Ortho-Para Conversion for H⁺ + H₂ Collision in Low Temperature: A Fully Close-Coupled Time-Dependent Wave Packet...

Area of Science:

• Physical Chemistry
• Astrochemistry
• Quantum Dynamics

Background:

• The H+ + H2 reaction is fundamental to understanding interstellar chemistry.
• Ortho-para conversion of molecular hydrogen influences its cooling and chemical evolution in cold environments.
• Accurate rate coefficients are needed to model these processes in the interstellar medium.

Purpose of the Study:

• To calculate collision-induced rate coefficients for ortho-para conversion in the H+ + H2 reaction.
• To investigate the influence of low collision energies and rotational states on the conversion process.
• To provide data for probing cold interstellar environments.

Main Methods:

• Coupled three-dimensional (3D) time-dependent wave packet (TDWP) formalism in hyperspherical coordinates.
• Ab initio ground adiabatic potential energy surface of H3+.
• Calculation of rotationally resolved reaction probabilities and cross-sections at low collision energies (0 < E_col ≤ 0.3 eV).

Main Results:

• Converged reaction probabilities and cross-sections were obtained for H+ + H2 (v=0, j=0-5) → H+ + H2 (v'=0, j').
• Rate constants for ortho-to-para (O-P) and para-to-ortho (P-O) conversions were determined.
• The ortho-para conversion ratio showed convergence with increased rotational states and agreement with quantum statistical methods, featuring a peak at lower temperatures.

Conclusions:

• The study provides accurate collision-induced rate coefficients for H+ + H2 ortho-para conversion.
• The calculated ratio offers insights into the lifetime of cold interstellar environments.
• A low-temperature peak in the conversion ratio is attributed to specific rotational state transitions.