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Sum Frequency Generation (SFG) Vibrational Spectroscopy

ADVANTAGES

Sensitive and selective to the orientation of molecules in the surface layer
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

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 (ω_IR) laser beam is mixed with a visible VIS (ω_VIS) beam to produce an output at the sum frequency (ω_SFG = ω_IR+ ω_VIS). 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.

Vibrational sum frequency generation (SFG) spectroscopy holds several important advantages over traditional spectroscopy methods for the molecular level analysis of interfaces, including (i) surface sensitivity, (ii) vibrational specificity, and (iii) the possibility to extract detailed information on the ordering and orientation of molecular groups at the interface by analysis of polarization-dependent SFG spectra.

SFG signal generation diagram (a) and the molecular energy level diagram for the SFG process (b)

SFG spectra of monoolein surface, 1 cm<sup>-1</sup> scan step, 200 acquisitions per step.

SFG spectra of monoolein surface, 1 cm^-1scan step, 200 acquisitions per step.

Water-air interface spectra, 200 acquisitions per step. <br> <i>Courtesy of University of Michigan</i>

Water-air interface spectra, 200 acquisitions per step.
Courtesy of University of Michigan

Comparison of Narrowband and Broadband SFG Spectrometers

NARROWBAND PICOSECOND SCANNING SFG SPECTROMETER

In order to get SFG spectrum during measurement wavelength of narrowband mid-IR pulse is changed point-by-point throughout the range of interest. Narrowband SFG signal is recorded by the time-gated photomultiplier. Energy of each mid-IR, VIS and SFG pulse is measured. After the measurement, the SFG spectrum can be normalised according to IR and VIS energy. Spectral resolution is determined by the bandwidth of the mid-IR light source. The narrower mid-IR pulse bandwidth, the better the SFG spectral resolution. Separate vibrational modes are excited during the measurement.

BROADBAND FEMTOSECOND SFG SPECTROMETER

A broadband mid-IR pulse is mixed with a narrowband VIS pulse. The result is broadband SFG spectrum which is recorded using a monochromator and a sensitive CCD camera. The full spectrum is acquired simultaneously by integrating signal over time. Spectral resolution is determined by the bandwidth of the VIS pulse and on the monochromator-camera combination. The narrower the bandwidth of VIS pulse, the better the SFG spectral resolution.

COMPARISON OF DIFFERENT SFG SPECTROMETRES

Narrowband Picosecond Scanning Spectrometer	Broadband Femtosecond High Resolution Spectrometer
Narrowband mid-IR excitation, only one band is excited. Coupled states can be separated.	Simultaneous exsitation and recording of broad vibration spectrum with high resolution.
High mid-IR pulse energy. Less influence of IR absorbtion in the air.	High mid-IR intensity at low pulse energy – suitable for biological or other water containing samples.
No reference spectrum needed, IR energy measured at each spectral point.	Optically coupled IR and VIS channels. Reduced complexity and increased stability of the system.
System is more simple, lower ambient conditions requirements, easier to maintain.	Hight repetition rate up to 1 kHz.

Diagram of Narrowband picosecond scanning SFG spectrometer

Diagram of Broadband femtosecond SFG spectrometer

SFG spectra of ethanol with 30 sec. acquisition time

SFG spectra of glucose with 1 sec. acquisition time

SFG spectra of water-air interface

Features and Design

Femtosecond broadband SFG (BB SFG) spectrometer allows fast SFG spectra acquisition since most vibrational modes can be resolved without scanning. The advantage of the broadband SFG system is that intense femtosecond pulses allow efficient sum frequency generation at low pulse energies thus reducing the possibility of sample modification. It is especially important for aqueous and biological samples.

The system is based on a femtosecond industrial FemtoLux® series laser with 500 fs pulse duration, more than 1 mJ pulse energy at 1030 nm and a 1 kHz repetition rate.

The main part of the laser radiation is directed to a broadband mid-IR OPA module. Broad bandwidth ((150 – 450) cm^-1) mid-IR radiation can be continuously tuned in a spectral range from 2.5 up to 10 μm, providing from 0.5 to 12 μJ energy transform-limited pulses for the IR channel. The VIS channel realisation depends on the system configuration. In standard setup, a part of laser output radiation is frequency doubled (515 nm) ~20 µJ and then spectrally filtered to produce <8 cm^-1 bandwidth pulses. High resolution version consists of optically synchronised femtosecond and picosecond lasers. The combination of broadband mid-IR and narrowband VIS radiation allows to get the broadband sum frequency signal with exceptionally high spectral resolution close to 3 cm^-1.

Schematic layout of BB SFG spectrometer

Mid-IR parametrical amplifier characteristics. Energy and spectral bandwidth versus central wavelength

Monoolein SFG spectra demonstrated spectral bandwidth of 3 cm^-1

Specifications

Version ¹⁾	SFG FS	SFG FS High resolution
SYSTEM (general)
Spectral range	1000 – 4300 cm^-1
Spectral resolution	<8 cm^-1	<3 cm^-1
Spectra acquisition method	Broadband accumulative
Sample illumination geometry	Top side, reflection (optional: bottom side, top-bottom side, total internal reflection)
Incidence beams geometry	Co-prop agating, non-colinear (optional: colinear)
Incidence angles	Fixed, VIS ~60 °, IR ~55 ° (optional: tunable)
VIS beam wavelength	515 nm	532 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 ²⁾
Model	FemtoLux series
Pulse energy	Optimised to pump broadband OPA
Pulse duration	500 ± 50 fs
Pulse repetition rate	1000 Hz
OPTICAL PARAMETRIC GENERATORS
IR source with standard linewidth (<6 cm^-1)	MID-IR OPA
For standard specifications please check the brochure of particular model
PHYSICAL DIMENSIONS (footprint)
Standard	2000 × 1500 mm	2200 × 1500 mm

Due to continuous improvement, all specifications are subject to change without advance notice.
Laser is optimised for pumping parametrical generator, maximum output energy may be different than specified for stand alone application.