Precision electron affinity measurement in DESIREE

The electron affinity of an element is defined as the amount of energy released when an additional electron is bound to the system, which is—of course—equal to the energy needed to remove the negatively charged ion. An efficient way to determine electron affinities is to expose an ensemble of negative ions to a laser field and then vary the wavelength of the light to find the threshold for a signal relating to the photodetachment process separating an electron from the now neutralized atom. The longest wavelength where this happens corresponds to a photon energy equal to the electron affinity, so by finding the threshold and transforming from wavelength to photon energy one can determine the electron affinity.

Moa Kristiansson and Gustav Eklund, the DESIREE Group
Moa Kristiansson and Gustav Eklund, the DESIREE Group

In the case of oxygen, the anion has fine structure and in the ion production process it is inevitable to also populate the upper fine structure level. This means that at the threshold for which we are looking, there will be a considerable signal caused by ions that were formed in the upper fine structure level. In DESIREE we can store the anions for a long time, but unfortunately the average lifetime of the upper fine structure level is several hours, which makes it impractical to wait for the decay to the ground state.

Stored oxygen anions in DESIREE

Several years ago, Dag Hanstorp from Gothenburg University, who is an expert on negatively charged atomic ions, proposed that we should store oxygen anions in DESIREE and expose them to an intense continuous laser field. The laser wavelength should be chosen such that the population in the upper fine structure level could be effectively removed while the ground state population remained unaffected. This way, we could eliminate the contribution from the upper fine structure level and perform a better electron affinity measurement.

Installation of a very precise wavemeter

For a first exploratory measurement in 2016, Dag brought his master’s student, Moa Kristiansson, and while that measurement was not successful, we managed to make Moa interested in joining our group as a PhD student. During her PhD work, Moa contributed to a number of other DESIREE experiments requiring laser expertise, while she worked on and improved the oxygen experiment. Meanwhile, we found that hardware upgrades were necessary to reach the ultimate resolution aimed for. The first step there was the installation of a very precise wavemeter, which is situated in our laser laboratory and is connected via an optical fiber to the laboratory of Markus Hennrich and co-workers. This arrangement allows for them to make use of the wavemeter and for us to receive an accurate calibration signal from their lab.

Continuous, tunable narrow-bandwidth laser

With the ability to determine the laser wavelength very precisely, we saw that our laser system was not sufficiently stable to perform the measurement with the high accuracy aimed for. For that reason (among others), we purchased a continuous, tunable narrow-bandwidth Titanium-Sapphire laser from M Squared. With that laser we have now performed the planned measurement, and reached a result, which constitutes a ten-fold improvement in precision for the oxygen electron affinity value. Obviously, this was a cornerstone of Moa’s thesis and Friday last week, this work was published in Nature Communications.


Henning Schmidt, on behalf of DESIREE and friends.

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