- Characterisation of vibrational bonds of molecules at surfaces or interfaces
- Intrinsically surface specific
- High spectral resolution
- Wide range of accessible (molecular) vibrations: 625-4000 cm⁻¹
- SFG Microscope
- Characterisation of vibrational bonds of molecules at surfaces or interfaces
- Intrinsically surface specific
- High spectral resolution
- Wide range of accessible (molecular) vibrations: 625-4000 cm⁻¹
- SFG Microscope
Sum Frequency Generation (SFG) Vibrational Spectroscopy
- Intrinsically surface specific
- Selective to adsorbed species
- Sensitive to submonolayer of molecules
- Applicable to all interfaces accessible to light
- Capable of high spectral and spatial resolution
- Investigation of surfaces and interfaces of solids, liquids, polymers, biological membranes and other systems
- Studies of surface structure, chemical composition and molecular orientation
- Remote sensing in hostile environment
- Investigation of surface reactions under real atmosphere, catalysis, surface dynamics
- Studies of epitaxial growth, electrochemistry, material and environmental problems
Sum Frequency Generation Vibrational Spectroscopy (SFG-VS) is powerful and versatile method for in-situ investigation of surfaces and interfaces. In SFG-VS experiment 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 is second order nonlinear process, which is allowed only in media without inversion symmetry.
At surfaces or interfaces inversion symmetry is necessarily broken, that makes SFG highly surface specific. As the IR wavelength is scanned, active vibrational modes of molecules at the interface give a resonant contribution to SF signal. The resonant enhancement provides spectral information on surface characteristic vibrational transitions.
Design of the SFG Spectrometer
Sum frequency generation (SFG) spectrometer is based on picosecond pump laser and optical parametric generator (OPG) with difference frequency generation (DFG) extension. Solid state mode-locked Nd:YAG laser featuring high pulse duration and energy stability is used in the system. Fundamental laser radiation splits into several channels in multichannel beams delivery unit. Two of these beams are used for pumping OPG and DFG. Small part of laser output beam, usually with doubled frequency (532 nm), is directed to VIS channel of SFG spectrometer. IR channel of spectrometer is pumped by DFG output beam.
The sizes of individual compartments, positions of apertures and beams heights are fitted. As a result SFG spectrometer takes less space in laboratory. Standard versions usually fit on 1000×2400 mm optical table. All beams among laser, harmonics module, parametric generator, SFG box are enclosed in tubes. For example beam dedicated for VIS channel passes through OPG compartment only to minimize the risk of accident with dangerous high intensity laser radiation. It makes Ekspla spectrometer substantially safer comparing to home-made SFG-VS setups. Also optical parameters, like beam diameter, pulse energy, delays between channels are perfectly matched. Motorised switch of IR Polarisation, motorisation of delay line are included in standard configuration.
We designed our spectrometer thinking about user friendly operation. Many components of the system are automated and controlled from PC. The opto-mechanical holders that need to be tuned often during routine operation are located around sample area and can be easily accessed without walking around the optical table. According to user needs different level of automation can be proposed, starting from most simple mechanical setup to most advanced fully motorized version.
Detection system consists of monochromator with high stray light rejection and gated PMT based SF signal detector. The feature of such design is ability to perform measurements in room lighting. Second parallel detection channel is available as an option. All system components are controlled from single dedicated software. Program contains many useful instruments for automatic SFG spectra recording, dynamics monitoring, X-Y sample mapping, azimuthal scan and system parameters monitoring.
Ekspla offers three common SFG spectrometer models for classical picosecond scanning SFG vibrational spectroscopy and several specialized models for most demanding users. Basic models are: SFG Classic, SFG Advanced and Double resonance SFG, Phase sensitive. They differ by IR beam tuning ranges and available VIS beam wavelengths (please see specifications page). Other models: Classic + Phase-sensitive SFG and SFG microscope provides unique features, which are described on next pages of this brochure.
- Picosecond mode-locked Nd:YAG laser
- Multichannel beam delivery unit
- Picosecond optical parametric generator
- Spectroscopy module
- PMT based signal detectors
- Data acquisition system
- Dedicated LabView® software package for system control
SFG Spectrometer Modifications and Options
- Double resonance SFG spectrometer – allows investigation of vibrational mode coupling to electron states at a surface
- Phase sensitive SFG spectrometer – allows measurement of the complex spectra of surface nonlinear response coefficients
- SFG microscope – provides spectral and spatial surface information with micrometers resolution
- Single or double wavelength VIS beam: 532 nm and/or 1064 nm
- One or two detection channels: main signal and reference
- Second harmonic generation surface spectroscopy option
- High resolution option – down to 2 cm-1
- Motorized VIS and IR beams alignment system
Double resonance SFG spectrometer
Both IR and VIS wavelengths are tunable in Double resonance SFG spectrometer model. This two-dimensional spectroscopy is more selective than single resonant SFG and applicable even to media with strong fluorescence. Double resonant SFG allows investigation of vibrational mode coupling to electron states at a surface.
