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Spectroscopic Technique

Sum Frequency Generation
This nonlinear spectroscopic technique is used to characterize interfacial molecules. Conceptually,
it is very simple. Two pulsed laser beams, one in the visible, w_VIS, and the other in the infrared, w_IR, are overlapped on a sample and light is emitted at the sum of these two incident frequencies, where w_SUM = w_VIS+w_IR.  In other words, two photons overlap temporally and spatially to produce one photon at their sum frequency.

The intensity of the light w_SUM changes if the IR laser is in resonance with a vibrational mode of a molecule at the surface being studied. Thus, scanning the frequencies of the IR laser yields a vibrational spectrum. Frequency-tunable infrared light is produced (by our optical parametric oscillator/amplifier.) The strength of the resonant sum frequency depends on the surface density of the molecule of interest, the IR transition moment, and the Raman transition strength. This last factor restricts the technique to the study of vibrational modes that are both infrared and Raman active. In other words, the electric dipole approximation forbids second-order nonlinear optical processes in mediums with inversion symmetry, but at an interface the symmetry is broken and these processes are allowed.

SFG is unique because the signal is not dominated by the overwhelmingly large number of bulk molecules. Thus, SFG is versatile because it affords better understanding of the static and dynamic features of molecular/atomic orientations as well as the nonlinear optical effects occurring at surfaces.

Thus, SFG provides molecular identity by producing a vibrational spectrum of interfacial molecules as the infrared light is scanned, shown below in Fig. 1.  For example, the free OH peak of water appears at 3710 cm^-1. An OH group is termed "free" when it is protrudes from the liquid surface into the vapor phase, free of hydrogen bonding.



 Figure 1. Normalized SFG spectrum of the free OH peak of interfacial water molecules for ssp polarization. Note only the infrared light ca. 3710 cm-1 is in resonance.

 

SFG is an ideal technique for probing surface molecules because it is a surface specific,c⁽²⁾ process. c⁽²⁾processes are forbidden in isotropic media but allowed at the surface where inversion symmetry is broken. Resonant enhancement only occurs in SFG if a specific mode of vibration is both infrared and Raman active. SFG yields information about bonding via these vibrational resonances.Thus, SFG determines molecular orientation by analyzing the signal intensity for different input and output polarization combinations.

The polarization of the input visible and infrared beams, and the direction of the IR and Raman transition moments of the surface molecules, determines the SFG intensity observed. The ssp polarization combination probes surface vibrational modes with a projection dipole moment perpendicular to the interface. The sps and pss polarization combinations access modes that have transition dipoles with components parallel to the surface. SFG intensity with ppp polarization is dependent on all of the tensor elements, such that vibrational modes with components both perpendicular and parallel to the surface plane are present in the SFG spectra.

To generate SFG light, crucial components of the experimental setup include visible and tunable infrared sources of high intensity. The primary light source is a Spectra-Physics GCR 150 Nd:YAG laser. This generates 700 mJ/pulse of light at a fundamental frequency of 1064-nm light with a pulse width of about 9-nsec and a repetition rate of 10-Hz. The YAG pumps a LaserVision Optical Parametric Oscillator/Optical Parametric Amplifier (OPO/OPA) that generates the visible and infrared light, as descibed in that section.

The 532-nm and infrared beams are directed around the laser table. Halfwave plates and polarizers are strategically placed to control power and polarization of the input and SF beams. The detection procedure of the SFG signal includes focusing it onto an entrance slit of a monochromator after filtering, and detecting it with a photomultiplier tube. Before computer processing, the signal is amplified and sent to a gated boxcar/averager.