Press Release:
The 1986 Nobel Prize in ChemistryPress Release:
The 1986 Nobel Prize in ChemistrySummary
This year's Nobel Prize in Chemistry has been awarded to Dudley R. Herschbach,
Yuan T. Lee and John C. Polanyi for their contributions concerning
the dynamics of chemical elementary processes. Their research has been of great
importance for the development of a new field of research in chemistry - reaction
dynamics - and has provided a much more detailed understanding of how chemical
reactions take place.
Dudley R. Herschbach has developed the method of crossed molecular beams,
directed and well-defined fluxes of molecules, to and beyond the point where
detailed studies of chemical reactions have been possible. He has also elucidated
the dynamics of the basic types of reaction. Yuan T. Lee, who initially
worked in cooperation with Herschbach, has developed the method of crossed molecular
beams further towards its use for general reactions. Most notably, he has used
this method for the study of important reactions for relatively large molecules.
John C. Polanyi has developed the method of infrared chemiluminescence,
in which the extremely weak infrared emission from a newly formed molecule is
measured and analysed. He has used this method to elucidate the detailed energy
disposal during chemical reactions.
Background
The molecules and atoms in all substances are in perpetual motion, and collisions
between the molecules in a gas or a liquid thus occur continuously. When molecules
come in close enough contact with each other, redistribution of the atoms can
take place between or within them. New molecules form so-called product molecules,
which means that a chemical reaction takes place. To effect a reaction, the
colliding molecules are often required to have some special property such as
high velocity or large internal energy.
The classical description of how chemical reactions occur, and how rates of
chemical reaction are measured, belongs to the field of chemical reaction kinetics.
This field has developed rapidly during the last few decades, especially regarding
experimental methods. The 1967 Nobel Prize in Chemistry was awarded to M. Eigen,
Federal Republic of Germany , R.G.W. Norrish and G. Porter, Great Britain, for
their studies of extremely fast chemical reactions. In many respects however,
fundamental understanding of what molecular features influence the rate of chemical
reactions has been slow in developing.
The directions and velocities of the molecular motion in a gas or a liquid are
mainly random. Consequently, the collisions between the molecules are ill-defined
as regards, for example, the kinetic energy in the collision. The details of
the reaction thus become blurred and cannot be observed precisely enough. This
problem had not been solved satisfactorily before the development described
here.
It was finally possible to solve the problem by using molecular beams formed
of directed and spatially well-defined molecular fluxes of low density, often
also with well-defined velocities. When two molecular beams are caused to cross
each other, the details of the reactions between molecules can be studied. The
crossed molecular beam technique is thus one of the most important advances
within the field of reaction dynamics.
Dudley R. Herschbach took part in the development of this method almost
from the start. His extremely important achievements concerned for example studies
of short-lived direct reactions, especially of the two main types, the "rebound"
and the "stripping" reaction. He supplemented the commonly-used procedure of
detecting the product molecules by deflecting them in magnetic and electric
fields, thus circumventing one of the largely-overlooked problems inherent in
the early experiments. The discovery of the first long-lived reaction complexes
in crossed beams was soon followed by a theoretical description of their formation
and decay. The great importance of angular momentum was observed for the first
time in these reactions. Subsequent, more extensive studies by Lee, among others,
have clearly shown that this type of long-lived reaction is of great general
importance.
During this first stage of the development of the field of crossed molecular
beams, reactions between alkali atoms and other molecules were almost the only
ones which could be studied, due to the method of detection used at that time.
Several research groups developed crossed-beam machines for more general reactions.
One of the most sophisticated of these was developed at Herschbach's laboratory,
first of all by Yuan T. Lee. This so-called "supermachine" employed two
well-defined crossed molecular beams and a moveable mass spectrometric detector,
incorporating electron impact ionization and several stages of differential
pumping.
Both Lee and Herschbach, as well as other researchers, have used this type of
molecular-beam apparatus for detailed studies of a large number of chemical
reactions. Lee has led the development towards chemically important reaction
systems by investigating reactions between organic molecules and fluorine or
oxygen atoms. Short-lived direct reactions as well as longlived reactions have
been observed for large systems such as these. This confirms the universal validity
of the early results from studies of alkali-containing reaction systems. Extremely
important reactions, of immediate significance for combustion chemistry and
atmospheric chemistry, have been studied by Lee during recent years.
Another very important method for the detail study of chemical reactions has
been developed by Polanyi, the infrared - (IR) - chemiluminescence method.
This development took place concurrently with the formation of the crossed molecular
beam field. This complementary method resembles the crossed molecular beam method
in many respects, but involves measurement and analysis of the extremely weak
infrared emission from the product molecules in some chemical reactions. Excess
energy from the reaction is deposited as internal energy in the product molecules,
which after some delay emit the energy in the form of infrared light. Spectroscopic
analysis of this light reveals directly the quantum mechanical states which
the product molecules occupied. This gives indirect but extremely important
information on the multi-dimensional surface describing the potential energy
for the system. The potential energy surface is the fundamental, but in most
cases largely unknown factor, which determines the detailed behaviour of a chemical
reaction.
Polanyi has to a large extent combined a description of the potential energy
surface for the reactions studied with the experimental findings. He has for
example described how the existence and location of an energy barrier on the
potential energy surface modifies the dynamics of the reaction. Further, he
has observed that the product molecules in some cases belong to two different,
well separated, classes with respect to the internal energy distribution. The
method which he has developed can be considered as a first step towards the
present more sophisticated, but also more complicated, laser-based methods for
the study of chemical reaction dynamics.