Adsorbate vibrations

The bond strengths of adsorbed species can be affected by a change of the electric field at the interface. In this case, shifts of the adsorbate vibrational frequencies are also observed [21]. According to Eq. (1.3) the frequency shift gives rise to bipolar bands (i.e., bands exhibiting both negative and positive parts). 

This wide range of questions is to be elucidated in the present chapter. The bulk of attention is given to the effects induced by the collectivization of adsorbate vibrational modes whose low-frequency components are coupled to the phonon thermostat of the substrate. This coupling gives rise to the resonant nature of low-frequency collective excitations of adsorbed molecules (see Sec. 4.1). A mechanism underlying the occurrence of resonance (quasilocal) vibrations is most readily... 

Here it is our intention to show that for a system constituted by substrate phonons and laterally interacting low-frequency adsorbate vibrations which are harmonically coupled with the substrate, the states can be subclassified into independent groups by die wave vector K referring to the first Brillouin zone of the adsorbate lattice.138 As the phonon state density of a substrate many-fold exceeds the vibrational mode density of an adsorbate, for each adsorption mode there is a quasicontinuous phonon spectrum in every group of states determined by K (see Fig. 4.1). Consequently, we can regard the low-frequency collectivized mode of the adsorbate, t /(K), as a resonance vibration with the renormalized frequency and the reciprocal lifetime 7k-... 

Payen and coworkers (Le Bourdon et al., 2003) investigated Pd/y-A1203 catalysts under DeNO reaction conditions with Raman and IR spectroscopy. Their instrument allows the quasi-simultaneous recording of both IR and Raman spectra. Several adsorbed NO species were identified (ranging from nitrates, nitrito, nitrate, and nitro species) by exploiting the high sensitivity of IR spectroscopy for adsorbate vibrations and the specificity of Raman spectroscopy for the vibrations of catalyst— adsorbate bonds. 

Beckerle JD, Cavanagh RR, Casassa MP, Heilweil EJ, Stephenson JC. Subpicosecond transient infrared spectroscopy of adsorbates vibrational dynamics of CO/Pt(lll). J Chem Phys 1991 95 5403-5418. 

The term has two components a resonant contribution from the adsorbate vibrations x (incorporating the resonance condition (ffliR-m )) and a nonres-onant contribution Xnr from the surface itself. In many cases, the applied light frequencies are far from resonances of the surface, and the response of the surface is therefore usually modeled by a frequency-independent nonresonant susceptibility... 

This is a surface vibrational spectroscopic technique that involves the irradiation of the adsorbate-metal interface with a beam of low-energy (2 to 10 eV) electrons and the measurement of the energies of the backscattered electrons energy losses below 0.5 eV are due mainly to inelastic interactions with metal-surface phonons and adsorbate vibrational excitations. The extremely high sensitivity of HREELS makes possible measurements of adsor-... 

At the time of a recent review [9], there remained very few examples of vibrational studies of adsorbate, or localised substrate modes, at metal oxide surfaces. By far the majority of studies concerned the characterisation by HREELS of phonon modes (such as Fuchs-Kliewer modes) pertaining to the properties of the bulk structure, rather than the surface, or to electronic transitions. Such studies have been excluded from this review in order to concentrate on the vibrational spectroscopy of surface vibrations on well-characterised metal oxide surfaces such as single crystals or epitaxially grown oxide films, for which there is now a substantial literature. Nevertheless, it is important to briefly describe the electronic and phonon properties of oxides in order to understand the constraints and difficulties in carrying out RAIRS and HREELS with sufficient sensitivity to observe adsorbate vibrations, and more localised substrate vibrational modes. 

In the first part of this chapter, we investigate the desorption of NO and CO molecules from a platinum surface to leam about energy flow between the substrate and the adsorbate. By using a stepped surface we unravel how the chemical dynamics are affected by surface defects [2], and demonstrate that the chemical dynamics depend critically on the precise adsorption site. In the second part we will follow diffusion of CO over the surface in real-time by using the adsorbate vibration-sensitive SFG method in a pump-probe manner. The stepped surface allows us to distinguish between molecules on the step and the terrace sites and therefore makes the diffusion from step to terraces sites visible [1]. 

The flow of energy from laser-heated snbstrate electrons into the adsorbate results in the excitation of adsorbate vibrational modes. For desorption of diatomics from various surfaces, it has been snggested that excitation of the frustrated rotation is responsible for the desorption process [9,14, 39]. Our results are consistent with those observations. We cannot, strictly speaking, dismiss (a contribution from) the Pt-CO stretch vibration, but the frustrated translational mode can be exclnded based on the independently determined electron-coupling times found to be 2.5 and 4 ps for terrace-and step-adsorbed molecnles, respectively (see the Sect. 10.3.2). These coupling times are much longer than those describing the very rapid desorption process. 

Budde F, Heinz TF, Kalamarides A, Loy MMT, Misewich JA (1993) Vibrational distributions in desorption induced by femtosecond laser pulses Couphng of adsorbate vibration to substrate electronic excitation. Surf Sci 283 143... 

Zou, S., and Weaver, M.J. (1998) Surface-enhanced Raman scattering on uniform transition-metal films toward a versatile adsorbate vibrational strategy for solid-non vacuum interfaces. Analytical Chemistry, 70, 2387-2395. 

When CoX was similarly treated with ammonia vapour a band at 1312 cm, similarly associated to C0-NH3 symmetric vibration, was observed (see Figure 8). When compared to other adsorbates (pyridine, acetone and water vapour) for their ability to shift the Metal-Zeolite bands (at 896 cm for CuX and 918 cm for CoX), the band positions for each of the metal-adsorbate vibrations was distinct (see Table 2). 

The use of RAIRS has recently been extended from its regular mid-IR characterization of adsorbates on metals into other exciting and promising directions. For one, changes in optics and detectors have allowed for an extension of the spectral range towards the far-IR region in order to probe substrate-adsorbate vibrations [35]. The use of intense synchrotron sources in particular looks quite promising for the detection of such weak modes [36]. Thanks to the speed with which Fourier-transform spectrometers can acquire complete IR spectra, kinetic studies of surface reactions can be carried out as well. To date this has only been done in a few cases, usually for reactions that take seconds or more to occur [37], but the advent of step scanners promises... 

---