surface sensitive analytical techniques.
Structure of the Fundamental Lipopeptide Surfactin at the Air/Water Interface Investigated by Sum Frequency Generation Spectroscopy
The lipopeptide surfactin produced by certain strains of Bacillus subtilis is a powerful bio-surfactant possessing potentially useful antimicrobial properties. In order to better understand its surface behaviour, we have used surface sensitive Sum Frequency Generation (SFG) vibrational spectroscopy in the C-H and C=O stretching regions to determine its structure at the air/water interface. Using surfactin with the leucine groups of the peptide ring perdeuterated we have shown that the majority of the SFG signals arise from the 4 leucine residues. We find that surfactin forms a robust film, and that its structure is not affected by the number density at the interface or by pH variation of the sub-phase. The spectra show that the ring of the molecule lies in the plane of the surface rather than perpendicular to it, with the tail lying above this, also in the plane of the interface.
Quantitative Sum-Frequency Generation Vibrational Spectroscopy of Molecular Surfaces and Interfaces: Lineshape, Polarization, and Orientation
Sum-frequency generation vibrational spectroscopy (SFG-VS) can provide detailed information and understanding of the molecular composition, interactions, and orientational and conformational structure of surfaces and interfaces through quantitative measurement and analysis. In this review, we present the current status of and discuss important recent developments in the measurement of intrinsic SFG spectral lineshapes and formulations for polarization measurements and orientational analysis of SFG-VS spectra. The focus of this review is to present a coherent description of SFG-VS and discuss the main concepts and issues that can help advance this technique as a quantitative analytical research tool for revealing the chemistry and physics of complex molecular surfaces and interfaces.
Probing the Orientation and Conformation of α-Helix and β-Strand Model Peptides on Self-Assembled Monolayers Using Sum Frequency Generation and NEXAFS Spectroscopy
The structure and orientation of amphiphilic α-helix and β-strand model peptide films on self-assembled monolayers (SAMs) have been studied with sum frequency generation (SFG) vibrational spectroscopy and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. The α-helix peptide is a 14-mer, and the β-strand is a 15-mer of hydrophilic lysine and hydrophobic leucine residues with hydrophobic periodicities of 3.5 and 2, respectively. These periodicities result in the leucine side chains located on one side of the peptides and the lysine side chains on the other side. The SAMs were prepared from the assembly of either carboxylic acid- or methyl-terminated alkyl thiols onto gold surfaces. For SFG studies, the deuterated analog of the methyl SAM was used. SFG vibrational spectra in the C-H region of air-dried peptides films on both SAMs exhibit strong peaks near 2965, 2940, and 2875 cm-1 related to ordered leucine side chains. The orientation of the leucine side chains was determined from the phase of these features relative to the nonresonant gold background. The relative phase for both the R-helix and β-strand peptides showed that the leucine side chains were oriented away from the carboxylic acid SAM surface and oriented toward the methyl SAM surface. Amide I peaks observed near 1656 cm-1 for the R-helix peptide confirm that the secondary structure is preserved on both SAMs. Strong linear dichroism related to the amide π* orbital at 400.8 eV was observed in the nitrogen K-edge NEXAFS spectra for the adsorbed β-strand peptides, suggesting that the peptide backbones are oriented parallel to the SAM surface with the side chains pointing toward or away from the interface. For the α-helix the dichroism of the amide π* is significantly weaker, probably because of the broad distribution of amide bond orientations in the α-helix secondary structure.
Second harmonic generation (SHG) is a second order nonlinear optical effect where two photons of frequency ω are converted to one photon of frequency 2ω. SHG is allowed only in media without inversion symmetry.
SHG is a sensitive method used to characterize molecules at surfaces or interfaces because inversion symmetry is broken at the interface.
SHG measurements provide information about: surface coverage, molecular orientation, adsorbtion-desorbtion processes, and reactions at interfaces.
