Association reactions at low pressure. III. The C2H2+/C2H2 system

Area of Science:

• Physical Chemistry
• Chemical Kinetics
• Mass Spectrometry

Background:

• Ion-molecule reactions are fundamental in astrochemistry and plasma physics.
• Understanding reaction mechanisms and rate coefficients is crucial for modeling chemical processes.
• Acetylene (C2H2) is a key molecule in various chemical environments.

Purpose of the Study:

• To investigate the association reactions of C4H2+ and C4H3+ with C2H2.
• To determine the rate coefficients for bimolecular and termolecular association pathways.
• To elucidate the stabilization mechanisms (radiative vs. collisional) of the resulting ion-molecule complexes.

Main Methods:

• Utilized an ion cyclotron resonance mass spectrometer.
• Conducted experiments at low pressures (8 x 10(-8) to 1 x 10(-4) Torr) and controlled temperature (298 K).
• Analyzed reaction products and determined rate coefficients under varying pressure conditions and with different third bodies (C2H2, N2, Ar, Ne, He).

Main Results:

• Observed two distinct association mechanisms: bimolecular at low pressures and termolecular at higher pressures.
• Quantified bimolecular rate coefficients (k2) for C4H2+ and C4H3+ with C2H2.
• Quantified termolecular rate coefficients (k3) with C2H2 and other third bodies, indicating collisional stabilization.
• Determined lifetimes of the collision complexes ((C6H4+)* and (C6H5+)*) and the extent of radiative stabilization.

Conclusions:

• Bimolecular association is primarily driven by radiative stabilization, while termolecular association relies on collisional stabilization.
• The internal energy of product ions influences their propensity for radiative association.
• Provides critical kinetic data for ion-molecule reactions involving acetylene, relevant for atmospheric and interstellar chemistry.