Quantum Chemistry

Even at ultracold temperatures close to absolute zero, molecules can undergo chemical reaction. These reactions follow the principles of quantum physics and manifest themselves in fascinating and yet unexplained phenomena.

Ultracold diatomic ground-state molecules are ideal candidates for the search of quantum chemical phenomena. They possess a rich internal level structure which gives rise to many interesting effects. One intriguing effect is the formation of a long-lived complex, when two molecules collide. Due to resonant effects of the internal level structure, the two colliding molecules (e.g. NaK+NaK) stick together (Na₂K₂*) for a significantly long time before they detach again. During the enhanced collision time, the sticky complex might change its quantum state, for example by photoexcitation of light (Na₂K₂**) or spontaneous spin-relaxation (Na₂K₂).

Another way to investigate ultracold chemical reaction is to induce chemical exchange reactions in otherwise chemically stable molecular ensembles. Bialkali ground-state molecules, such as NaK, can be excited selectively to a higher vibrational state of the internal level structure. In a collision, those excited molecules NaK*+NaK* can undergo the exchange-reaction to the products Na₂ + K₂. This simple model reaction exhibits only a limited amount of reactant and product states which can be used to study and model the fundamental physics of chemical reaction on the basis of quantum mechanics.