Research projects | Molecular ions

SAMURAI – How do you make the first antimatter molecule?

Tue, 28 Oct 2025 00:00:00 +0000

Synthesizing Anti-Matter Using Radiative AssociatIon – an NWO Vidi project

A reaction nobody has watched

Take a proton and a hydrogen atom and let them meet. Every so often the pair emits a photon, loses just enough energy, and stays together as H$_2^+$, the simplest molecule there is. This process, radiative association, is how the first molecules formed in the early universe, long before there was dust on which chemistry could happen more easily. Astrophysical models depend on how often it happens.

As far as we know, nobody has ever measured it.

The same question has a second, more exotic side. Antihydrogen atoms are now made and trapped routinely at CERN, and the next step is to bind them into molecules: $\bar{\mathrm{H}}_2^-$ would allow more sensitive tests of the symmetry between matter and antimatter (CPT) than single atoms do. Three routes have been proposed, and all three are reactions between hydrogen atoms and ions. Antiparticles are scarce, so it matters a great deal which route is the most efficient. Because the reactions should run in exactly the same way for matter and antimatter, we can find out with ordinary hydrogen.

Collisions involving hydrogen species. H₂* and H₂⁺* are the short-lived complexes through which the reactions pass. Reaction 8 → 7 is radiative association.

Half a collision

The usual way to study a reaction is to aim two beams at each other and count what comes out. For hydrogen this has been done down to collision energies of about a millielectronvolt, but the energy resolution is not good enough to see the narrow resonances, where the colliding pair is briefly caught in a quasi-bound state and the reaction probability can rise sharply.

We approach it from the other side. A collision passes through a transition complex: the moment when the two atoms are close together and form, for a short time, a molecule. Instead of colliding atoms to reach that state, we start from a hydrogen molecule and put it there with lasers, choosing its energy and angular momentum, and then watch it fall apart. This is the second half of a collision. Because the quantum mechanics is the same forwards and backwards in time, two such half collisions tell us about the full one.

Full and half collision. In a half collision, laser excitation prepares the transition complex in a chosen partial wave and at a chosen energy, and only the separation of the fragments is observed.

Resonances that beam experiments can easily miss, or that stay hidden in the measured cross sections, show up in this picture as sharp spectral lines. Their positions, widths and branching ratios can be measured with spectroscopic precision, and they pin down the collision far more tightly than a cross-section measurement alone.

Three steps

The three objectives of SAMURAI.

H$_2$. Using two tunable extreme-ultraviolet lasers built in our lab, we excite H$_2$ to highly excited states above the H$^+$+H$^-$ and H(2$\ell$)+H(2$\ell$) thresholds and image the fragments – protons, H$^-$ ions, excited atoms and electrons – with velocity-map imaging. This also gives us access to excited electronic states of H$_2^+$ that have never been observed. Edward Teller calculated the first of them in his PhD thesis in 1930.
H$_2^+$. We trap H$_2^+$ ions prepared in vibrational levels just below the H$^+$+H(1s) threshold and use XUV and millimetre-wave radiation to drive them across it. The last bound level of the excited A$^+$ state is predicted to be bound by only a few MHz. Once the hyperfine structure is included, one component stays bound and the other becomes a Feshbach resonance, which we can tune with a magnetic field.
H$^+$ + H. Finally, we cool H$_2^+$ sympathetically with laser-cooled Be$^+$ ions in a Coulomb crystal, break it apart gently into a cold proton and a cold hydrogen atom, and try to put the two back together with light. That would be the first observation of radiative association, and it brings the toolbox of ultracold chemistry to hydrogen, which has so far resisted it.

Alongside the experiments, we develop a multichannel quantum-defect and scattering model that treats bound states, resonances, ionization and dissociation on the same footing, and compare it with ab initio calculations.

What you would work with

Most of the hardware exists: two narrow-band XUV sources (60–205 nm), dye lasers and an OPO, velocity-map imaging spectrometers, and a home-built wire trap for H$_2^+$ and Be$^+$. The work combines lasers, vacuum, ion traps, electronics and quantum theory, and you will learn all of them.

We are looking for a PhD student for this project. See the vacancy or contact m.beyer@vu.nl

Scattering theory for ionization and dissociation of diatomics

Mon, 01 Jan 2024 00:00:00 +0000

Precision tests of scattering theory for photoionization and photodissociation of molecular hydrogen

We develop scattering methods to predict the photoionization and photodissociation spectra of the most fundamental molecule – molecular hydrogen – from highly accurate ab initio calculations. The predictions are compared to experimental resonance positions, lifetimes and branching ratios obtained using molecular Rydberg spectroscopy and velocity-map imaging. Comparing theory and experiment allows us to push the understanding of molecular fragmentation processes to unprecedented levels using multichannel quantum-defect theory and coupled-channel scattering calculations.

