Adsorption of ammonia molecules on ice surface

Chemical reactions in the atmosphere are usually heterogeneous in nature.

One of the many important processes is adsorption and reaction of air-bone

species on the small ice particle surfaces in the polar stratosphere clouds (PSC).

Ammonia is the only significant basic species in the atmosphere.

Therefore, its adsorption and subsequent reactions on ice surface is very

important for a better understanding of the acid-base equilibrium and reactions

in the atmosphere. At the present time, I am using vibrational sum frequency

generation (SFG) to study adsorption of ammonia on ice surface under ambient

pressure. Sum frequency spectroscopy is a non-linear laser spectroscopy with

high sensitivity to study the vibrational resonance of adsorbents on surfaces.

It is capable of providing details of the adsorption states, bonding, surface structure

of adsorbed ammonia molecules. It has been found that ammonia molecules adsorb

on ice surface (0001) with its C₃ axis perpendicular to the surface. The surface

density of NH₃ is roughly 25% as that of surface dangling OH bonds, which

also suggests that NH₃ adsorbs by forming N---HO hydrogen bond to water

molecules on the ice surface.

Future studies on the adsorption processes and subsequent reactions

of hydrogen chloride, ammonia and other species are in progress.

(A presentation of my recent research can be found here as a power point file,

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