SFG spectrum

Figure Bl.5.15 SFG spectrum for the water/air interface at 40 °C using the ssp polarization combination (s-, s- and p-polarized sum-frequency signal, visible input and infrared input beams, respectively). The peaks correspond to OH stretching modes. (After [ ].)...

Figure 12.19 Potential-dependent SFG spectra from atop CO on a Pt(lll)/Ru electrode in 0.1 M H2SO4 at 1 mV/s in 0.1 M H2SO4 (see Fig. 12.18). The scan potential for each spectrum is shown on the right. Spectra from Ru-Pt(lll)-CO and Pt(lll)/Ru-CO are shown, including the CO phase transition at > 0.17 V (vs. Ag/AgCl). The inset shows a blow-up of the SFG spectrum from Pt(lll)/Ru-CO at 0.17 V.

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. 

For example, Fig. 6b shows the raw SFG spectrum of CO on Pt(l 11) in the presence of 200 mbar of CO at 300 K (open circles) together with the applied gas-phase compensation curve (solid line) and the corrected spectrum (black dots). To prevent the undesired attenuation of the IR beam by atmospheric CO2 and water before entering the SFG cell, all beam lines were encapsulated and purged with dry nitrogen. 

In contrast, when the H-precovered Pd(l 11) surface was exposed to CO at 150 K (Figs 29d and 31a), a different structure was observed, including bridge (1966 cm ) and on-top (2090 cm ) bonded CO, typical of approximately 0.65 ML of CO. The SFG spectrum is even identical to a corresponding measurement without preadsorbed hydrogen, suggesting that surface hydrogen was absent—that is, there... 

Fig. 39. SFG spectrum of CO on Rh(l 11) at 1 mbar and 300 K. The inset shows an AE spectrum of the clean surface, (b) SPG spectrum taken after increasing the temperature to 680 K. (c) SFG spectrum taken after increasing the pressure to 100 mbar at 680 K. The inset shows the AE spectrum after cooling of the sample to 300 K and evacuation adapted from Pery et al. (314). Copyright (2002) The Combustion Institute.

SFG using femtosecond lasers allows all the resonances within the broad (-200 cm" ) bandwidth of the IR pulse to be probed simultaneously, without scanning the infrared source. To obtain spectral resolution in an SFG spectrum, the IR polarization is upconverted with a narrowband (-8 cm" ) visible beam, which is prepared by pulse shaping the output of a femtosecond laser. Only the frequency components of the pulse that interact resonantly with the vibrational modes are enhanced, resulting in an SFG spectrum [28, 29]. Owing to the use of femtosecond... 

Figure 6. a) SFG spectrum of the Pt(lll) surface during ethylene hydrogenation with 100 Torr Hj, 35 Torr 615 Torr He at 295 K b) The vibrational spectrum of the same system after the evacuation of the reaction cell c) SFG spectrum under the same conditions as a), but on a surface which was pre-covered in UHV with 0.52 monolayers of ethylidyne. 

Here Uab is the Raman transition moment, fic is the infrared transition moment, g and V refer to ground and excited vibrational states, coir is the input infrared frequency, coq is the resonance frequency of the adsorbate, and T is a damping factor [8, 14—17]. Thus, the SFG intensity is related to the product of an (anti Stokes) Raman transition and an infrared transition. The SFG intensity is enhanced when the input infrared wavelength coincides with a vibrational mode of the adsorbate and the result of an SFG spectrum corresponds to the vibrational levels of the molecule. This situation is shown schematically in Fig. 5.1. From (5), non-zero SFG intensity will occur only for transitions that are both Raman and IR allowed. This situation occurs only for molecules lacking inversion symmetry [19]. 

Since the intensity of IR light for s-polarization is very small at the metal surface, only vibrations with a transition dipole component along the surface normal are detected. This necessitates using p-polarized light for the IR beam. The transition dipole vector of the molecule and electric field vector of the IR light must have a non-zero projection on each other, the consequence is that molecules with their transition dipole parallel to the metal are not seen in the SFG spectrum. This effect is referred to as the IR dipole surface selection rule [45,46]. 

The broadband configuration pioneered by Richter et al. [52] and van der Ham et al. [53] makes use of broadband IR pulses generated by OPAs pumped by Ti sapphire lasers. These typically collect the entire SFG spectrum over a ca. 200-300 cm window at one shot as the SFG is dispersed onto a CCD. The... 

Fig. 5.10 Vibrational (SFG) spectrum of CO/Pt(l 11) with CO in solution. Peak position and site assignment o = 0.0 mV, = 400 mV vs Pd/H2 reference electrode, ssp polarization [103].

Studies of the energy of the CO stretches on Pt electrodes are complemented by polarization measurements. Using Equations 5 and 6, the relative magnitude of polarizability tensor elements of CO can be determined by analysis of the polarization-dependent SFG spectrum shown in Fig. 5.11. SFG experiments determined that the polarizability of GO adsorbed at a three-fold site (1780 cm ) is larger than CO at a top site (2070 cm ) and that both of these polarizabihties are greater than that of the free CO molecule [67]. This conclusion is based on the appearance of the CO peaks in both ssp- and ppp-polarized spectra (Fig. 5.11). In the SFG results, the increased polarizability found for adsorbed CO arises from electron donation from the metal into the n orbital of CO. 

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