The Cold and Ultracold Molecules Group

We are proud to share that our article, “Magneto-optical trapping of aluminum monofluoride”, has been selected for an Editor's Choice highlight in Physical Review Letters!

We are delighted to announce that our article "Magneto-optical trapping of aluminum monofluoride” has been accepted for publication in Physical Review Letters. In our article, we show trapping of up to 60,000 AlF molecules in three different rotational levels, and find a temperature of around 15 mK. This is, at present, the shortest wavelength MOT of any atom or molecule.

We are excited to annouce that we have observed the first magneto-optical trap of AlF. Molecules are laser slowed from a cryogenic buffer gas molecular beam and captured about 50 cm donwstream. Trapping appears to work for three rotational levels, by adjustment of the laser slowing and trapping frequencies. This is a clear demonstration of the advantage of molecules with ¹Π ←¹Σ⁺ transitions. The photograph shows team members present in the lab, including Stefan Truppe who was visting from Imperial College London at the time. 

Having previously observed that AlF molecules from our continuous beam collide and thermalise with a room-temperature vacuum walls of our experiment, we decided to look in more detail. We can now see that a single collision is enough to rotationally thermalise molecules, and directly observe the Doppler-sensitive fluorescence spectrum of outgoing molecules leaving the surface. The rotational and translational temperature is very near room temperature. It is not clear how many molecules survive a surface collision event on average, but we expect a number much larger than 10%. We hope this will enable developing a continuous, compact vapour source for future trapping of AlF. 

As a molecule with a ¹Π ←¹Σ⁺ transition, a rotationally closed optical cycle exists for all levels in AlF with J > 0. We have now shown that chirped frequency laser slowing is effective for J=1, 2, 3 with a buffer gas cooled AlF molecular beam. Molecules can be slowed from about 100 m/s to below 50 m/s, where we expect it will be feasible to capture into a MOT. 

During Jionghao Cai's visit to the institute in February, we were able to demonstrate convincing laser slowing of our buffer gas beam of AlF molecules. Starting from an initial velocity of 160m/s, we applied chirped frequency laser slowing to decelerate our molecular beam to about 90m/s. This is approximately halfway the expected capture velocity of a deep ultraviolet magneto-optical trap. Well done Jionghao! 

Congratulations to Simon Hofsäss, who successfully defended his PhD thesis “Laser cooling of atoms and molecules in the deep ultraviolet”, at Radboud University (Nijmegen). His thesis describes experiments demonstrating optical cycling in AlF molecules and magneto-optical trapping of cadmium atoms, taking on the challenges of working with deep ultraviolet laser light. Simon has been a member of the AlF group almost from the beginning and many of our results have relied upon his hard work. Well done Simon! 

Whilst AlF can be made very efficiently in an effusive molecular beam, such a source requires around 1000K and therefore makes using such a source in trapping experiments challenging. Recently, we observed buffer gas cooling of our continuous beam of AlF molecules by injection into a buffer gas cell containing cold Neon. We hope to improve this source and make it suitable for loading a magneo-optical trap in future. 

Cadmium atoms serve as an ideal test system for laser cooling and trapping of AlF molecules. We have tested chirped laser slowing of our Cd buffer gas beam using deep ultraviolet laser light, showing efficient slowing down to 20m/s. This method of laser slowing is commonly used to load magneto-optical traps of molecules. 

David Röser, a graduate student at the University of Bonn and a UVQuanT collaborator, visited us for two weeks to measure isotope shifts in atomic Zn using our buffer gas beam. He brought along an ultrastable Menlo ORC cavity, with which we were able to ensure a narrow laser linewidth (~10 kHz) and highly linear detection laser scan.

Congratulations to Max Doppelbauer, who successfully defended is his PhD thesis at Radboud University, Njimegen! Max's thesis is titled "The AlF molecule as a candidate for laser cooling and trapping”, and brings together much of the work in the group over the past five years. He will continue to work as a Postdoctoral Researcher in the group. 

Whilst AlF can be produced at high temperatures via a chemical reaction, this method has so far not been used to generate a continuous molecular beam in our group. After a few different attempts, we found a method of generating a stable beam of AlF molecules capable of operating for several days. This enabled us to take a 2 THz-wide spectrum of the rovibrational structure of the A-X transition with a precision of 10 MHz, containing an enormous amount of information (more than 300 resolved rovibrational lines!). We hope that this source will prove useful for further experiments with AlF.

We are excited to have Russell Thomas from the UK joining the institute, where we will work as a laser engineer in our group and support others across the Molecular Physics department. He has extensive extensive experience in installing, testing and optimising DUV laser systems relevant for laser cooling AlF molecules. Welcome Russell!

As part of an investigation into molecule production in buffer gas sources, we produced a beam of cold MgF molecules in the same machine we use to generate AlF. Like AlF, MgF is a laser cooling candidate, but much less is known about its first electronically excited state. We undertook a study to measure the rotational and hyperfine structure of the first electronically excited state, and its dipole moment. We found that excited states of opposite parity are much closer in energy than would be naively expected, and this has consequences for optical cycling with MgF.

We are delighted that Jose Eduardo Padilla-Castillo from Cozumel, Mexico has joined us as a new PhD student. Eduardo completed his Masters at Mexico City studying quantum degenerate gases of ⁶Li₂ Feshbach molecules, making the first Bose Einstein Condensate ever produced in Mexico. He will work on laser cooling and trapping of cadmium atoms and AlF molecules. Welcome Eduardo!

We loaded our first magneto-optical trap (MOT) of cadmium atoms, by direct loading from a cryogenic buffer gas beam. We are able to separately trap all stable isotopes using the strong deep ultraviolet transition near 228 nm. This is a convenient test platform for future experiments on the AlF molecule. 

We have designed and assembled an upgraded cryogenic molecular beam source. This enables faster thermal cycling of the machine, more detailed measurements of the molecules in and outside of the buffer gas cell, and straightforward comparison of the production efficiency of various species.