Skip to main content

Chemistry of the Quantum Kind

Final Report Summary - QUCC (Chemistry of the Quantum Kind)

Interstellar space is the natural environment where reactions take place at temperatures as low as several Kelvin. Importantly, at such low temperatures wave-particle duality becomes important and quantum effects start dominating reaction processes. For example, reactants that do not have enough energy to overcome a potential energy barrier classically may proceed to react “using” the matter-wave property that allows tunneling under the barrier. It is important to understand such fundamental processes in order to describe reactions and collisions taking place deep in the quantum regime. Together with the state-of-the-art theoretical approaches this enables the most stringent tests of our current modeling of various processes taking place at low temperatures.
Recreating such cold conditions in laboratory has been a long standing goal and a grand challenge in the field of molecular science. Whereas success of laser cooling of atoms brought a revolution to atomic physics, cooling molecules proved to be far more complicated task. Interestingly, a simple method already existed for more that fifty years that did allow cooling of almost any gas. It was used by the chemical physics pioneers Dudley Herschbach and Yuan T. Lee both of whom received the Nobel prize in 1986. The cooling method is based on an adiabatic expansion of gas from high pressure to vacuum. Although during expansion gas cools to temperatures below 1K, the gas “cloud” also accelerates to very high velocities. Whenever two such cold gas “clouds” carrying reactants collide they meet together at very high relative velocities that can be translated to temperatures above 300K. We have solved this problem by doing our collisional experiment in the moving frame of reference. We merged two clouds using a very high magnetic field gradient. This simple step allowed us to reduce the collision energy by a factor of 1000 as compared with the earlier efforts. This allows us now to routinely perform experiments that unveil different aspects of matter-wave properties in molecular collisions.
We have experimentally demonstrated the elusive phenomena of quantum resonances in low energy reactive collisions. At low energies particles that tunnel through a potential energy barrier may become “stuck” at short separations while orbiting each other. These special states can be observed by measuring reaction rate probability as a function of collisions energy. Whenever collision energy matched the resonance position we observed a very strong, in some cases a 10-fold enhancement in reactivity. Surprisingly at resonance energy reactions that were strongly suppressed at sub-Kelvin energies proceeded as fast as at room temperature.
Using the quantum resonances as highly sensitive probes of molecular interaction we have discovered new and interesting effects that are particularly important to the most abundant molecule in the interstellar space, hydrogen. We demonstrated that molecular rotation structure plays a key role in interactions with other objects. Molecular hydrogen in the lowest rotational state behaves a symmetric “ball” whereas molecular hydrogen in the excited rotational state takes a different shape and reacts with a different rate. We have also found a new isotope effect that may dramatically change reaction rates that take place through quantum effects at low collision energies.