Double resonance enables the use of another wavelength for VIS beam if the sample has strong absorption at 532 nm and 1064 nm. Two outputs PL2230 laser is used for this spectrometer.
SFG-VS spectroscopy combined with micrometers spatial resolution provides unique ability to investigate spatial and chemical variations across the surface as a function of time. An example of such application is chemical imaging of corrosion. SFG microscopy reveals presence of highly-coordinated complexes of molecules at particular stage of this process.
SFG spectrometer offered by Ekspla uses far-field image formation technique. Illuminated area on the sample surface is substantially bigger than in regular SFG spectrometer.
Using blazed grating and unique design optical system, image of surface plane is translated to matrix of ICCD camera. This way we can record distribution of SF signal at particular wavelength. For complete spectral and spatial information it is necessary to record multiple surface pictures at different wavelength. Integrated software package provides ability to visualize measured data making various cross sections: position-, wavelength- or time-dependent.
Courtesy of University of Houston
Phase-sensitive SFG spectrometer
Phase sensitive measurements with spectral resolution up to 6 cm⁻¹ (2 cm⁻¹).
In conventional SFG-VS intensity of SF signal is measured. It is proportional to the square of second order nonlinear susceptibility ISF ~ | χ(2) |2. However, χ(2) is complex, and for complete information, we need to know both the amplitude and the phase. This will allow us to determine the absolute direction in which the bonds are pointing and characterize their tilt angle with respect to the surface. Measurement of the phase of an optical wave requires an interference scheme. Mixing the wave of interest with a reference wave of known phase generates an interference pattern, from which the phase of the wave can be deduced.
In practice Phase-sensitive SFG experimental setup includes two samples generating SF signal simultaneously. One sample (usually called local oscillator) has well known and flat spectral response. Second one is investigated sample. The excitation beams are directed to first sample, where SFG beam is generated. Later all three beams are retranslated to the second sample, where another SFG beam is generated. Due to electromagnetic waves coherence both SFG beam are interfering. Setup contains the phase modular located on the SFG beam path between samples. We are able to change the phase of SFG beam by rotating it. This way we are recording two-dimensional interfererogram with wavelength and phase shift on x and y axis. Using fitting algorithms we are able to calculate the amplitude and phase of SF signal.
Phase sensitive SFG + Classic SFG Spectrometer in one unit
Interference measurements of SFG signals from reference sample and the investigated sample for Phase-sensitive configuration.
Switchable setup. Phase sensitiv / “Classic” (“Advanced”) ; Top/ Bottom configuration. Switch: VIS beam manual. IR mirrors motorised, BaF₂ lens manual. Path length to the sample is same in all configuration. Motorised polarisation control. VIS beam 532 nm. IR 2.3 – up to 10 (16) µm.
- Spectrometer has “classic” and “Phase-sensitive” properties
- Easy switching between setups
- Adjustable spot size for classic configuration
Tunable beam size for IR beam. Beams are Focused with Lens. (BaF₂ lens for IR beam). “Classic” configuration. IR 2.3-10 µm (up to 16 µm).
Fixed beams sizes on the sample. VIS and IR beams. Beams are Focused with Parabolic mirrors. Interference configuration for Phase measurement. IR 2.3-10 µm.
Narrowband SFG system <2 cm-1
Spectral resolution in of narrowband SFG is determined by light source – OPA. Monochromator is used only as filter.
Light source for IR: PG511. Line width of mid-IR < 2 cm-1.
Synchronously pumped optical parametric generator with OPO with long focal length resonator.
Components & Optional Accessories
Picosecond mode-locked Nd:YAG laser
The heart of the spectrometer is solid-state picosecond laser. Its reliability is critical to perfect spectrometer operation and relevance of measured data. Two standard models of high energy lasers are dedicated for SFG spectrometers. Model PL2230 is fully diode pumped, which means that master oscillator and all following amplification stages are diode pumped. It features great long term parameters stability and minimal maintenance requirements.
This model provides up to 40 mJ per pulse output energy, which in most cases is enough for pumping OPG and VIS channel of SFG spectrometer. Model PL2230 is available for double resonance SFG. This model usually is used for pumping of two independent OPG’s. Such configuration is used in double resonance SFG version.
Multichannel beams delivery unit
Fundamental laser radiation needs to be split into several channels and converted to different wavelenghts. Tunable IR radiation is generated in picosecond optical parametric generator (OPG). Large portion of laser output is converted into second or third harmonics and used for OPG pumping. Residual beam is spatially filtered, delayed and directed into SFG spectrometer as VIS channel. Usually it is converted into second harmonic (532 nm), but in some cases can be used also at fundamental wavelength (1064 nm) or tunable in visible range, when second OPG is used.
Multichannel beams delivery unit SFGHX00 series provides all these features. Additionally it contains automatized VIS channel input energy monitoring and control. The VIS channel wavelength (if double wavelenght option is included) is changed manually. Setup also includes all needed separators and filters to block residual radiation and prevent it from reaching a sample.
