Direct Observation of a Feshbach Resonance by Coincidence Detection of Penning Ionization Products

  Baruch Margulis  ,  Julia Narevicius  ,  Ed Narevicius  
Weizmann Institute of Science

Intermolecular interactions stir the outcome of atomic and molecular collisions. The details of interactions are accessible in molecular collision experiments by detection of the products` quantum state distributions and energies.

We present a new experimental approach for the state resolved investigation of ion-neutral dynamics. Our approach utilizes an autoionization process known as Penning Ionization (PI) reaction as a state preparation step of the ion neutral system with the energy of ejected electron serving as a mark for the formed quantum state. Any post-ionization energy transitions are translated to kinetic energy of the Penning ion. Therefore, the correlation between electronic and ionic energy distributions serves as a state to state mapping of the post ionization ion-neutral dynamics.

For the realization of the presented approach, we have designed and constructed a Coincidence Double Velocity Map Imaging (CDVMI) apparatus where the 2D momentum of both ions and electrons products of PI were detected, with complete mapping between images.

As a first demonstration of this new experimental capability, we have studied the PI and Associative Ionization (AI) reaction channels of the He*(3S,1S)+Ar collision. By correlation between measured VMI images of electrons and ions we were able to directly observe a Feshbach resonance arising from the coupling between a bound electronically excited HeAr+ state to the scattering electronically ground He + Ar+(3P1/2) state. The lifetime of the bound HeAr+ state was measured as well.