Research Topics

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Spectroscopy of HD⁺ and tests of fundamental physics

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Spectroscopy of HD⁺ and tests of fundamental physics

This experiment aims to measure the vibrational frequencies of one of nature's most simple molecules, the molecular ion HD⁺, with extremely high precision using laser spectroscopy. The experimental frequencies can subsequently be compared with theoretical predictions based on Relativistic Quantum Mechanics and Quantum Electrodynamics (QED). Depending on the outcome, such a comparison between theory and experiment can have interesting implications. If good agreement is found, this confirms that the most advanced theoretical models provide an accurate description of the physical world. While QED is normally used to describe elementary particles and atoms, our experiments allow verifying QED also in more complex forms of matter. Interestingly, the theory of HD⁺ has become so accurate that the uncertainty of theoretical predicitions is no longer limited by the quality of the theoretical model, but rather by the fundamental constants which enter the theoretical calculation - in particular the proton to electron mass ratio, m_p/m_e. This implies that we can use theory to translate an accurately measured vibrational frequency back to a new value of m_p/m_e.

Conversely, if we do not find good agreement between theory and experiment, there is a possibility that hitherto unrecognized phsyical phenomena are at work. We recently computed the effect of possible 'fifth forces' and compactified higher dimensions, which allows converting a potential discrepancy to the strength and range of a fifth force, or to the radius of rolled-up higher dimensions.

Recently we published new experimental results, showing that experiment and theory agree at the level of 1.1 ppb. From this, we could infer a value of m_p/m_e with an uncertainty of 2.9 ppb, and put a better constraint on the (non)existence of fifth forces. Previously we published more details regarding the experimental setup, laser systems and the calculation of systematic effects.

Together with the group of Laurent Hilico at LKB Jussieu, Paris, France, we have demonstrated the feasiblity of two-photon spectroscopy to highly excited vibrational states, which may lead to spectrscopy of HD⁺ at the 10^-14 accuracy level. The experimental setup is currently being upgraded to perform such spectroscopy.

Together with Vladimir Korobov, Jean-Philippe Karr and Laurent Hilico, we published improved theoretical calculations of the hyperfine structure of the molecular hydrogen ion. The results were in excellent agreement with experimental observations, for the first time within the 1 ppm experimental error margin, thereby concluding a nearly 50-year-long theoretical quest to explain the experimental observations within their uncertainty.

Motivated students who are looking for an undergraduate project in a stimulating and high-level experimental research environment are welcome to inquire about the possibilities!

This work was supported by NWO through Veni and Vidi grant, a FOM Projectruimte grant, and is part of FOM Program 125 'Broken Mirrors and Drifting Constants'.