Picosecond optical parametric generator
PG501 series picosecond optical parametric generator (OPG) feature high pulse energy and narrow linewidth. It is used for generation of tunable wavelength in broad spectral range. In SFG spectrometer it provides middle infrared radiation for IR channel. DFG stage extends tuning range to mid IR, which corresponds to molecular vibrational fingerprints. Depending of OPG model, DFG output can cover spectral range 2.3 – 10 µm or 2.3 – 16 µm. All residual wavelengths are carefully filtered preventing residual radiation from reaching a sample.
Visible laser pointer is installed inside each unit and aligned in-line with IR beam. It helps to manage invisible mid IR radiation and direct it through multiple optical elements into a sample. Some SFG-VS studies require better than 6 cm-1 spectral resolution. In such cases Ekspla offers unique design PG511 series OPG. In this system seed is generated in synchronously pumped optical parametric oscillator (SPOPO), which is temporally synchronized with laser regenerative amplifier. In this configuration radiation spectral width is narrowed down to 2 cm-1 in mid IR range.
However, in some experiments one layer of the sample can be transparent only for VIS beam, but not for IR beam and vice versa. In such case experimental setup requires different geometries. This problem can be solved, if we can access interface from different sides, for example directing VIS beam from the top and IR beam from the bottom. Ekspla offers several standard geometries: top side, bottom side, top-bottom side and total internal reflection. All of them can be implemented in single spectroscopy unit and easy interchangeable. The special design of SFG spectrometer provides possibility to change angles of interaction. This feature together with different polarization combinations helps better understand molecular dipoles orientation.
In our spectrometer we use large aperture parabolic mirror. The sample is places in focal point of parabolic mirror. Such solution makes optical system extremely simple in operation, because it guarantee the same beams position on the sample surface and perfect overlap, when incidence angle is changed.
Sample surface and beams overlap can be monitored using camera installed above sample area. This utility is integrated into every SFG spectrometer. On a special request sample visualization system can be combined with motorized beams adjustment. This allows to align SFG spectrometer from PC, even being physically far from it. It essentially solves safety issues and opens new possibilities for multiple long time experiments without accessing spectroscopy box.
SFG Spectrometer Accessories
- Six axis sample holder
- Sealed temperature controlled sample chamber
- Langmuir trough
- Motorisation of polarisation central of VIS beam, polarisation analyser
|Version 1)||SFG Classic||SFG Advanced||SFG Double Resonance||SFG Phase Sensitive|
|Spectral range||1000 – 4300 cm-1||625 – 4300 cm-1||1000 – 4300 cm-1||1000 – 4300 cm-1|
|Spectral resolution|| <6 cm-1|
(optional <2 cm-1)
|<10 cm-1|| <6 cm-1
(optional <2 cm-1)
|Spectra acquisition method||Scanning|
|Sample illumination geometry||Top side, reflection (optional: bottom side, top-bottom side, total internal reflection)|
|Incidence beams geometry||Co-propagating, non-colinear (optional: colinear)||non-colinear|
|Incidence angles||Fixed, VIS ~60 deg, IR ~55 deg (optional: tunable)||not tunable|
|VIS beam wavelength||532 nm|
(optional: 1064 nm)
|Tunable 420 – 680 nm|
(optional: 210 – 680 nm)
|Polarization (VIS, IR, SFG)||Linear, selectable “s” or “p”, purity > 1:100|
|Beam spot on the sample||Selectable, ~150 – 600 µm||Fixed|
|Sensitivity||Air-water spectra||Solid sample|
|Pump lasers 2)|
|Model||PL2230||PL2230 for Double resonance||PL2230|
|Pulse energy stability||<0.5 %|
|Pulse duration||28±3 ps|
|Pulse duration stability||±1.0 ps|
|Pulse repetition rate||50 Hz|
|Optical parametric generators|
|IR source with standard linewidth (<6 cm-1)||PG501-DFG1P||PG501-DFG2||PG501-DFG1P|
|IR source with narrow linewidth (<2 cm-1)||PG511-DFG||PG511-DFG2||–||PG511-DFG|
|UV-VIS source for Double resonance SFG||–||–||PG401 (optional: PG401-SH)||–|
|For standard specifications please check the brochure of particular model|
|Physical dimensions (footprint)|
|Standard||2400 × 1000 mm||3000 × 1500 mm||2600 × 1200 mm|
|Extended (with special options or large accessories)||2700 × 1200 mm||3000 × 1500 mm||2700 × 1200 mm|
- Due to continuous product improvements, specifications are subject to changes without advance notice.
- Laser is optimised for pumping parametrical generator, maximum output energy may be different than specified for stand alone application.
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.
Probing the Orientation and Conformation of α-Helix and β-Strand Model Peptides on Self-Assembled Monolayers Using Sum Frequency Generation and NEXAFS Spectroscopy
Related applications: SFG
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.
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.
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.
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.