Köhler theory

Sum frequency generation spectroscopy (SFG) is a technique used to analyze surfaces and interfaces. This nonlinear laser spectroscopy method was developed in 1987 and rapidly applied to deduce the composition, orientation distributions, and some structural information of molecules at gas–solid, gas–liquid and liquid–solid interfaces. In a typical SFG setup, two laser beams mix at a surface and generate an output beam with a frequency equal to the sum of the two input frequencies. SFG has advantages in its ability to be monolayer surface sensitive, ability to be performed in situ (for example aqueous surfaces and in gases), and not causing much damage to the sample surface. SFG is comparable to second harmonic generation (SFG is a more general form) and Infrared and Raman spectroscopy.

Theory

IR-visible sum frequency generation spectroscopy uses two laser beams that overlap at a surface of a material or the interface between two materials. An output beam is generated at a frequency of the sum of the two input beams. The two input beams have to be able to access the surface, and the output beam needs to be able to leave the surface to be picked up by a detector. One of the beams is a visible wavelength laser held at a constant frequency and the other is a tunable infrared laser. By tuning the IR laser, the system can scan over resonances and obtain the vibrational spectrum of the interfacial region.

As a nonlinear optical process, the polarization which generates the output depends on the electric fields of the two input beams.

$P=\epsilon _{0}(\chi ^{(1)}:E+\chi ^{(2)}:EE+\chi ^{(3)}:EEE+\dots )$

As a second-order nonlinear process, SFG is dependent on the 2nd order susceptibility χ⁽²⁾. The fact that the 2nd order susceptibility, a third rank tensor, becomes zero in centrosymmetric media, limits what samples are accessible for SFG. Centrosymmetric media include the bulk of gases, liquids, and most solids under the assumption of the electric-dipole approximation, which neglects the signal generated by multipoles and magnetic moments. At an interface between two different materials or two centrosymmetric media, the inversion symmetry is broken and an SFG signal can be generated. This suggests that the resulting spectra represent a thin layer of molecules. A signal is found when there is a net polar orientation.

SFG intensity

The output beam is collected by a detector and its intensity is measured. The intensity of the beam is given by

$I(\omega )\propto |\chi ^{(2)}|^{2}I_{1}(\omega _{1})I_{2}(\omega _{2})$

Here, intensity is directly proportional to the susceptibility squared and the product of the intensities of the incoming beams. The IR frequency is given as ω₂ and the visible frequency is given as ω₁. The constant of proportionality varies across literature, many of them including the product of the square of the output frequency, ω² and the squared secant of the reflection angle, sec²β. Other factors include index of refractions for the three beams.

The second order susceptibility has two contributions

$\chi {=}\chi _{nr}+\chi _{r}$

where χ_nr is the non-resonating contribution and χ_r is the resonating contribution. The non-resonating contribution is assumed to be from electronic responses. Although this contribution has often been considered to be constant over the spectrum, because it is generated simultaneously with the resonant response, the two responses must compete for intensity. This competition shapes the nonresonant contribution in the presence of resonant features by resonant attenuation. Because it is not currently known how to adequately correct for nonresonant interferences, it is very important to experimentally isolate the resonant contributions from any nonresonant interference, often done using the technique of nonresonant suppression.

The resonating contribution is from the vibrational modes and shows changes in resonance. It can be expressed as a sum of a series of Lorentz oscillators

$\sum _{q}{\frac {A_{q}}{\omega _{2}-\omega _{q}+i\Gamma _{q}}}$

where A is the strength or amplitude, ω is the frequency or energy, Γ is the damping or linewidth coefficient, and each q is a resonance mode. The amplitude is a product of μ, the induced dipole moment, and α, the polarizability. Together, this indicates that the transition must be both IR and Raman active.

The above equations can be combined to form

$\chi =\chi _{nr}e^{i\phi }+\sum _{q}{\frac {A_{q}}{\omega _{2}-\omega _{q}+i\Gamma _{q}}}$

which used to model the SFG output over a range of wavenumbers. When the SFG system scans over a vibrational mode of the surface molecule, the output intensity is resonantly enhanced. In a graphical analysis of the output intensity versus wave number, this is represented by peaks. Depending on the system, inhomogeneous broadening and interference between peaks may occur. The Lorentz profile can be convoluted with a Gaussian intensity distribution to better fit the intensity distribution.

Orientation information

From the second order susceptibility, it is possible to ascertain information about the orientation of molecules at the surface. χ⁽²⁾ describes how the molecules at the interface respond to the input beam. A change in the net orientation of the polar molecules results in a change of sign of χ⁽²⁾. As a rank 3 tensor, the individual elements provide information about the orientation. For a surface that has azimuthal symmetry, only four of the twenty seven tensor elements are nonzero:

χ_zzz, χ_xxz = χ_yyz, χ_xzx = χ_yzy, and χ_zxx = χ_zyy.

The tensor elements can be determined by using two different polarizers, one for the electric field vector perpendicular to the plane of incidence, labeled S, and one for the electric field vector parallel to the plane of incidence, labeled P. Four combinations are sufficient: PPP, SSP, SPS, PSS, with the letters listed in decreasing frequency, so the first is for the sum frequency, the second is for the visible beam, and the last is for the infrared beam. The four combinations give rise to four different intensities:

$I_{PPP}=|f'_{z}f_{z}f_{z}\chi _{zzz}^{(2)}+f'_{z}f_{i}f_{i}\chi _{zii}^{(2)}+f'_{i}f_{z}f_{i}\chi _{zii}^{(2)}+f'_{i}f_{i}f_{z}\chi _{iiz}^{(2)}|^{2}$

$I_{SSP}=|f'_{i}f_{i}f_{z}\chi _{iiz}^{(2)}|^{2}$

$I_{SPS}=|f'_{i}f_{z}f_{i}\chi _{zii}^{(2)}|^{2}$

$I_{PSS}=|f'_{z}f_{i}f_{i}\chi _{zii}^{(2)}|^{2}$

where indice i is of the interfacial xy-plane, and f and f’ are the linear and nonlinear Fresnel factors.

By taking the tensor elements and applying the correct transformations, the orientation of the molecules on the surface can be found.

Experimental setup

Since SFG is a higher order function, one of the main concerns in the experimental setup is being able to generate a signal strong enough to detect, with discernible peaks and narrow bandwidths. Pico-second and femto-second pulse width lasers are used due to being pulsed lasers with high peak fields. Nd:YAG lasers are commonly used. However, the bandwidth is increased with shorter pulses, forming a tradeoff for desired properties.

Another limitation is the tunable range of the IR laser. This has been augmented by optical parametric generation (OPG), optical parametric oscillation (OPO), and optical parametric amplification (OPA) systems.

Signal strength can be improved by using special geometries, such as a total internal reflection setup which uses a prism to change the angles so they are close to the critical angles, allowing the SFG signal to be generated at its critical angle, enhancing the signal.

Common detector setups utilize a monochromator and a photomultiplier for filtering and detecting.

References

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