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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