Watching Chemical Reactions on the Quantum Level
State-to-state Measurements of Ultracold Three-body Recombination
- Fig. 1: Three-body recombination process and state selective ionisation of molecules. (1) Three atoms come close enough to undergo a chemical reaction. (2) In the reaction a molecule is formed, and the binding energy is released as kinetic energy. (3) With the help of a laser the molecule is quantum state-selectively excited via an optical transition. (4) A second laser ionises the excited molecule which can then be detected very efficiently.
- Fig. 2: Measured population distribution of molecular reaction products over a range of quantum states, after three-body recombination. The plot shows the probability that a molecule is produced with specific vibrational and rotational quantum numbers v and R, respectively. The electronic and nuclear spin quantum numbers are also precisely determined, but are not listed here.
Everywhere in our lives we encounter chemical reactions. They belong to the most fundamental processes in nature and represent a main economic pillar in society. Despite more than a hundred years of research, reaction mechanisms are still not fully understood when it comes to the most fundamental level.
This is partially due to the fact that the full determination of reaction paths on the quantum level was essentially not possible so far, because of the experimental challenges in preparation and detection. Recently, however, these limitations are being overcome step by step.
Ultracold quantum gases have proven to be ideal model systems for this purpose. Ultracold means that atoms or molecules are cooled down to temperatures below one microkelvin using laser cooling and other cooling methods. At these temperatures the particles can be controlled very well allowing for preparation of the reactants in a precisely-defined quantum state. In addition, the collision energy and the interaction strength of the colliding particles can be tuned on demand.
In general, molecules are very complex objects featuring, besides vibration and rotation, additional internal degrees of freedom such as electronic and nuclear spins. All these parameters are described by a set of quantum numbers. Now, what is the role of these degrees of freedom in a reaction? Which are the quantum states the molecules are produced in in a chemical process? Those are the questions that are tackled at the Institute of Quantum Matter at the University of Ulm.
Collisions in an Ultracold Rubidium Gas
For this purpose, a novel method was developed to detect molecules resolving their quantum numbers [bit.ly/GLJ-Denschlag]. One central component of this method is to ionize a freshly formed molecule in a quantum-state selective way and to subsequently detect the ion.
To demonstrate this method, the molecular products of a three-body recombination process were investigated. Initially, an ultracold cloud of approximately four million rubidium atoms was prepared and trapped in a small volume with the help of laser light. Every now and then, three atoms of the cloud collide spontaneously (Fig.
1.1) and a Rb2-molecule can be formed (Fig. 1.2). In this experiment the formed molecule is ionised in two steps. A first laser transfers the molecule state-selectively into a pre-determined electronically excited intermediate state (Fig. 1.3). From this state it is ionized with a second laser (Fig. 1.4). Afterwards, the individual molecular ions are detected with almost 100% efficiency in a Paul trap. Besides its high selectivity this method is also characterized by essentially background-free signals.
The measurements (Fig. 2) reveal that in the given ultracold recombination reaction the molecules are mainly produced in vibrationally highly-excited states, corresponding to small binding energies. In fact, about 50% of all molecules formed by three-body recombination populate the state with the highest vibrational quantum number, v = 40 (see Fig. 2). A molecule in this state has a binding energy of only 24 MHz x h (where h is the Planck constant) and has a size of about 100 Bohr radii.
For symmetry reasons based on quantum statistics, only products with even rotational quantum numbers can be formed (Fig. 2). Generally, in these experiments, only molecules with rather low rotational excitation of up to six quanta (R = 6) were observed. However, such an excitation is still remarkable, since initially the system of three colliding ultracold atoms does not have any rotational angular momentum. In a classical picture, at first the freshly formed molecule is nonrotating. At the final stage of the reaction, however, it gets an asymmetric little kick from the leaving third atom which gives rise to the rotational excitation.
Surprisingly, for a given vibrational state the production rates of molecules for increasing rotational quanta R do not progress in a simple, monotonic manner (Fig. 2). Instead, the population distribution for the rotational states features variations, which could be explained by interference effects between the product channels. Hence the production of molecules in certain states is either amplified or supressed.
Interestingly, in these experiments neither the electronic nor the nuclear spins seem to flip during the reaction. When a weakly bound molecule is formed, the involved atoms are typically separated by large distances which might partially explain the weak interaction of the spins. Another reason might be the particular choice of our reactant atom, the 87Rb isotope, which is already known for spin-flip suppression in two body collision.
Thus, it is going to be interesting to extend the presented research to more deeply bound states, other atomic and molecular species, and different chemical reaction processes. The apparatus and the detection method are flexible enough in this respect, and the basic requirements for such experiments are already available in several laboratories worldwide. It might be possible to even use a modified version of the developed method for studies of thermal ensembles, e.g. by making the detection scheme insensitive to the Doppler shift. Some day in the near future, reaction processes might be understood to such an extent that they can be fully controlled on the quantum level, e.g. by using external electromagnetic fields or the exploiting of interference effects between the reaction channels. This may allow for suppression of unwanted reaction products and for an increased production efficiency of desired products.
Joschka Wolf1, Markus Deiß1, Johannes Hecker Denschlag1
1Institut für Quantenmaterie, Universität Ulm, Ulm, Deutschland
Joschka Wolf, Markus Deiß, Artjom Krükow, Eberhard Tiemann, Brandon P.Ruzic, Yujun Wang, José P. D’Incao, Paul S. Julienne, and Johannes Hecker Denschlag, State-to-state chemistry for three-body recombination in an ultracold rubidium gas, Science 2017, 358, 921; DOI: 10.1126/science.aan8721.
Trapping Moss Spores in a Paul Trap: