Getting to know ammonia
That year I learned about the microwave spectrum of the
ammonia molecule—an equilateral triangle of three hydrogen
atoms with a nitrogen atom off to one side. It has a hindered
vibration wherein the nitrogen atom tunnels through a small
potential barrier in the plane of the hydrogen atoms and comes
out the other side. The resulting spectrum is called the inversion spectrum, and it occurs in the microwave region. Of
course, since the nitrogen is heavier, the hydrogens do most
of the moving. The molecule has various rotational states that
modify the inversion frequency.
The rotational angular momentum of the molecule is
denoted by the quantum number J. The projection of J on the
molecular axis is labeled K, and the projection of J on some
laboratory axis, as provided, for example, by an electric field, is
labeled M. If K=J, the rotation is mainly around the molecular
axis; then the hydrogen atoms are pulled apart, and the inversion frequency is increased. Conversely, if K=0, the rotation is
mostly perpendicular to the molecular axis, and the inversion
frequency is lowered. The inversion line we settled on was the
J=K= 3 line, the strongest one at room temperature.
In the presence of an electric field, the energy levels of the
J=K= 3 inversion transition are split. This splitting is called the
The figure below shows the basic design of the first working maser. The maser had three main elements. On the left is
the ammonia beam source. Ammonia from a room temperature tank was allowed to effuse out of a source consisting of an
array of fine tubes, which at the appropriate pressure should
result in a beam of molecules more or less directed at the
focuser. The focuser consisted of four cylinders held in place
by a Teflon structure.
Field picture of attenuation: The loss is proportional to
N1-N2. How can that be?
N1 > N2
Why the maser worked
One of the reasons I became convinced that our experiment had realistic chance of succeeding is
illustrated in the figure above. It is a field picture of the
familiar process of attenuation. A wave impinges on a
lossy medium. Molecules resonant at the frequency of
the incident wave have level populations, N1 and N2,
respectively, in the lower- and upper-energy states of
the transition. Usually N1 >N2. The wave comes out of
the lossy medium diminished in amplitude. But the loss
is proportional to N1 – N2. This can only be true if the
processes of loss and gain are competing coherent processes (where the oscillations of the many molecules are
correlated in phase with the field).
Loss is provided by the N1 molecules in the lower
state. The incoming wave induces in these molecules
a dipole moment oscillating at the wave frequency in
quadrature with the incoming wave, in the phase that
absorbs energy from the field, thus increasing the energy
of the molecules. These dipoles in turn emit a forward-going wave that destructively interferes with the outgoing
wave, thereby reducing its amplitude. Gain is provided by
the N2 molecules in the upper state. Since the net loss is
proportional to N1 – N2, it is clear what these molecules
must do, and indeed what they actually do.
The incident wave causes in these molecules a dipole
moment that oscillates at the wave frequency in quadrature with the incoming wave, in the phase that emits
energy into the field, thus reducing the energy of the molecules. These dipoles in turn emit a forward-going wave
that constructively interferes with the outgoing wave, thus
increasing its amplitude.
It was pretty obvious to me that this picture was not
confined to plane wave fields, but that it would work with
any other field configuration, such as the field of a cavity
resonator. Thus, induced emission must be the inverse of
absorption as well as a coherent process. Microwaves,
like other forms of energy, have an annoying habit of
sometimes acting like waves and other times like particles,
depending on what you look for.
At the end of 1952, George Dousmanis had left the project, and Herb
Zeiger’s post-doc had come to the end. He departed in early 1953 for
Lincoln Laboratory. The oscillator was then my baby.