Optics & Photonics News

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 Stark effect.

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.

Output
guide
Input
guide

V

Source NH3

V

Cavity Focuser

V

End view
of source
Focuser
cross section

V

Loss

--N2--

--N1--

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.