A new approximation for atom-diatom rotational-relaxation cross sections.

Area of Science:

• Chemical Physics
• Theoretical Chemistry
• Computational Chemistry

Background:

• The infinite-order-sudden (IOS) approximation is a common method for calculating S-matrix elements in molecular collisions.
• Traditional methods for collision integrals can be computationally intensive.
• Accurate calculation of energy-transfer effective cross sections is crucial for understanding chemical reaction dynamics.

Purpose of the Study:

• To introduce a novel semiclassical approximation to the S matrix within the infinite-order-sudden approximation framework.
• To develop a purely classical approximation for energy-transfer effective cross sections, analogous to existing methods.
• To incorporate constraints on energy and angular momentum transfer into the new approximation.

Main Methods:

• Developed a semiclassical approximation for the S matrix of the infinite-order-sudden approximation.
• Derived a classical approximation for energy-transfer effective cross sections.
• Included energy and angular momentum transfer constraints.
• Performed numerical evaluations alongside traditional collision integral calculations.

Main Results:

• The new approximation yields results comparable to full classical trajectory calculations.
• Differences were within 15% for tested systems including H-N(2), He-N(2), He-CO, and Ar-CO(2).
• The method is computationally feasible alongside traditional collision integral calculations.

Conclusions:

• The introduced semiclassical approximation provides an efficient and accurate method for calculating energy-transfer effective cross sections.
• This approach offers a valuable alternative to full classical trajectory calculations, especially for complex systems.
• The method's accuracy and computational tractability make it suitable for broad application in molecular collision studies.