Reflection-Absorption Infrared Spectroscopy
(RAIRS/IRAS)

This spectroscopic technique is used to probe the gas-solid interface. Infrared radiation (IR) is directed onto the sample and reflected from an underlying metal surface. The signal can be absorbed by both the bulk ice molecules, surface ice molecules and the adsorbate molecules on the ice surface. The gold surface is chemically inert, so it does not have a direct effect on the behaviour of the ice, but it will produce characteristic features in the IR spectrum. However a direct absorbtion spectrum is obtained by ratioing the ice spectrum with a 'background' spectrum of the gold film alone. This also eliminates spurious atmospheric signals from the spectrum.

Infrared radiation is used to probe vibrational molecular motion. Only those vibrations whose dipole moment lies perpendicular to the metal surface can be observed. This is because the incident and reflected p-polarised components of the radiation superimpose constructively (add together), enhancing the signal, whereas the s-polarised components cancel each other out, (as they undergo a phase change on reflection from the metal surface).

RAIR spectra show bulk and surface features. These are used to characterise the structure of the ice and the ice surface and to evaluate the nature of the bond between molecules in the as as well as the ice and the adsorbate.

Dangling OH bonds at Ice Surfaces

In water ices one of the most distinguishing spectral features is the OH stretching vibration. The bulk OH stretching vibration dominates an ice spectrum. Just on the shoulder of this feature a second signal exists, (see spectrum oposite) which is attributable to the OH bonds that are dangling from the surface of the ice. In a thick ice sample this feature is quite weak as there are far fewer OH bonds at the surface compared with the bulk. It is also a more ditinctive feature than that of bulk OH, since the OH itself is less tightly bound in the lattice.

In water ice the dangling OH stretch appears at

3692cm^-1 in crystalline ice

3696cm^-1 in 3-coordinate ice

3720cm^-1 in 2-coordinate ice

The positions of these peaks shift according to the adsorbate on the ice.

Dangling OH bonds at Ice Surfaces
	In water ices one of the most distinguishing spectral features is the OH stretching vibration. The bulk OH stretching vibration dominates an ice spectrum. Just on the shoulder of this feature a second signal exists, (see spectrum oposite) which is attributable to the OH bonds that are dangling from the surface of the ice. In a thick ice sample this feature is quite weak as there are far fewer OH bonds at the surface compared with the bulk. It is also a more ditinctive feature than that of bulk OH, since the OH itself is less tightly bound in the lattice. In water ice the dangling OH stretch appears at 3692cm^-1 in crystalline ice 3696cm^-1 in 3-coordinate ice 3720cm^-1 in 2-coordinate ice The positions of these peaks shift according to the adsorbate on the ice.

In this experiment the IR spectrum is recorded using a Fourier Transform (FT) spectrometer, which encompasses a 60^o Michelson Interferometer. Interferometeric techniques greatly enhance the speed and sensitivity at which this type of spectrocopy is conducted.

The IR data ties in with IR regions observed by the astronomical community. They have already observed ice features in astronomical spectra taken in lines-of-sight towards reddened stars where visible light has been obscured by dust.

The FTIR spectrometer was purchased from Biorad

The Surface Astrophysics experiment is part of

Nottingham Astronomy Research Forum
and is based in the Surface Science Group in the School of Chemistry at the University of Nottingham

These pages were last updated 14/10/01

Reflection-Absorption Infrared Spectroscopy (RAIRS/IRAS)

Dangling OH bonds at Ice Surfaces

Reflection-Absorption Infrared Spectroscopy
(RAIRS/IRAS)