First direct laser cooling of a homonuclear diatomic molecule – the helium dimer

Tue, 21 Nov 2023 00:00:00 +0000

Ultracold helium dimers – a platform for fundamental physics and ultracold chemistry

Molecules at standstill

To reach high precision in spectroscopic measurements, we would like to trap our molecules. This is necessary because when molecules move around, they experience the laser radiation used in the measurements differently. That’s the Doppler effect, which is also known in everyday life: it causes an ambulance siren to sound higher in pitch when it is approaching than when it is receding. Having the molecules at one location also makes it easier to determine and compensate for interactions with the environment, which might otherwise distort the measurements. While trapping is relatively easy for molecular ions (we use radio-frequency traps for our measurements on H$_2^+$), it is much more complicated for neutral molecules.

Laser cooling

Within this ERC Starting Grant, we want to laser-cool the simplest neutral molecule possible. And as it turns out, this is not molecular hydrogen, but molecular helium. He$_2$ is the simplest molecule for which laser cooling to near absolute zero temperature seems feasible – which is the main goal of this project. At near-zero temperatures, the molecules hardly move due to the vanishing kinetic energy and therefore allow very precise measurements.

Platform for fundamental physics and chemistry research

If successful, this will be the first homonuclear molecule ever to be directly laser-cooled and trapped – every molecule laser-cooled to date has been heteronuclear. Compared to all other molecules laser-cooled so far, the simplicity of He$_2$ makes very accurate calculations of its collision properties possible. This will allow us to study ultracold collisions and chemical reactions with unprecedented accuracy. Being able to understand and control the collision properties is crucial for producing a molecular Bose-Einstein condensate. With such a system, applications range from testing quantum electrodynamics to quantum information and many-body physics.

Open PhD and postdoc positions. Please contact m.beyer@vu.nl

Weakly Bound Molecular Ion Clocks

Fri, 04 Dec 2020 02:32:34 +0100

Summary

Most existing molecular ion clocks operate starting from the vibrational ground state. This is because the lowest vibrational level is usually the only one that can be prepared with some selectivity either by photoionization or electron impact ionization. For heteronuclear molecules with a non-zero dipole moment an unselective preparation using electron impact ionization can be used, because after a certain time (on the order of milliseconds) the molecule will have decayed to the vibrational ground state via fluorescence. Homonuclear molecules have a vanishing dipole moment and can only decay via electric-quadrupole emission – with lifetimes on the order of days. In this case, photoionization with a laser source tuned just above the lowest ionization threshold is used to selectively create ions in the lowest rotational state. Selective ionization to produce excited vibrational states has so far not been possible.

Weakly bound molecular ion clocks are interesting tools that show an enhanced sensitivity to peculiar physical effects. This is caused by the degeneracies of these states close to the dissociation threshold. The usual hierarchy of motion – the basis of the Born-Oppenheimer approximation – can be completely reversed: the rotational motion is faster than the vibrational motion, which is faster than the electronic motion.

Weakly bound molecular states

Various laser sources in the visible and (vacuum) ultraviolet wavelength range are used to excite Rydberg states of the hydrogen molecule. Using small electric fields, the Rydberg states are field-ionized to selectively produce H$_2^+$ in the desired weakly bound states. The molecular ions are held in an ion trap (a linear Paul trap) to precisely measure their ‘clock’ frequency in the microwave range. Furthermore, this project will take advantage of the extremely high precision of the theoretical models describing this simple molecule, a precision which far exceeds that of all other molecules and even most atoms.

Molecular ion clock setup

Together, the theoretical framework and results of the clock measurements form a unique system that will shed light on several intriguing pending issues in physics, ranging from the (still not understood) role of nuclear-spin-induced symmetry breaking within the framework of quantum electrodynamics, to the possible existence of time-varying fundamental constants and hypothesized spin-dependent ‘fifth forces’. The obtained experimental results will serve as benchmarks for existing and emerging molecular quantum chemistry calculations. The results may also lead to the first detection of hydrogen molecular ions in space (through the Atacama Large Millimeter/submillimeter Array, ALMA), where this species is believed to play a fundamental role in the chemistry of the interstellar medium and the formation of the first stars in the universe. Furthermore, the techniques developed in this project hold promise for future production of molecules made of anti-matter and new CPT invariance tests.