Infrared Spectroscopy

Infrared (IR) spectroscopy is a powerful tool to study the interaction of molecules with surfaces. Molecular vibrations are excited by the absorption of IR light. The vibration frequency gives detailed information about chemical bonds and thus about the interaction of the molecule with the metal surface [105–109]. The vibration frequency of covalent bonds in some adsorbates, such as CO, does not only depend on the metal, but also on the specific site to which they are attached. Therefore, CO adsorption is frequently used to characterize the morphology of a surface (see chapter 3.2.1). This kind of information can also be obtained under reaction conditions, providing information which sites are populated during a chemical conversion. Moreover, in some cases reaction intermediates can be identified, which gives insights into the mechanisms of the reaction.

Molecular Vibrations

The excitation of molecular vibrations by IR light can be described as a pure time-dependent perturbation, since the electromagnetic field is approximately constant over the size of the excited dipole [110]. Thus, the Hamilton operatorH⁽¹⁾ of the perturbation can be described as

H⁽¹⁾=−~µ·E~ (2.12)

with ~µ the electric dipole moment of the molecule and E~ the electric field vector of the electromagnetic radiation [111].

According to Fermi’s Golden Rule, the probability for an excitation is given by W ∝ ^Dψ_f~µ·E~ψi

E

(2.13)

2.2 Experimental Methods

withψf and ψi the eigenfunctions in the exited and the ground state. The Born-Oppen-heimer approximation (BOA) allows to break the eigenfunctions into its electronic and nuclear (vibrational)ν_k compound.

ψ=, ν_k⁰ |~µ|, ν_k (2.14)

withν_k and ν_k⁰ representing the eigenfunction of the vibration before and after the exci-tation.

Within the BOA, both compounds can be treated separately. Thus, the probability for a vibrational excitation along the normal coordinateQ_k is given by

ν_k⁰ |~µ|ν_k= Therefore, a vibration can only be IR-active, if it involves a dynamic dipole moment:

∂~µ

∂ ~Q_k 6= 0 (2.16)

Infrared Spectroscopy on Metal Surfaces

Vibrational spectra of molecules on metal surfaces can be obtained byinfrared reflection-absorption spectroscopy (IRAS) in a reflection mode. In this case, themetal surface selec-tion rule (MSSR) has to be taken into account [106, 112, 113]. According to the MSSR, only the components of the dynamic dipole moments perpendicular to the surface can be detected because dipoles parallel to the surface are compensated by a mirror dipole in the metal. Moreover, IR light with polarization parallel to the surface (s polarization) is reflected with a phase shift of 180^◦ leading to almost complete vanishing of the s polarized field. In contrast, the effective field of the p polarized light is almost doubled at angles close to grazing incidence. Therefore, IRAS measurements are typically performed only with p polarized light. Taking the MSSR into account, adsorption geometries of molecules on metal surfaces can be deduced from characteristic intensity distributions of IR absorption peaks.

IRAS of adsorbed Molecules

The vibration frequencies of adsorbed molecules can be significantly different from gas phase species, due to the interaction with the metal surface or inter-adsorbate interactions [105, 106, 113].

Frequency shifts by metal–adsorbate interaction There are four important effects, which are responsible for frequency shifts of an isolated adsorbate.

Mechanical renormalization. Adsorption of a diatomic molecule (e.g. CO) to a rigid

The shift can be estimated from a simple model of masses and springs. In case of the metal–

C–O system, the C–O stretching shifts of 50cm⁻¹ to higher wavenumbers as compared to the gas phase. In case of a vibrating substrate, additional renormalization appears [114].

The renormalization model predicts a frequency shift to higher wavenumbers; however, experimental results indicate mostly lower frequencies for adsorbed molecules. This indi-cates that this model alone is not sufficient to discribe the experimental results.

Chemical shifts. Chemical shifts arise from chemical interaction between the molecule and the substrate. Chemical shifts were found to be responsible for the appearance of several IR vibrations of CO adsorbed on supported transition metals. A theoretical de-scription of this phenomenon has been given by Blyholder [115, 116]. In this model, a chemical bond between CO and the metal is formed by charge transfer from the 5σorbital of CO into the metal (σ bonding) and from the metal d-bands into the unoccupied 2π^∗ orbital of CO (π backbonding). The σ bonding increases the C=O bond stength while the π backdonation weakens the C=O bond. Since the π backdonation is dominant on transition metal surfaces, the C=O bond is weakened. Thus, the degree of backdonation into the antibonding 2π^∗is directly reflected by the lowering of the C=O stretch frequency.

