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.