Reflection Absorption Infrared Spectroscopy (RAIRS)

2.5.4.1 RAIRS Theory

Vibrational spectroscopy is a very powerful tool for identifying the bonding and

orientation of surface species upon molecular adsorption and the species generated during

a surface reaction.13-15 Reflection absorption infrared spectroscopy is widely used for vibrational studies on surfaces having low surface areas (e.g. single crystals). In this

technique, an IR beam exits from the IR generating source and is then focused onto a

reflective metallic surface. Then the beam reflects off the crystal surface and is collected

by a detector. In the gas phase, IR absorption is based on the interaction between the electric

field of the incoming IR beam and the dipole moment of the molecules but on metal surfaces

the process is dominated by the dielectric behavior of the metal. The theory of reflection at

metal surface was developed by Greenler in 1966.16 In this theory, the importance of surface selection rule is demonstrated and also it has been shown that the best sensitivity

for IR measurement on metallic surface is obtained using a grazing-incident reflection of

the IR light.

When a molecule with a dynamic dipole moment is adsorbed on a metal surface,

direction to the real dipole of the metal. If the vibration due to the adsorbed molecule is

parallel to the surface then the image dipole and the molecular dipole cancel and there is

no net dipole. However, if the vibration is perpendicular to the surface, then the image

dipole and the molecular dipole is additive and the net dipole is greater than the

molecular dipole which gives rise to infrared absorption.

Figure 2-11 The effect of orientation of a dipole on a conducting surface in the creation of an image dipole

The origin of surface selection rule is due to this image-dipole effect. According

to the selection rule the vibrations which are perpendicular to the surface (p-polarized

light) are infrared active while the vibrations which are parallel to the surface (s-

polarized) are not infrared active.

When an IR beam reflects from a clean metal surface (Figure 2-12), the interaction between the light and the surface is described by the Fresnel equations. The amplitude and phase changes during reflection depend on the direction of the electric field vectors. The electric field vector E is split into two components, one perpendicular to the plane of

Figure 2-12 illustrates the incident and reflected vectors of s- and p-polarized radiation.

The phase between the incident (𝐸𝑠𝑖) and reflected (𝐸𝑠𝑟) s-polarized light, changes by nearly

180° at all angles of incidence θ, resulting in a destructive interference and a negligible

electric field parallel to the surface (Es). On the other hand, the phase change in the p-

polarized light generates a constructive interference (Ep) that strongly depends on the

incident angle θ. Assuming the absorbance is proportional to the square of the electric field at the surface and to the surface area (number of molecules involved), which increases as 1/cos𝜃, we can conclude that RAIRS sensitivity is proportional to 𝐸𝑝2/cos𝜃. This function is

sharply peaked close to grazing angles, demonstrating the practical requirement of a high angle of incidence.

Figure 2-12 The reflection geometry showing the s and p components of the electric fields of incident and reflected radiation

Since this is an optical technique, it can be carried out both in vacuum and

ambient conditions. The major problem concerning this spectroscopy is the sensitivity.

Typically the sample area is 1cm2 with less than 1015 adsorbed molecules. With modern FT-IR spectrometers, however such small signals (0.01%-2% adsorption) can be recorded

2.5.4.2 RAIRS Chamber

The RAIRS chamber mainly consists of two parts. The main body of the chamber

is a 12-inch diameter bell jar, which is pumped by a combination of a mechanical pump,

a turbomolecular pump and an ion pump (250 L/s). This part of the chamber is equipped

with an ion gun for sample cleaning, a mass spectrometer, and several ports for dosing

sources and leak valves, an ion gauge for measuring the background pressure and a single

pass CMA. The other part of the chamber mainly consists of an IR cell and a transfer arm

which is separated from the main body of the chamber by a gate valve. The IR cell

consists of six-way 2 ¾” cube. The sample is mounted on a differentially pumped coaxial

manipulator. This manipulator can be moved horizontally and can be rotated 360° along

its axis of translation. The sample is usually moved to the main chamber for sample

cleaning or performing Auger electron spectroscopy and retracted to the IR cell for

infrared analysis. The sample can be cooled to liquid nitrogen temperature in the IR cell.

This is achieved by introducing a thin tube into the transfer arm through which liquid

nitrogen passes. The sample is connected to the heating wires via insulated feedthroughs.

By cooling down the feedthrough (located on the non UHV side) to liquid nitrogen

temperatures, the sample can be cooled down. The sample can also be heated resistively

Figure 2-13 Schematic diagram of the RAIRS Chamber 2 2.5.4.3 RAIRS Instrumentation

The RAIRS chamber is equipped with a commercial Bruker Vertex FT-IR

spectrometer. The IR beam exits the spectrometer and is reflected through a series of

gold-coated mirrors and then through a polarizer oriented perpendicular to the sample,

and finally through a KBr window into the UHV IR cell. The beam is then focused onto

the single crystal at a grazing angle and the reflected beam exits the IR cell through a

second KBr window. The beam is finally focused onto a liquid-nitrogen cooled mercury-

cadmium-telluride (MCT) detector. The entire light path, between the spectrometer and

the MCT detector, is enclosed in plexiglass boxes continuously purged with dry air or

liquid nitrogen boil off to eliminate the background IR signals due to carbon dioxide and

water. Data collection was achieved by a dedicated PC running OPUS software. The

same software is used for baseline correction of the original spectra if needed and finally