Aaron Abma
Optical Methods for Trapping Atoms and Making Cold Molecules
I spent the summer working under Professor Walhout with fellow student Jonathan Shomsky on a
research project studying the molecular interactions of Krypton atoms in higher lying energy states. The
fact that noble gases such as Krypton have full valence electron shells seems to indicate that they do not
form molecules. However, Krypton atoms can be made to form molecules for short periods of time
through photoassociaton, a process by which two atoms are bound together by light. Photoassociation
can be used to study the molecular energy levels of gases, but only at sub-milliKelvin temperatures, so the
atoms must first be cooled. To do this, physicists often employ what is known as a magneto-optical trap,
an array of lasers and magnetic fields arranged to form a cloud of cold atoms at their center, where the
magnetic field has a zero and where all the trapping lasers intersect. If the atoms move away from the
center in any direction, they are pushed back towards that center by the lasers and magnetic fields. We
used such a setup for our experiment.
Our trap was made up of lasers, magnetic fields, optical elements, various electronics, and
computers all arranged to form and observe clouds of Krypton atoms. To begin, we directed a beam of
krypton atoms down the center of an evacuated pipe. We used magnetic fields to shift the energy levels
of the atom, tuning them to remain resonant with the frequency of a slowing laser beam directed up the
pipe, counter to the motion of our Krypton atoms. This slowing was possible because photons have
momentum, and so with a laser tuned to the proper frequency, we were able to transfer the momentum of
our photons to our atoms. These atoms were slowed to the point where they could be trapped in a cloud
at the center of an array of laser beams and at the zero point of a magnetic field. This cloud was then
cooled to temperatures below 1 milliKelvin by the trapping beams. Having formed the cold cloud, we
directed another laser through it in the hopes of performing photoassociation. Tuning this laser to various
frequencies, we used cameras and ion sensors to observe and record the cloud’s behavior as it reacted to
the differing frequencies. This in turn gave us clues as to what was occurring on an atomic and molecular
level. Ions are formed more frequently when there are more collisions among the atoms, so our ion signal
told us how much our atoms were interacting; more collisional interactions means molecules are more
likely to form, because atoms must be fairly close together to form molecules. Atoms can be lost from
the trap for various reasons, but we were interested in atoms lost to molecular interactions. Trap loss can
be indicative of molecule formation because atoms which form molecules are more likely to escape the
trap. We scanned our laser over a range of frequencies and took data on trap brightness and ion count,
which corresponded to the number of atoms in the trap and the number of atomic collisions, respectively.
We looked for wavelengths where there were unexpected dips in our data, where fewer ions were
produced and where there was trap loss, because this indicated that molecules may have been forming.
We already knew of five wavelengths, corresponding to atomic transitions, which cause
significant trap loss. The three of these in the range of our laser we were able to verify with our data, but
we were unable to find any unambiguous evidence of molecular interactions. While we did find several
other wavelengths that caused significant trap loss, we were not able to confirm that they corresponded to
molecular interactions. It is possible that the trap loss at these wavelengths was due to other factors, such
as our photoassociation laser including extra, unintended wavelengths of light, known as modes.
However, we saw this trap loss at the same wavelengths even when using different photoassociation
lasers. Because different lasers do not generally have the same mode structure, it is still very possible that
these wavelengths are of atomic or molecular interest, and so they merit further exploration.
Through my involvement in this research project I developed both laboratory instrumentation
skills and skills with machinery such as lathes and mills. I was introduced to optical equipment for the
first time. I learned concepts from atomic, molecular and optical physics, and I was introduced to more
concepts than I was able to learn. Perhaps most valuably, I got a taste for what research entails, which
will likely prove a vital lesson when deciding what vocational direction I might be best suited for.