SFG Spectrometer

The SFG Spectrometer is an ideal laser spectroscopy tool for in-situ investigation of surfaces and interfaces. The system operates from 4300 to 625 cm-1 and provides < 6 cm-1 spectral resolution. The heart of the system is a picosecond Nd:YAG laser generating 25 ps pulses that pump an OPG/DFG delivering up to 300 µJ per pulse in the IR range

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Features

  • Intrinsically surface specific
  • Selective to adsorbed species
  • Sensitive to submonolayer of molecules
  • Applicable to all interfaces accessible to light
  • Nondestructive
  • Capable of high spectral and spatial resolution

Applications

  • 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

Principle of SFG spectroscopy

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.

 

Model 1) SFG Classic SFG Advanced SFG Double resonance
Spectral range 1000- 4300 cm-1 625- 4300 cm-1 1000- 4300 cm-1
Spectral resolution <6 cm-1(optional <2 cm-1) <6 cm-1(optional <2 cm-1) <10 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)
Incidence angles Fixed, VIS ~60 deg, IR ~55 deg (optional: tunable)
VIS beam wavelength 532 nm (optional: 1064 nm) 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
Sensitivity Air-water spectra
Pump lasers PL2231 PL2251B
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) PG551-DFG - -
UV-VIS source for Double resonance SFG - - PG401 (optional: PG401-SH)
Monochromators
Model MS200 2xMS350
Type Czerny-Turner with single grating turret (optional: four grating turret)
Focal length 200 mm 2x350 mm
Slits 0-2.0 mm, manual
Stray light rejection 10-5 10-5 10-10

1) Due to continuous product improvements, specifications are subject to changes without advance notice.

Modifications

  • 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

Options

  • 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

Optional Accessories

  • Six axis sample holder
  • Sealed temperature controlled sample chamber
  • Langmuir trough

Double resonance model

Both IR and VIS wavelengths are tunable in Double resonance SFG spectrometer model. This twodimensional 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. In Double resonance SFG spectrometer model second OPG is used to generate tunable VIS beam in UV and visible range.

SFG microscope

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.

Phase-sensitive SFG spectrometer

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.

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TOPAG Lasertechnik GmbH
Nieder-Ramstädter Str. 247
64285 Darmstadt, Germany
Phone: +49 6151 4259 78
Fax: +49 6151 4259 88
E-mail: info@topag.de