Quantum-State Controlled Reaction Channels in Chemi-ionization Processes: Radiative (Optical-Physical) and Exchange...

Area of Science:

• Physical Chemistry
• Chemical Physics
• Theoretical Chemistry

Background:

• Chemi-ionization reactions, involving charge transfer (CT), are fundamental to many natural and industrial processes, including flames, plasmas, and astrochemistry.
• Despite their significance, a unified theoretical description for these reactions, characterized by electronic rearrangements in collisional complexes, has been lacking.

Purpose of the Study:

• To introduce a novel general theoretical methodology for describing chemi-ionization reactions.
• To demonstrate that these reactions are prototypes of gas-phase oxidation processes occurring via two distinct microscopic mechanisms.
• To elucidate how the relative importance of these mechanisms varies with collision energy and separation distance.

Main Methods:

• Development of a new general theoretical methodology for chemi-ionization reactions.
• Application of the methodology to collisions involving excited Neon (Ne*) and noble gases (Ng).
• Analysis of reaction mechanisms at different collision energies (thermal, hyperthermal, subthermal) and separation distances.

Main Results:

• Identified two distinct microscopic mechanisms for chemi-ionization: a chemical oxidation pathway (dominant at higher energies) and a physical radiative photoionization pathway (dominant at subthermal energies).
• Demonstrated that these mechanisms compete in the thermal energy regime, with their relative importance dependent on collision energy and distance.
• The new method is consistent with prior theoretical descriptions and reproduces key experimental findings over the past 40 years.

Conclusions:

• Chemi-ionization reactions can be unified under a single theoretical framework, revealing a transition from chemical oxidation to photoionization as collision energy decreases.
• The methodology accounts for factors like orbital alignment and angular momentum couplings, providing a detailed description of the collision complex.
• The theoretical approach is extensible to other reaction types, including redox and acid-base reactions in condensed phases.