FT RAIRS

Some of the techniques in Tab. 1.1 have angle-resolved variants, witli die prefix AR, e. g. ARUPS, or use Fourier transform mediods, with die prefix FT, e. g. FT-RAIRS. 

The extent of lattice oxygen incorporation into the decomposition products is linked to the surface redox properties of the oxide. Recent SSIMS and FT-RAIRS studies have provided initial evidence for the transient formation of oxygen vacancies on the surface of TiO2(110). Henderson has proposed that surface oxygen vacancies may explain the formation of trace amounts of formaldehyde from formic acid on Ti02(l 10) (figure 3) [41]. Both reactions (6) and (7) are proposed to occur below 500 K the water produced from formic acid exposure desorbed at 475 K, before the onset of formate decomposition to form CO. Formaldehyde and CO2 were produced in minor quantities relative to the production of CO [41]. Formaldehyde was formed from formic acid on reduced... 

Fig.7 FT-RAIRS spectrum of Rh2C04Cl2 on Ti02(l 10) [56], Measurements are carried out at (p=83 , and can be compared directly with the predictions of the model used for an isotropic film on an isotropic, non-absorbing substrate (Fig.6). The lines indicate the expectation values for the modes in the condensed phase.

Fig. 8 FT-RAIRS spectrum of a physisorbed layer of Mo(CO)6 on TiO2(110) [58], The net dipole coupling in the thin film shifts the singleton (gas phase) frequency coo either up or down in energy for normal and parallel components respectively.

Fig. 10. FT-RAIRS spectrum of rhodium dicarbonyl Rh(CO)2 on TiO2(H0) [56, 69, 70] following dissociative adsorption of Rh2C04Cl2 (a). The molecule is reformed after desorption of the CO at 450K following exposure to lOOL of CO (b). Reaction of Rh(CO)2 with hydrogen forms the monocarbonyl Rh(H)CO [71].

Since the parallel components of the dynamic dipole are active in RAIRS, it is possible to use the azimuthal dependence to obtain the orientation of the adsorbate at the surface. A similar technique has been applied to adsorbates on metals in HREELS measurements made off specular in order to observe parallel modes through impact or resonant scattering processes. This was first demonstrated for the Rh(CO)2 molecule on anisotropic TiO2(110) surface [72]. The results of this study also allow a test of the three layer model theory (Fig.5,6) as applied to S-polarised radiation. Fig. 11 shows the FT-RAIRS spectrum for 1/3 monolayer of Rh(CO)2 on Ti02(l 10) measured with P and S polarised radiation. 

Fig. 11. Azimuthal dependence of FT-RAIRS spectra for TiO2(110)-Rh(CO)2 [72], The azimuthal angle (j) is defined as 0° when the incident radiation is aligned in a plane parallel to the <110> direction. The Vsym(C-O) dynamic dipole is aligned normal to the surface and couples to Pn (transmission band), and Vasym(C-O) is aligned parallel to the surface in the <110> direction, and couples to Pt (absorption band). Two possible adsorption geometries consistent with the FT-RAIRS azimuthal dependence are shown for the gem-dicarbonyl.

Fig.15. FT-RAIRS spectra of CO adsorbed at 90K on alumina supported palladium particles, as a function of mean particle size [85] The two sets of spectra correspond to particles formed by depositing palladium at 90K or 300K. FT-RAIRS spectra are also shown for CO adsorbed at lOOK on silica supported Pd (pre-exposed to oxygen) following the annealing of the surface to various temperatures [86].

Fig. 16. FT-RAIRS of CO adsorbed at saturation coverage at 300K on Ti02(l 10) supported Pd particles [59, 77]. The bands associated with bridge bonded CO (B and B2) were used as indicators of particle morphology. Linear bands (L) were associated with edge or terrace sites. Transition bands were associated with small Pd particles, and absorption bands with larger Pd particles.

