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An international team of scientists led by VU University in Amsterdam physicist Jeroen Koelemeij has developed a new method for measuring vibration frequencies in the molecular hydrogen ion with a level of accuracy four hundred times higher than before.

This enables us to know more about the laws of nature and particles such as the proton. The research results were published in Science last week.

Vibrations in a three-part molecule

The research team investigated the precise vibration frequency of the simplest molecule that exists, the three-part molecular hydrogen ion (HD+). Jeroen Koelemeij, senior author of the article in Science, says: “This vibration frequency is determined by two things. First, the mass and diameter of the core particles – the proton and deuteron – and the mass of the electron. For this we use values derived from already-existing measurement methods. But after some recent values turned out to deviate strongly from older measurement values, there is discussion about the reliability of those values and methods.”

Koelemeij continues: “The second aspect is the interaction between the two core particles and the electron. This can be described using quantum electrodynamics, a theory that has been successful in describing loose electrons and the hydrogen atom (a nucleus particle plus an electron). We know this because earlier measurements of loose electrons and atoms proved to be in accordance with the theory. However, the question is whether this also applies to more complex particles such as molecules.”

Catching particles in an ion trap

The new method, developed with support from the Dutch Research Council (known by its Dutch acronym NWO) by Koelemeij and colleagues at the VU LaserLaB, uses an ion trap in a vacuum chamber. Approximately 100 HD+ ions are trapped in this chamber and cooled to a ten thousandth of a degree above absolute zero (-273.15 degrees Celsius) using lasers. The molecular vibrations are then excited and measured with high purity using lasers specially developed for this purpose.

The measured frequency is compared to the theoretical vibration frequency as predicted by quantum electrodynamics, calculated by physicists from France and Russia. Theory and experiment appear to be consistent, which enables scientists to determine the mass of the proton relative to the electron – a common natural constant – with unprecedented precision.

Possible fifth natural force

This new method can lead to more insights. Koelemeij: “Physics is approaching a turning point. In the last century, experimental and astronomical observations could invariably be explained by Einstein’s theory of relativity on the one hand and the standard model of particle physics on the other. However, over the last 40 years there have been increasing indications that 95% of the universe consists of unknown dark matter and dark energy. No one knows what it is.”

There are speculations that dark matter and energy are related to as yet undiscovered particles and forces of nature that can also influence vibrations in HD+. This could lead to a significant deviation between theory and experiment in even more precise experiments. Koelemeij explains: “Such a deviation has not been established in our experiment, but we can give a much better upper limit for the strength of a possible fifth natural force, and the mass of possible undiscovered particles.

Koelemeij and his colleagues are already thinking about improved follow-up experiments: “It all looks a bit like the game Mastermind. You stick a laser of a certain color into your experiment and see what information the experiment produces. Then you try it again with a different color, and again – until you have all the necessary information on the table to decipher the laws and properties of particles.”