When one thinks about chemistry, one doesn't usually consider quantum mechanics to play a part. Yet it does. When it boils down to it, all matter is a combination of a handful of subatomic particles and the forces holding them together. Chemistry, is in essence, applied physics. For decades, scientists have been trying to determine how to follow a chemical reaction from its initial state through all of its quantum states to its products. The hope was that, by doing so, researchers could understand the quantum dynamics that drive these reactions. Until now, it has mostly been speculation. However, a recent paper published in Nature by Liu et al. suggests that this may be possible.
An Ultracold Reaction
The paper explicitly studies the reaction of a diatomic compound. The compound consists of one rubidium atom and one potassium atom (KRb). The reaction was performed at ultra-low temperatures, and the researchers were able to track the reaction from the single compound to its final state. In monitoring the reaction, Liu et al. followed the resulting products through all fifty-seven of its possible quantum states. The results established the validity of quantum-statistical models of reactions.
Why an Ultracold Reaction?
As any high-school chemistry student can tell you, reactions increase with temperature. Temperature lends energy to molecules, allowing them to run into each other at an increasing volume. With each collision, a reaction occurs. Ultracold reactions, therefore, mean a slower movement of the molecules involved. Recent years have seen ultracold prepared molecules being sued to study the underlying quantum elements of a reaction. Ultracold diatomic molecules of alkali metals provide a perfect window to observe these phenomena. The metals are in their absolute ground state. The journal Molecular Physics notes that ultracold molecules usually have very low electronic, rotational, and vibrational states. The molecules also have an almost non-existent translational kinetic energy - the energy required to move through space.
Ultracold reactions allow researchers to delve deeper into chemical reactions that involve reactants in a single internal quantum state. Reactants in a single internal quantum state demonstrate the same rotational, vibrational, and electronic states. This situation represents the best test-bed science has for collecting data about the quantum-dynamical models of chemical reactions. Once enough data is collected and calculations done, the data can be used to model more complex interactions with a certain degree of accuracy. Potassium and rubidium were used for this reaction because of the properties of two KRb molecules interacting with each other. The KRb reaction is exoergic - meaning it releases energy. This release of energy sustains the reaction, allowing it to proceed even though the component molecules are at a state of near-zero energy.
A Low-Energy Interaction
When two KRb molecules interact, they increase the rotational and translational energy of each other. There is not enough energy involved to affect the vibration of the molecules. Thus, if a team were to measure the results of the KRb reaction, they would need to determine the rotational energies of the resulting Rb2 and K2 molecules. How do they manage that? Until recently, no one had measured the quantum states of reaction products from ultracold reactions. However, some of the authors of this same paper previously worked on a method for using mass spectrometry to measure the velocities of beams of molecules. They took to adapting this methodology for use in inspecting the products of these reactions. The real challenge lay in determining if the rotation of the measured molecules was produced in the same reaction event.
Using a technique that most high school physics students would recognize, the team relied on the principle of conservation of momentum. When a reaction occurs between two KRb molecules, the results fly off at roughly the same momentum as the incoming particles had. By using spatial correlation, the research team was able to pinpoint which molecules resulted from what reactions. They could then track the progress of those products through their differing quantum states.
Not About the Numbers
While this methodology doesn't really give us hard numbers to work with, it does prove that a quantum-statistical model is viable for predicting the quantum states of the products of a reaction. The probability of the result depends on the state's degeneracy. Liu's team doesn't measure each state degeneration but gives us a probability for finding a particular quantum state in a random selection of the products.
More Interesting Results
The precise control of the energetic states of the molecules allowed the team to work out that the minimum translational energy needed for threshold states was around 1mK (approximately 10^-7 eV). The authors' experimental results differ from the model, and for good reason. At 1mK, matter can be viewed as a wave, allowing it to "tunnel" across the centrifugal barrier. This tunneling is responsible for the discrepancy in readings. Whether we can use this fine control over quantum states in more complex reactions remains to be seen. Liu and his team have proven that we can at least generate a quantum-statistical model that gives us a feel for some reactions.