Research on Water

Water is one of the most important chemicals on earth. It is the
best solvent and most chemical reactions in nature take place in water.
However, these reactions do not happen in pure water, rather, they
take place at the interfaces where water is in contact with other substances,
such as air, rock, oil, ice, proteins, and cell membranes.
These reactions are called heterogeneous reactions.

During the time when people want to learn more about these reactions,
they find that a better understanding of the properties of the interfaces
between water and other substances is obviously very crucial.
Interfacial structures are important because they are the very place
for chemical reactions to happen. Chemical reactions are usually very sensitive to
the microscopic environments. Hence, even a minor change of the physical and
chemical properties of these interfaces will greatly change the corresponding
reactions taking place there.

The idea of using laser spectroscopy to study water interfaces originated
in our group in later 1970's. We developed the technique called sum frequency
generation (SFG) which is a surface specific laser spectroscopy. This technique
works specifically at the interface because interface has broken symmetry and this
property facilitates the generation of signal whose frequency is the
sum of the two input laser beams. At the beginning, we could only detect
vibrational resonance of the species at the interfaces using a tunable infrared
(IR) laser system. Recently, we managed to implement the probing of
electronic resonance of the speices at the surfaces, as well as vibrational resonance.

The numerous number of systems we have studied so far include water/air, ice/air,
water/mineral, alcohol/air, metal/gas, and many other interfaces. A brief introduction of what we do
can be found here. Currently, I'm working on the adsorption of ammonia at the ice/air
interface. This is an important problem related to atmospheric chemistry.

Ammonia (NH3) is the only significant alkaline species in the air. Therefore, it is very important for
acid-base equilibrium in the atmosphere. Most reactions in nature are heterogeneous and
ice surface serves as a playground for these heterogeneous reactions. A detailed study on the
interaction of ammonia with ice/air interface seems to be indispensable for a deeper understanding
of the chemical/physical processes involving ammonia. The study we are performing
is unique because of the following:

1. We are studying a model system that closely resembles the real conditions in the atmosphere;
2. We are able to derive the exact orientations of the molecules at the interface.

A summary and a presentation of our recent findings are available.

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