This contrasts with the case of FT-RAIRS on metal supported on single crystal oxide surfaces where the local dielectric response, and the associated electric fields in IR, depend on the particle size or film thickness. It should also be mentioned that on thin film oxide substrates, because the underlying metal will always screen the parallel fields, molecules adsorbed on particle facets which result in e.g. v(C-0) being orientated parallel to the macroscopic surface will be screened and not be observed in the RAIRS spectra. 

FT-RAIRS spectra of adsorbed CO on palladium supported on TiO2(110) has been combined with photoemission and LEED used to deduce particle morphology [59, 77]. A series of spectra have been selected (Fig. 16) from CO coverage dependent measurements made on Pd particles produced at various... 

Fig. 18. FT-RAIRS spectra of CO adsorbed at 300K on IML and 20ML films of palladium deposited by metal vapour deposition on TiO2(110) [56]. The switch from a local titania dielectric (transmission band) to that of the metal (absorption band) takes place at about lOML of palladium. The singleton frequency and the coverage dependent dipole shift are similar for both palladium layers indicating little perturbation of the CO adsorption behaviour on the palladium by the Ti02(l 10) substrate.

FT-RAIRS measurements of CO have also been used to identity facets of oxide supported Cu particles [78, 82]. The low sensitivity of RAIRS on single crystal ZnO(OOOl) prevented the observation of adsorbed CO or CO2, despite their observation in NEXAFS [78], although the local metallic dielectric allowed CO to be observed on the Cu particles. There appear to be no examples of HREELS being used to carry out vibrational spectroscopy of adsorbates on oxide supported metal particles. A HREELS study of Ag on MgO(lOO) films [95] was used only to characterise the Ag induced attenuation in the substrate Fuchs-Kliewer phonons, and the appearance of the metal/oxide interfacial plasmon at higher energies. HREELS has also been used to characterise the oxide/oxide interface between NiO and thin film MgO(lOO) [96]. Similar measurements of substrate phonon attenuation were made in HREELS studies on Pt films grown on ZnO(OOOl) [97]. 

FT RAIRS (Fourier transform reflection-absorption infrared spectroscopy)... 

Figure 4. (a) FT-RAIRS spectrum of PLL-g-PEG chemisorbed as a monolayer on a natural-oxide-covered titanium surface, (b) FT-RAIRS spectrum of PLL-g-PEG physisorbed as multilayers on a natural-oxide-cover titanium surface. 

FT-RAIRS is closely related to high-resolution electron energy loss spectroscopy (HREELS) (see Section 27,7.3), and they are often used together in the same system. The two techniques are complementary. since FT-RAIRS has high energy resolution ( 2cm ) and speed of acquisition, but... 

The main objectives of our experiments were to determine if the monolayer was still present after the evaporation process to determine if the SAM had undergone any modifications to establish if these changes are reversible to determine whether increased electrical defects in the SAM result from evaporation and to determine the nature of the perturbations induced by the copper coating. The SAMs were characterized both as deposited, and after the removal of the copper overlayer with nitric acid, using FT-RAIRS and electrochemistry. Control experiments were performed to confirm that the nitric acid solutions used for the stripping had no effect on either the organization or the coverage of the SAM. 

F. Bensebaa, Ch. Bakoyannis, T.H. Ellis, "Order and disorder in self-assembled monolayers probed by FT-RAIRS", Mikrochim. Acta [Suppl], 1997,14,621-623. 

Interferometry It is true to state that the renaissance of the RAIRS technique as a general use method in surface science laboratories came about through the use of Fourier-transform (FT) IR spectrometers. This advance was pioneered by Chesters et al. [41-43], who demonstrated that FT-RAIRS could obtain vibrational data on an adsorbed adlayer over a broad wavenumber regime of 4000—700 cm, with sensitivity to fractions of a ML of adsorbate (<0.01 ML of CO, <0.1 ML of hydrocarbon), and with high spectral resolution (typically l-4cm ). 

---