ReMi-Lab
Dynamics in interacting Rydberg systems and novel trilobite molecules are investigated in this laboratory. In a Rydberg atom, one or more electrons are in a very highly excited state, making them up to 10,000 times larger than ground state atoms. This makes Rydberg atoms very sensitive to external influences such as electric or magnetic fields. This sensitivity also leads to a strong interaction over large distances, which can be either attractive or repulsive. The resulting motions can be analyzed with our unique momentum spectrometer, which determines the full three-dimensional momentum.
Current Research
Inelastic collisions between ground and Rydberg atoms are also investigated. In these collisions, it was observed for the first time that the state of the Rydberg atom changes over a large range. The energy released from the transition to a lower-energy state is converted into kinetic energy for both collision partners. By measuring the momentum, a state-resolved measurement of the inelastic collisions is possible. These state-changing collisions build a dominant decay channel in so-called ultra-long-range Rydberg molecules.
This type of Rydberg molecule consists of a Rydberg atom and at least one ground state atom, in which the bond differs from any other type of bond known from chemistry. The ground state atom is bound to the Rydberg atom by a scattering interaction with the Rydberg electron. A new class of Rydberg molecules are the so-called trilobite molecules. In these molecules, the mixture of high angular momentum states leads to a high probability of the electron staying close to the base atom. As a result, the center of charge is not together, which means that trilobite molecules have the highest permanent electric dipole moments ever measured. This makes this type of molecule interesting for strongly correlated many-body systems or quantum computers. Furthermore, the fundamental scattering property of the electron at extremely low energy can be investigated.
Technical
In this experiment, an ultracold gas cloud of rubidium atoms is generated using laser cooling. To increase the density of the atomic cloud, around 100,000 atoms are transferred into a crossed dipole trap with a maximum density of 1013 atoms per cm3. This requires a very good vacuum of 10-10 mbar, which is achieved by two ion getter pumps, an NEG element and a titanium sublimation pump. A special feature of our setup is the Rydberg excitation using three photons. In addition to P, this also makes it possible to excite hard-to-reach F angular momentum states. Various laser systems are available, which are stabilized using different methods, such as Doppler-free saturation spectroscopy, an ultrastable high-finesse resonator or a frequency comb. In order to determine the pulses, the Rydberg atoms or molecules must be ionized, which is done using a short CO2 laser pulse. This utilizes the fact that photons have only a very low momentum despite their high energy, which means that the initial momentum of the atom is not distorted. In a weak, homogeneous electric field in Wiley-McLaren configuration, the ions are accelerated onto a spatially and time-resolved detector, which consists of a multi-channel plate (MCP) and a Deley line detector. From this, the three-dimensional momentum of the atom at the moment of ionization can be determined. In addition, the electrons on the opposite side can be detected with a channel electron multiplier, which enables a coincidence measurement.
Publications
Max Althön, Markus Exner, Richard Blättner and Herwig Ott
"Exploring the vibrational series of pure trilobite Rydberg molecules"
Nature Communications 14, 8108 (2023)
In trilobite Rydberg molecules, an atom in the ground state is bound by electron-atom scattering to a Rydberg electron that is in a superposition of high angular momentum states. This results in a homonuclear molecule with a permanent electric dipole moment in the kilo-debye range. Trilobite molecules have previously been observed only with admixtures of low-l states. Here we report on the observation of two vibrational series of pure trilobite Rubidium-Rydberg molecules that are nearly equidistant. They are produced by three-photon photoassociation and lie energetically more than 15 GHz below the atomic 22F state of rubidium. We show that these states can be used to measure the electron-atom scattering length at low energies in order to benchmark current theoretical calculations. In addition to measuring their kilo-Debye dipole moments, we also show that the molecular lifetime is increased compared to the 22F state due to the high-l character. The observation of an equidistant series of vibrational states opens the way to observe coherent molecular wave packet dynamics.
Philipp Geppert, Max Althön, Daniel Fichtner and Herwig Ott
"Diffusive-like redistribution in state-changing collisions between Rydberg atoms and ground state atoms"
Nature Communications 12, 3900 (2021)
Exploring the dynamics of inelastic and reactive collisions on the quantum level is a fundamental goal in quantum chemistry. Such collisions are of particular importance in connection with Rydberg atoms in dense environments since they may considerably influence both the lifetime and the quantum state of the scattered Rydberg atoms. Here, we report on the study of state-changing collisions between Rydberg atoms and ground state atoms. We employ high-resolution momentum spectroscopy to identify the final states. In contrast to previous studies, we find that the outcome of such collisions is not limited to a single hydrogenic manifold. We observe a redistribution of population over a wide range of final states. We also find that even the decay to states with the same angular momentum quantum number as the initial state, but different principal quantum number is possible. We model the underlying physical process in the framework of a short-lived Rydberg quasi-molecular complex, where a charge exchange process gives rise to an oscillating electric field that causes transitions within the Rydberg manifold. The distribution of final states shows a diffusive-like behavior.