Self-image shifts. The adsorbate can interact with its own image dipole in the metal.

This effect tends to lower the vibration frequency in the case of adsorbed CO.

Charge transfer. Charge transfer between the substrate and the adsorbate results in electrostatic interaction and therefore causes a frequency shift. Theoretical calculations showed a shift of 10-20 cm⁻¹ for a single adsorbed non-polar molecule, such as CO [117].

The effects of the substrate–adsorbate interaction on vibrational frequencies is widely used in surface science to identify adsorption sites. For example, the C=O stretch fre-quency generally decreases with increasing coordination number. In this work, the Pd surface was saturated with CO in order to probe different surface sites.

Frequency shifts by adsorbate–adsorbate interaction While the vibrational frequency of an isolated molecule typically undergoes a shift to lower wavenumbers by means of the substrate–adsorbate interaction (redshift), an increase of the frequencies is typically ob-served with increasing surface coverage (blueshift). This blueshift arises from the lateral interaction between adsorbates. Here, we will summarize three major effects:

Dynamic dipole–dipole coupling. Vibrational coupling between adsorbates can appear as through-space dipole-dipole coupling [118]. As the distance between adsorbates decreases, this effect becomes increasingly important. The dynamic dipole–dipole interaction in-creases the vibration frequency.

Chemical shifts. Chemical shifts have been discussed as a phenomenon of the adsorbate–

metal interaction before. However, with increasing coverage, adsorbates compete for d-electrons of the metal, which decreases the strength of theπ backdonation.

2.2 Experimental Methods

Static dipole–dipole interaction. The vibrational frequency of an adsorbate is affected by an electric field, which is created by neighboring static dipoles. This effect is typical for co-adsorbed dipoles with significantly different vibration frequencies. The shift depends on the orientation of the two dipoles: parallel orientation results in a blueshift, while an antiparallel orientation leads to a redshift [119].

Intensity changes The intensity of the IR absorption of adsorbed molecules is not only influenced by the MSSR, but also by adsorbate–adsorbate interactions. In the low cover-age limit, the intensity is proportional to the number of vibrating dipoles. With increasing coverage, the intensity is subject to the influence of the effects described above: static and dynamic dipole–dipole coupling and chemical effects. The impact of these effects on the IR absorption intensities is non-linear with increasing coverage.

One frequently observed effect is known as intensity borrowing. This effect can lead to difficulties in identification of species on a surface. If a surface is populated with two species with slightly different vibration frequencies, dipole–dipole coupling can result in intensity transfer from the IR adsorption at the lower frequency to the one at the higher frequency. This effect can result in strong changes in the intensity distribution between different species.

Fourier-Transform Infrared Spectroscopy

Fourier-transform (FT) IR spectrometers [108, 109, 120] are the most commonly employed IR spectrometers. They have a number of advantages over dispersive spectrometers. In dispersive spectrometers, the sample is exposed to monochromatic light and the absorption is detected at each wavelength individually. In FT-IR spectrometers the sample is exposed to light from a wide spectral range. The absorption data in the whole spectral range is collected simultaneously. An FT-IR spectrometer consists of an IR source, a Michelson interferometer, an IR detector and a computer. The principle is illustrated in Figure 2.8.

In the interferometer, the radiation from the source is passed through a beam splitter, which reflects half of the light to a fixed mirror and passes the other half to a movable mirror. The two beams are reflected from the mirrors and interfere again at the beam splitter. The recombination will be constructively or destructively, depending on the path difference. The position of the movable mirror (x) is altered and the detector collects the beam intensity as function of the mirror displacementI(x), which is called interfero-gram. For monochromatic light, the detected intensity is a cosine function of the mirror displacement. For polychromatic light, the interferogram is the sum of all interferences of each wavelength. With path difference x = 0, constructive interference of all waves oc-curs, resulting in a maximum intensity of the interferogram. With increasing distance to x= 0, most waves undergo partial or total destructive interference, which leads to rapidly decaying oscillations on both sides of the center. The intensity of the light at frequency I(ν) is obtained by Fourier transformation ofI(x).

∞

Figure 2.8: Schematic of an FT-IR spectrometer with Michelson interferometer.