FemtoLux 3 laser for the rapid wide-field second harmonic generation microscopy
Related products: FemtoLux series
FemtoLux 3 laser was used as an illumination source in the wide-field second harmonic generation (SHG) microscope. Relatively high laser pulse energy at a medium pulse repetition frequency allowed for a faster single image acquisition compared to that using laser-scanning. It was also possible to acquire images of relatively large sample areas, which paved the way for the rapid imaging of macroscopic sample areas with microscopic resolution.
In nonlinear optics, z-scan technique is used to measure the non-linear index n2 (Kerr nonlinearity) and the non-linear absorption coefficient Δα via the “closed” and “open” methods to measure both real and imaginary components of the nonlinear refractive index.
For measuring the real part of the nonlinear refractive index, the z-scan setup is used in its closed-aperture form. The sample is typically placed in the focal plane of the lens, and then moved along the z axis, defined by the Rayleigh length. In this form, since the nonlinear material reacts like a weak z-dependent lens, the far-field aperture makes it possible to detect small beam distortions in the original beam. Since the focusing power of this weak nonlinear lens depends on the nonlinear refractive index, it is possible to extract its value by analyzing the z-dependent data acquired by the detector and by interpreting them using an appropriate theory.
For measurements of the imaginary part of the nonlinear refractive index, or the nonlinear absorption coefficient, the z-scan setup is used in its open-aperture form. In open-aperture measurements, the far-field aperture is removed and the whole signal is measured by the detector. By measuring the whole signal, the beam small distortions become insignificant and the z-dependent signal variation is due to the nonlinear absorption entirely.
The main cause of non-linear absorption is two-photon absorption. Due to high pulse intensity and cost effectiveness, picosecond high energy lasers are the most appropriate choice for z-scan measurements.
LIDAR is an acronym for “LIght Detection And Ranging”. LIDAR sends out short laser pulses into the atmosphere, where all along its path, the light is scattered by small particles, aerosols, and molecules of the air and is collected by telescope for analysis. Due to the constant velocity of light, time is related to the scatter’s distance, therefore, the spatial information is retrieved along the beam path. LIDAR uses ultraviolet, visible, or near infrared light to image objects. It can target a wide range of materials, including non-metallic objects, rocks, rain, chemical compounds, aerosols, pollutants, clouds, and even single molecules. LIDAR especially helps in those cases where access with conventional methods is troublesome.
Laser-induced breakdown spectroscopy (LIBS) utilizes a high intensity short laser pulse to convert a very small amount of target material to plasma for optical analysis of the spectra. LIBS can be used on solid, liquid, or gas samples, and, depending on the spectrograph and detector, can detect all elements. LIBS is non-contact, so it can be used in a wide variety of environments, including remote analysis and micro-sampling.
When coupled with appropriate optics and stages, elemental maps of a surface can be created. Multiple LIBS scans can effectively resolve material composition throughout the volume, building a full three dimensional elemental map.
Adjusting the catalytic properties of cobalt ferrite nanoparticles by pulsed laser fragmentation in water with defined energy dose
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Highly active, structurally disordered CoFe2O4/CoO electrocatalysts are synthesized by pulsed laser fragmentation in liquid (PLFL) of a commercial CoFe2O4 powder dispersed in water. A partial transformation of the CoFe2O4 educt to CoO is observed and proposed to be a thermal decomposition process induced by the picosecond pulsed laser irradiation. The overpotential in the OER in aqueous alkaline media at 10 mA cm−2 is reduced by 23% compared to the educt down to 0.32 V with a Tafel slope of 71 mV dec−1. Importantly, the catalytic activity is systematically adjustable by the number of PLFL treatment cycles. The occurrence of thermal melting and decomposition during one PLFL cycle is verified by modelling the laser beam energy distribution within the irradiated colloid volume and comparing the by single particles absorbed part to threshold energies. Thermal decomposition leads to a massive reduction in particle size and crystal transformations towards crystalline CoO and amorphous CoFe2O4. Subsequently, thermal melting forms multi-phase spherical and network-like particles. Additionally, Fe-based layered double hydroxides at higher process cycle repetitions emerge as a byproduct. The results show that PLFL is a promising method that allows modification of the structural order in oxides and thus access to catalytically interesting materials.
