What is ultracold chemistry?

By taking advantage of the latest technical developments in physics that allow us to produce very slowly-moving atoms and molecules in the gas phase, we are developing novel experiments to study the bimolecular chemical reactions of these species under such extreme low temperature conditions.


What does ‘cold’ or ‘ultracold’ imply for chemistry?

¨      Molecules moving slowly
The mean speed of gas phase NH3 molecules:
               at 298 K = 600 m s-1
               at 30 mK = 6 m s-1    (cold)
               at 3 x 10-6K = 6 cm s-1  (ultracold)

but this is a purely classical perspective: at low temperature, wave-particle duality becomes important.


Maxwell Boltzmann distribution

Wave effects may dominate the collisional behaviour if the deBroglie wavelength (p = h/λ) is long compared to molecular dimensions (range of interaction)

NH3 at 300 K   λ = 0.023 nm
       at 3 K      λ = 0.23 nm
       at 30 mK   λ = 2.3 nm
       at 3 K       λ = 230 nm

Thus at 30 mK the de Broglie wavelength is already an order of magnitude longer than molecular dimensions
  
Low temperature for molecules also means population of few internal quantum states (vibration or rotation). Our goal in studying chemical reactions at very low temperatures is to investigate how the transition to the quantum regime leads to novel aspects of the collision dynamics.

What is different about chemical collisions below 1K?

  • Tunnelling and quantum reflection
  • Importance of long-range potential (KE comparable with PE)
  • Highly restricted and quantised angular momentum of collision - Threshold Laws
  • Scattering resonances - Rotational and translational energy comparable
  • Electromagnetic field control

 

Interest from chemists’ viewpoint

Our ability to test experimentally quantum mechanical calculations of reactive processes is still very limited, so experiments are needed in a regime where quantum effects dominate. Ultracold reactions offer the possibility to develop new methods of controlling chemical processes (e.g., using electromagnetic fields)

Focus on barrierless or low-barrier reactions

  • Radical – radical or some radical-non-radical (insertion)  e.g., CH + ND3
  • Atom-molecule O + OH
  • Reactions of electronically excited species (e.g., metastable N2 or CO, metastable He, Rydberg species)
  • Ion – neutral: e.g., Ca+ + H2O -> CaOH+ + H
  • Photodissociation
  • Surface adsorption

Experimental challenges for ultracold chemistry below 1K

General methods need to be developed for deceleration and cooling molecules and atoms.

Methods are needed that cool sufficient molecules to detect products of reactions (or can trap cold molecules on sufficient timescale)


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