Low temperature chemistry has been predicted to be dominated by quantum effects in the collision coordinate, such as shape resonances, since the wavelike properties of colliding particles become resolved as their de-Broglie wavelength increases. Observation of these quantum effects provides valuable insight into what governs scattering processes. Our recent advances in the control of neutral supersonic molecular beams, namely merged beam experiments, have enabled continuous tuning of collision energies from classical room temperature down to 0.01 kelvin, where quantum descriptions of the dynamics are necessary.
I will discuss our use of this technique to study how the dynamics change when molecules are involved in collisions, since new internal degrees of freedom are introduced such as rotation and vibration of the molecule. We have found their effect at low energies to be profound. I will demonstrate how ground and excited rotational states of a molecule change the effective atom-molecule interaction. For slow processes, which are dominated by resonances, different rotational states lead to shifts of the resonance energies allowing us to probe the interaction with spectroscopic precision, separating the anisotropic and isotropic components. In the unitary process limit, where every collision is inelastic, the rate is determined only by the long-range forces leading to Langevin universal power law scaling. Therefore measuring the power law gauges the type of long-range force directly.
Interestingly, we have found that in cold collisions the state of quantum molecular rotor selects the type of long-range force the colliding particles encounter, where the effect originates in the symmetry of the molecular wavefunction.
I will also present our latest preliminary results where we study ion-neutral interactions by velocity map imaging. We observe quantum effects in atom-molecular-ion interactions likely caused by Feshbach resonances coupled to different vibrational states of molecular-ion.