Enhancement of Laser-Induced Breakdown Spectroscopy (LIBS) Detection Limit Using a Low-Pressure and Short-Pulse Laser-Induced Plasma Process
Laser-induced breakdown spectroscopy (LIBS) technology is an appealing technique compared with many other types of elemental analysis because of the fast response, high sensitivity, real-time, and noncontact features. One of the challenging targets of LIBS is the enhancement of the detection limit. In this study, the detection limit of gas-phase LIBS analysis has been improved by controlling the pressure and laser pulse width. In order to verify this method, low-pressure gas plasma was induced using nanosecond and picosecond lasers. The method was applied to the detection of Hg. The emission intensity ratio of the Hg atom to NO (IHg/ INO) was analyzed to evaluate the LIBS detection limit because the NO emission (interference signal) was formed during the plasma generation and cooling process of N2 and O2 in the air. It was demonstrated that the enhancement of IHg/INO arose by decreasing the pressure to a few kilopascals, and the IHg/INO of the picosecond breakdown was always much higher than that of the nanosecond breakdown at low buffer gas pressure. Enhancement of IHg/INO increased more than 10 times at 700 Pa using picosecond laser with 35 ps pulse width. The detection limit was enhanced to 0.03 ppm (parts per million). We also saw that the spectra from the center and edge parts of plasma showed different features. Comparing the central spectra with the edge spectra, IHg/INO of the edge spectra was higher than that of the central spectra using the picosecond laser breakdown process.
Overview of the application of laser-based techniques in plasma-wall interaction research program at IFPiLM
The purpose of this paper is to give an overview of the progress in the application of the laser-based techniques which has been achieved in the research in the field of plasma-wall interaction (PWI) at the Division of Laser-Produced Plasmas (DLPP), Institute of Plasma Physics and Laser Microfusion (IFPiLM, Warsaw, Poland) since 2005. The evolution of the experimental set-up which started in a simple configuration for the laser ablative co-deposit removal is presented with stress on the milestones which led to subsequent modifications, namely installation of laser induced breakdown spectroscopy (LIBS), fast HR (high resolution) CCDs, pulsed fiber-laser and the common triggering system.
Laser Induced Breakdown Spectroscopy and Applications Toward Thin Film Analysis
Laser induced breakdown spectroscopy (LIBS) provides the opportunity to analyze almost any element, from any material, in any environment. Among the many applications of LIBS is the analysis of thin lms and multilayered structures. An automated system was designed and built to conduct LIBS using Nd:YAG and Ti:Sapphire lasers, broadband and high-resolution spectrometers and detectors. This system incorporates the sample manipulation as well as laser and spectrometer control and timing. A series of experiments were conducted to analyze the ability of nanosecond and femtosecond lasers to detect Mg impurities in thin TiO2 lms using LIBS. It was determined that optimal detection occurs early in the plasma ionic/atomic emission with detection capabilities in the parts-per-million range. Another series of experiments were conducted using LIBS to analyze thin transparent organic lms, with specic emphasis on the eect of lm thickness and interplay between lm and substrate. The challenges of ablating and measuring multiple layers have also been explored using various laser wavelengths. The eectiveness of LIBS has been demonstrated for depth proling of CIGS solar cells. Ablation crater and ablation threshold analysis aided in understanding and overcoming some of the obstacles in depth proling. One of the challenges with LIBS is the identication and mitigation of matrix eects. This problem was explored using a Mg tracer element and various compositions of the suspected elements Si, Ca, and Sr which can cause errors in LIBS analysis. The goal of this dissertation is to investigate the ability of LIBS to conduct detailed thin lm analysis for a variety of materials and potential applications. This includes analyzing trace elements from a traditionally noisy background, measuring dicult to ablate thin lms, and the unique challenges associated with multilayered structures.