Sum frequency generation spectroscopy
The sum frequency spectroscopy (SFS, English sum frequency spectroscopy, and sum frequency generation spectroscopy, SPE, vibrational sum frequency spectroscopy, VSFS, or sum frequency vibrational spectroscopy, SFVS, ) is an molekülspektroskopisches procedure followed in on a non- linear optical second-order effect of the sum-frequency generation (English sum frequency generation, SFG ) is based. It is closely related to the Raman and infrared spectroscopy.
The method for recording all spectra was described in 1986 by XD Zhu, a staff member of the research group of Yuen - Ron Shen,. The group had been looking for a selective method for the study of surfaces of centrosymmetric materials such as liquids, gases and optically isotropic solids in the infrared spectral range and had some years before, reported a similar measurement system. At that time, however, was still not a tunable laser for recording spectra available, so that initial investigations could be carried out only at a rate specified by the two laser frequency.
Description
For the observation of the phenomena of non-linear optical effects, a very high radiation intensity is needed, how it can be achieved for example by laser with a high peak power. At the sum frequency spectroscopy, similar to the non-linear Raman spectroscopy (English coherent anti - Stokes Raman spectroscopy CARS), irradiating the sample with a laser beam of fixed frequency (usually in the visible or near infrared light ). Additionally, a second, pulsed laser is a frequency in the infrared range adjustable ( tunable ) is also irradiated to the sample. Both laser pulses penetrate the sample and be coordinated so that they overlap in time and space on the surface to be examined / add. The result is a sum frequency signal with the frequency:
Corresponds to the frequency of the infrared laser excitation of the interface, there is an elevation of the resonant signal. In this three-photon interaction, an optical second-order process (similar to the difference frequency generation ( engl. difference frequency generation, DFG) or the second harmonic generation (English second harmonic generation, SHG) is exploited that the second-order susceptibility χ (2 ) is not zero. this effect occurs at high field strengths in the vicinity of symmetry breaks ( the material), such as at an interface. through this spatial limitation of the signal, the extraordinary sensitivity of the SFS at the interface of two inversion symmetric materials explained.
A spectrum is obtained in essentially a superposition of the non-resonant background signal with the resonance vibration of the molecules at the interface of air or solvent.
Application
The fact that sum or difference frequency generation are only possible if a symmetry-breaking exists, the SFS makes a surface sensitive technique. The method is sensitive to monolayer on the surface and can be used in contrast to other surface analytical methods without vacuum conditions. Thereby can be examined under conditions as heterogeneous catalytic processes at the first surface, corresponding to those of the technical application.
The sum frequency spectroscopy is an experimental complex method that does compared to alternative methods such as electron energy loss spectroscopy and infrared spectroscopy, the advantage of a specific interface sensitivity. The reason for this is partly in the fact that the resulting SFG signal is usually in a spectral region where there are powerful photomultiplier with sensitivity in the range of single photons. The SFS is therefore used, among others, material analysis, it is suitable for example for the characterization of surface coverage and adsorbate, both under ultrahigh vacuum and under pressure.