CUI: Advanced Imaging of Matter
Cluster of ExcellenceCUI: Advanced
Imaging of Matter
Imaging of Matter
Physicists employ ion microscope to observe unusual collisions in slow-motion
24 September 2024
Photo: UHH, AG Schmelcher
Atoms in excited Rydberg states have large dipole moments and thus interact with an ion (green sphere) over large distances. This interaction mixes different internal states of the Rydberg atom with differing dipole moments, giving rise to a manifold of collision channels (blue cylinders). Fast colliding pairs of particles (red sphere) can undergo diabatic transitions between neighboring collision channels, whereas initially slow ion-Rydberg pairs (blue sphere) do not transition between channels and instead fall rapidly inward.
With the help of a novel ion microscope experiment, researchers at the Universities of Stuttgart and Hamburg have observed and understood the collisional behavior between highly excited and charged particles inside an ultracold gas. They report on their experiment and theory collaboration, which is important for understanding fundamental processes in nature, in the journal Physical Review Letters.
In particular, they observed collisions between a positively-charged ion and a Rydberg atom, whose highly-excited valence electron means that the two particles interact over unusually large distances of several micrometers - comparable to the width of a human hair! The long-range nature of this interaction further means that the collisions take place on time scales of several microseconds, approximately six orders of magnitude slower than the internal dynamics of more familiar molecular systems, such as the vibrational dynamics of a water molecule.
“The ion-Rydberg collisions take place in a regime in which the well-known Born-Oppenheimer approximation breaks down,” says Peter Schmelcher, who is a professor at the University of Hamburg and a researcher in the Cluster of Excellence “CUI: Advanced Imaging of Matter”. This approximation is a cornerstone of modern theoretical chemistry, yet due to the dense energy level structure of the Rydberg atom, it is no longer valid. As a result of this break down, the pair of particles may collide along many different possible channels (denoted by the blue cylinders in the figure) and have a probability to transition between neighboring channels whenever the energies of a pair of channels come sufficiently close.
The unique shape leads to unusual properties
The unique shape of the collision channels in this system leads to a further unusual property: particles which start at t=0 with a slower velocity (blue sphere in the figure) are less likely to transition between collision channels, causing them to experience a large inward acceleration as they reach the steep strongly-polar region of the collision channel. Thus, these particles collide faster than those starting with a higher velocity (red sphere in the figure) which have a greater probability to transition between channels and thereby remain longer on the weakly-polar regions of the collision channels. “In other words,” Schmelcher says, “by cooling the system down, one observes faster overall collisions.”
The scientists are able to further influence which channels the particles follow by changing the frequency of their excitation laser, which precisely controls the initial separation of the ion-Rydberg pair as well as the energy of the Rydberg atom’s valence electron.
The experimental observations made in Stuttgart are supported by simulations of a semi-classical molecular dynamics model performed in Hamburg.
The study of physical systems such as this has the potential to shed greater light on the role of physics beyond the Born-Oppenheimer approximation, which is important for understanding a variety of fundamental processes in nature, such as photosynthesis and the breakdown of DNA due to UV light.
Original Publication
M. Berngruber, D.J. Bosworth, O.A. Herrera-Sancho, V.S.V. Anasuri, N.Zuber, F. Hummel, J. Krauter, F. Meinert, R. Löw, P. Schmelcher and T. Pfau
In Situ Observation of Non-Polar to Strongly Polar Atom-Ion Collision Dynamics
Physical Review Letters 133, 083001 (2024)
