Sum-frequency-generation

Optical sum frequency generation (SFG) is a speciality in vibrational spectroscopy that probes adsorbates at interfaces in real time and in situ. The complicated set-up 

The strong point of SFG is that the process is forbidden in centrosymmetric media (i.e. media with an inversion center). Therefore it occurs only at interfaces, where the sum-frequency response forms within a region of typically one nanometer thickness. Hence, neither the bulk of the catalyst nor the molecules in the gas phase contribute to the SFG spectrum. 

The intensity of SHG is low but readily detectable for a photomultiplier. Note that although the sensitivity increases quadratically with laser intensity, the possibility of damage requires that the energy per unit area remains below a certain threshold value 

Some examples are given to demonstrate experimental realizations of the sum-frequency mixing technique [565-576]. 

A novel device for efficiently generating intense radiation at wavelengths around 202 nm is shown in Fig. 6.16. A laser diode-pumped Nd YV04 laser is frequency doubled and delivers intense radiation at A = 532 nm, which is again frequency doubled to A = 266 nm in a BBO crystal inside a ring resonator. The output from this resonator is superimposed in a third enhancement cavity with the output from a diode laser at A = 850 nm to generate radiation at A = 202 nm by sum-frequency mixing. This 202-nm radiation is polarized perpendicularly to that at the two other waves and can be therefore efficiently coupled out of the cavity by a Brewster plate [576]. 

The lower-wavelength limit for nonlinear processes in crystals (SHG or sum-frequency mixing) is generally given by the absorption (transmission cutoff) of the crystals. 

For shorter wavelengths sumfrequency mixing or higher-harmonic generation in homogeneous mixtures of rare gases and metal vapors can be achieved. Because 

Because of the lower densities of gases compared with solid crystals, the efficiency I co)/I a)) is much smaller than in crystals. However, there is no short-wavelength limit as in crystals, and the spectral range accessible by optical mixing can be extended far into the VUV range [579]. 

An obvious disadvantage of using second harmonic generation for the spectroscopy at adsorbate-covered surfaces is the missing molecule specificity. This problem can be overcome if one mixes a fixed-frequency laser pulse with a pulse of variable frequency ( sum frequency generation , SFG). Since also vanishes in dipole approximation in media with inversion symmetry, this method has high surface sensitivity, too, and shows resonance enhancement. 

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Recently, in situ studies of catalytic surface chemical reactions at high pressures have been undertaken [46, 47]. These studies employed sum frequency generation (SFG) and STM in order to probe the surfaces as the reactions are occurring under conditions similar to those employed for industrial catalysis (SFG is a laser-based teclmique that is described in section A 1.7.5.5 and section BT22). These studies have shown that the highly stable adsorbate sites that are probed under vacuum conditions are not necessarily tlie same sites that are active in high-pressure catalysis. Instead, less stable sites that are only occupied at high pressures are often responsible for catalysis. Because the active... 

Shen Y R 1998 Sum frequency generation for vibrational spectroscopy applications to water interfaces and films of water and ice Solid State Commun. 108 399... 

These effects correspond, respectively, to the processes of sum-frequency generation (SFG), SFIG and optical rectification. 

Shen Y R 1989 Surface-properties probed by second-harmonic and sum-frequency generation Nature 337 519-25... 

Petukhov A V 1995 Sum-frequency generation on isotropic surfaces general phenomenology and microscopic theory for ]ellium surfaces Phys. Rev. B 52 16 901 -11... 

Zhu X D, Suhr H and Shen Y R 1987 Surface vibrational spectroscopy by infrared-visible sum frequency generation Phys. Rev. B 35 3047-59... 

Lin S H and Villaeys A A 1994 Theoretical description of steady-state sum-frequency generation in molecular absorbates Phys. Rev. A 50 5134-44... 

Richter L T, Petralli-Mallow T P and Stephenson J C 1998 Vibrationally resolved sum-frequency generation with broad-bandwidth infrared pulses Opt Lett. 23 1594-6... 

Figure Bl.22.8. Sum-frequency generation (SFG) spectra in the C N stretching region from the air/aqueous acetonitrile interfaces of two solutions with different concentrations. The solid curve is the IR transmission spectrum of neat bulk CH CN, provided here for reference. The polar acetonitrile molecules adopt a specific orientation in the air/water interface with a tilt angle that changes with changing concentration, from 40° from the surface nonnal in dilute solutions (molar fractions less than 0.07) to 70° at higher concentrations. This change is manifested here by the shift in the C N stretching frequency seen by SFG [ ]. SFG is one of the very few teclnhques capable of probing liquid/gas, liquid/liquid, and even liquid/solid interfaces.

Unlike linear optical effects such as absorption, reflection, and scattering, second order non-linear optical effects are inherently specific for surfaces and interfaces. These effects, namely second harmonic generation (SHG) and sum frequency generation (SFG), are dipole-forbidden in the bulk of centrosymmetric media. In the investigation of isotropic phases such as liquids, gases, and amorphous solids, in particular, signals arise exclusively from the surface or interface region, where the symmetry is disrupted. Non-linear optics are applicable in-situ without the need for a vacuum, and the time response is rapid. 

FIG. 27 Sum frequency generation spectra in ssp polarization of a deuterated water (D2O) film on mica as a function of the relative humidity (RH) at room temperature (296 K). Above 40% RH, the spectrum is very similar to that of ice. The free OD stretching mode can be seen above 91% RH. The bottom trace corresponds to the spectrum of the bulk waver/vapor interface. (From Ref. 72.)... 

Spectroscopic evidence that ethylene preferably hydrogenates directly via the jt-bonded adsorbate and not via the more stable di-<7 bonded state has been provided by Somorjai and coworkers by in situ sum-frequency generation measurements [P.S. Cremer, X. Su, Y.R. Shen and G.A. Somorjai,y. Am. Chem. Soc. 118 (1996) 2942]. 

The polarizability expresses the capacity of a system to be deformed under the action of electric field it is the first-order response. The hyperpolarizabilities govern the non linear processes which appear with the strong fields. These properties of materials perturb the propagation of the light crossing them thus some new phenomenons (like second harmonic and sum frequency generation) appear, which present a growing interest in instrumentation with the lasers development. The necessity of prediction of these observables requires our attention. 

Vidal, F. and Tadjeddine, A. (2005) Sum-frequency generation spectroscopy of interfaces. Rep. Prog. Phys., 68, 1095-1127. 

Buck, M. and Himmelhaus, M. (2001) Vibrational spectroscopy of interfaces by infrared-visible sum frequency generation. J. Vac. Sci. Technol. A, 19, 2717-2736. 

Ye, S., Nihonyanagi, S. and Uosaki, K. (2001) Sum frequency generation (SFG) study of the pH-dependent water structure on a fused quartz surface modified by an octadecyltrichlorosilane (OTS) monolayer. Phys. Chem. Chem. Phys., 3, 3463-3469. 

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