Cold chemistry with electronically excited Ca+ Coulomb crystals

Area of Science:

• Atomic, Molecular, and Optical (AMO) Physics
• Chemical Physics
• Quantum Chemistry

Background:

• Understanding ion-molecule reactions at low temperatures is crucial for astrochemistry and precision measurements.
• Laser cooling techniques enable the study of fundamental chemical processes at ultralow collision energies.

Purpose of the Study:

• To measure bimolecular rate constants for reactions between laser-cooled Ca(+) ions and polar molecules (CH(3)F, CH(2)F(2), CH(3)Cl).
• To investigate the influence of Ca(+) electronic states on reaction dynamics at low collision energies.

Main Methods:

• Doppler laser cooling of Ca(+) ions to form Coulomb crystals.
• Reaction of trapped ions with cold polar molecule beams or gas.
• Monitoring reaction progress via Ca(+) ion fluorescence imaging.
• Product ion identification using resonance-excitation mass spectrometry.
• Modeling electronic state populations using optical Bloch equations.

Main Results:

• Bimolecular rate constants were extracted for ground and excited states of Ca(+).
• Ground state reaction rate constants show suppression due to potential energy barriers.
• Excited state reaction rate constants are consistent with capture theory predictions.

Conclusions:

• The presence of barriers on the ground state potential surface explains suppressed reaction rates.
• Capture theories accurately describe reaction dynamics for excited Ca(+) states.
• This work provides insights into low-energy ion-molecule reaction mechanisms.