Buffer gas cooling of polyatomic ions in rf multi-electrode traps.

Area of Science:

• Atomic, Molecular, and Optical Physics
• Chemical Physics
• Physical Chemistry

Background:

• Cooling molecules, clusters, and nanoparticles to ultracold temperatures is experimentally difficult.
• Existing methods like laser cooling or electric field deceleration are often insufficient for cooling vibrational or rotational molecular motion.
• Ultracold chemical reactions require effective cooling schemes for all degrees of freedom.

Purpose of the Study:

• To present a novel method for cooling charged particles in an rf trap using slow neutral beams.
• To demonstrate the preparation of internally cold molecular ions for spectroscopy and chemistry.
• To analyze the kinematic conditions for ultracold collisions and predict rate coefficients.

Main Methods:

• Utilizing a pulsed cold effusive beam of slow neutrals (H, He, H2, D2) to interact with ions in a multi-electrode rf trap.
• Employing a shutter to remove fast neutrals from the beam, ensuring interaction with only slow neutrals for sub-Kelvin temperatures.
• Developing methods to determine ion temperature and analyzing collision kinematics.

Main Results:

• Experimental results for hydrogen abstraction in collisions between slow H atoms and CH5+ at meV energies.
• Detailed analysis of kinematic conditions in the slow neutral beam-ion trap arrangement.
• Demonstration of potential for cooling ions like protonated methane and H3+ with slow H atoms.

Conclusions:

• The developed method enables cooling of charged particles to sub-Kelvin temperatures, facilitating ultracold chemistry.
• The study provides insights into hydrogen abstraction reactions at ultracold energies.
• Formation of weakly bound ions like H4+ and CH6+ is identified as a pathway to preparing collision complexes near the dissociation continuum.