Sum frequency spectroscopy

The sum frequency spectroscopy (SFS, English sum frequency spectroscopy , also sum frequency generation spectroscopy , SFGS, vibrational sum frequency spectroscopy , VSFS, or sum frequency vibrational spectroscopy , SFVS) is a molecular spectroscopic method that is based on a non-linear optical effect of the second order in the sum frequency generation ( english frequency generation sum SFG) based. It is closely related to Raman and infrared spectroscopy .

The method for recording entire spectra was described in 1986 by XD Zhu, a member of Yuen-Ron Shen's research group . At that time, the group was looking for a selective method to investigate the surfaces of centrosymmetric materials such as liquids, gases and optically isotropic solids in the infrared spectral range and had already reported on a similar measuring system a few years earlier. At that time, however, there was still no tunable laser available for recording spectra, so that initial investigations could only be carried out at a frequency determined by the two lasers.

description

To observe the phenomena of non-linear optical effects, a very high radiation intensity is required, as can be achieved, for example, by lasers with a high peak power. In sum frequency spectroscopy, similar to non-linear Raman spectroscopy ( coherent anti-Stokes Raman spectroscopy , CARS), the sample is irradiated with a laser beam of a fixed frequency (mostly in visible light or near infrared). In addition, a second, pulsed laser, the frequency of which can be set (tuned) in the infrared range, is also beamed onto the sample. Both laser pulses penetrate the sample and are coordinated in such a way that they overlap / add spatially and temporally on the surface to be examined. A sum frequency signal arises with the frequency : ${\ displaystyle \ omega _ {\ text {SF}}}$

{\ displaystyle \ omega _ {\ text {SF}} = \ omega _ {\ text {IR}} + \ omega _ {\ text {VIS}}}

If the frequency of the infrared laser corresponds to an excitation of the interface, there is a resonant increase in the 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 (engl. Second harmonic generation , SHG), is utilized, that the susceptibility of the second order χ ^{(2) is} not zero. This effect occurs at high field strengths near symmetry breaks (of the material), for example at an interface. This spatial limitation of the signal explains the extraordinary sensitivity of the SFS at the interface of two inversely symmetrical materials. ${\ displaystyle \ omega _ {\ text {IR}}}$

The spectrum obtained is essentially a superposition of the non-resonant background signal with the oscillation resonances at the interface of the molecules with the air or the solvent.

application

The fact that the generation of sum or difference frequencies is only possible if there is a symmetry break makes SFS a surface-sensitive method. The method is sensitive to monolayers on the surface and, in contrast to other surface analysis methods, can be used without vacuum conditions. In this way, for example, heterogeneous catalytic processes on surfaces can be investigated for the first time under conditions that correspond to those of technical use.

Sum frequency spectroscopy is an experimentally complex method, but compared to alternative methods such as electron energy loss spectroscopy or infrared spectroscopy , it has the advantage of a specific interface sensitivity. One of the reasons for this is the fact that the resulting SFG signal is usually in a spectral range in which there are powerful photomultipliers with sensitivity in the range of single photons. The SFS is therefore used, among other things, for material analysis; it is suitable, for example, for the characterization of surface coverings and adsorbates , both under ultra-high vacuum and under pressure.

literature

David A. Beattie, Colin D. Bain: Sum-frequency Spectroscopy . In: John M. Chalmers, Peter R. Griffiths (Eds.): Handbook of Vibrational Spectroscopy . John Wiley & Sons, 2001, ISBN 0-471-98847-2 .
Ulrich Bauer: Sum frequency spectroscopy in the medium infrared range at in situ interfaces and surfaces under UHV conditions . Munich 2005 ( d-nb.info [PDF] Dissertation, Munich, Technical University of Munich, 2005).

Web links

Sum frequency spectroscopy (SFG) . University of Heidelberg, accessed on September 16, 2010.

Individual evidence

↑ XD Zhu, Hajo Suhr, YR Shen: Surface vibrational spectroscopy by infrared-visible sum frequency generation . In: Physical Review B . tape 35 , no. 6 , 1987, pp. 3047-3050 , doi : 10.1103 / PhysRevB.35.3047 .
↑ JH Hunt, P. Guyot-Sionnest, YR Shen: Observation of CH stretch vibrations of monolayers of molecules optical sum-frequency generation . In: Chemical Physics Letters . tape 133 , 1987, pp. 189-192 , doi : 10.1016 / 0009-2614 (87) 87049-5 .
↑ CK Chen, TF Heinz, D. Ricard, YR Shen: Detection of Molecular Monolayers by Optical Second-Harmonic Generation . In: Physical Review Letters . tape 46 , no. 15 , March 13, 1981, pp. 1010 , doi : 10.1103 / PhysRevLett.46.1010 .

[1] XD Zhu, Hajo Suhr, YR Shen: Surface vibrational spectroscopy by infrared-visible sum frequency generation . In: Physical Review B . tape 35 , no. 6 , 1987, pp. 3047-3050 , doi : 10.1103 / PhysRevB.35.3047 .

[2] JH Hunt, P. Guyot-Sionnest, YR Shen: Observation of CH stretch vibrations of monolayers of molecules optical sum-frequency generation . In: Chemical Physics Letters . tape 133 , 1987, pp. 189-192 , doi : 10.1016 / 0009-2614 (87) 87049-5 .

[3] CK Chen, TF Heinz, D. Ricard, YR Shen: Detection of Molecular Monolayers by Optical Second-Harmonic Generation . In: Physical Review Letters . tape 46 , no. 15 , March 13, 1981, pp. 1010 , doi : 10.1103 / PhysRevLett.46.1010 .