Cold Chemistry
The Heather Lewandowski group investigates molecular dynamics and chemical reactions at temperatures of tens of milliKelvin. Lewandowski and her students want to learn more about the strong correlation between cold chemical reactions in the laboratory and the interstellar chemistry that takes place at temperatures of about 3 K. More importantly, they want to understand how chemical reactions proceed when all of the states of the molecules can be controlled in the quantum regime.
The Lewandowski group has begun studies of the control of cold chemical reactions with the precise manipulation of both the internal states of atoms and molecules and their external states (i.e., position, velocity, and orientation) during interactions. The control of external states of atoms and molecules at milliKelvin temperatures is much easier than at warmer temperatures. A wide distribution of their position, velocity, and orientation occurs at higher energies, making it nearly impossible to precisely control these states.
Lewandowski and her group are currently developing the tools they need to figure out how to precisely control external states of atoms and molecules at very cold temperatures. This control will allow them to meet the challenge of investigating how a change in a single internal or external parameter of a reactant can alter a chemical reaction. In the long term, this research is expected to lead to a precise and detailed understanding of specific chemical reactions on the quantum mechanical level.
Stark decelerator used to brings molecules to temperatures of tens of milliKelvin.
The group uses a technique known as Stark deceleration to cool and slow packets of molecules until the packets come to a stop at temperatures in the tens of milliKelvin range. At these temperatures, there’s simply not enough energy available to excite the atoms and molecules out of their ground electronic, rotational, or vibrational states. Once they are in this very low-energy state, the researchers can trap molecules and observe them for periods of 1–10 seconds—which is 3–5 orders of magnitude longer than scientists are able to observe chemical reactions in molecular beams.
The long observation times should allow the group to observe slow chemical reactions. One important goal is to observe and study quantum mechanical tunneling through the activation barrier that can inhibit chemical reactions. At very cold temperatures, quantum mechanical theory suggests that the system may tunnel through such barriers and initiate chemical reactions.
The Lewandowski group has embarked on an experimental project to understand the quantum mechanical nature of a simple chemical reaction, the charge-exchange reaction between NH3+ and Rb that yields NH3 and Rb+. This experiment is closely related to other experiments on cold molecule collisions in the Lewandowski lab.
Lewandowski and her co-workers are also working with theorist Chris Greene on a study of cold collisions between NH molecules and Rb atoms. In their analysis of Rb and NH scattering, Greene and his collaborators explored the consequences of an interesting accident of nature that has the experimentalists pretty excited: the first excited Rb state has almost the same energy as the first excited state of NH. This means that once you trap a gas of cold Rb atoms in the ground state (which is relatively straightforward to do), collisions between the cold Rb atoms and cold, excited NH atoms will produce cold ground-state NH atoms with very high efficiency. The Lewandowski group plans to use cold, ground-state NH molecules in a variety of cold chemistry investigations.
The Jun Ye group has been working on Stark deceleration for more than nine years. Recently, it investigated collisions of 80 K deuterium molecules (D2) with cold (70 mK) ground-state polar molecules such as hydroxyl (OH) free radicals and formaldehyde (H2CO) molecules. These experiments should bring new opportunities for control over molecular interactions at low temperatures. The group recently studied collisions between OH radicals and supersonic beams of either helium atoms (He) or D2 molecules. The more complex D2 molecules were excited by some of the OH molecules via a collisional resonance that set D2 molecules spinning. Similar resonant energy transfer may also occur in interstellar clouds between H2 and OH molecules, leading to the emission of coherent radio-wavelength photons, or masing. In contrast, collisions between He and OH molecules demonstrated that inelastic collision channels in OH (excited to higher rotational levels) can be shut off when the intermolecular collision energy can be tuned below particular threshold values. The use of a trapped sample (OH) also allows characterization of the absolute collision cross section, a feat difficult to achieve in a regular molecular beam experiment.
Building on the D2–OH experiments, the Ye group is gearing up to probe collisions between two dipolar molecules, OH and deuterated ammonia (ND3). Because both of these molecules can be oriented with electric fields, the researchers expect to see major changes in their collision behavior as compared to the D2–OH experiments. For instance, elastic collision rates should be much faster in the presence of an aligning electric field. With respect to inelastic collisions, the experimentalists aren’t sure what to expect. The underlying dynamics are expected to be much more complex and interesting. They’re awaiting new theory models under development by collaborators including the Alex Dalgarno group at the Institute for Theoretical Atomic, Molecular, and Optical Physics (ITAMP). In the meantime, the researchers are busy developing a direct buffer gas-cooling technique to cool ammonia molecules to about 14 K. The ND3 molecules will then be used in collision experiments with 70 mK OH molecules confined in magnetic trap. This lower temperature should render the upcoming collision studies somewhat easier to interpret. The buffer gas-cooling cooling technique is being developed in collaboration with John Doyle at Harvard.
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