Cold Collision Experiments Based on Atomic Fountain and Ion Velocity Map Imaging


Cold Collisions

Cold collisions refer to scattering processes between atoms or molecules where the collision energy lies between that of ultracold collisions (< 1 mK) and thermal collisions (≥1 K). In ultracold collisions, the presence of the centrifugal barrier often restricts the process to be dominated by partial waves with the lowest angular momentum (S-wave or P-wave). In thermal collisions, quantum mechanical effects are usually obscured by the contribution of a large number of partial waves, and the scattering process tends to follow semi-classical behavior. The cold collision regime, situated between these two extremes, involves multiple partial waves while still exhibiting significant quantum effects due to angular momentum quantization. Consequently, a rich array of quantum phenomena can be observed, such as discrete angular momentum states, interference between partial waves, shape resonances, and more.


Autoionizing Collisions

In collisions between electronically excited atoms and other atoms or molecules, an intermediate complex may form which subsequently undergoes autoionization, emitting an electron to yield atomic or molecular ionic products. Such collision processes are commonly observed in systems involving metastable rare gas atoms (Rg*) colliding with target atoms or molecules (A):

Rg*+A → Rg+A++e- (Penning ionization)

Rg*+A → RgA++e- (Associative ionization)

Our research focuses on the system comprising metastable Krypton (Kr*) atoms and Rubidium (Rb) atoms. Future studies are planned to extend this work to other metastable species such as Argon (Ar*) and Helium (He*), as well as a wider variety of target atoms or molecules.


Integration of Laser Cooling and Velocity Map Imaging Techniques

We have developed a novel experimental configuration that integrates the laser-cooled atomic fountain technique with ion velocity map imaging (VMI). This setup enables direct measurement of differential scattering cross-sections for Penning ionization products with high precision across a broad range of collision energies (15 mK ~ 8 K). The principal advantages of this innovative approach include:

  • The low collision energy spread inherent to laser-cooled atoms.
  • The high-resolution capability of ion velocity map imaging for detecting final-state velocities.

    •

    Quantum Phenomena

    Within the collision energy range of 15 mK to 1 K, we have observed interference between partial waves, potential signatures of scattering resonances, and a significant dependence of the scattering cross-section on the initial atomic angular momentum states.