Collisional energy dependence of the integral cross sections for O₂ excitation . a) Theoretical ICS calculated with modified PES; b) theoretical ICS convoluted with the experimental collision energy spread calculated with the original PES (dashed line) and the modified PES (solid line); experimental data (open circles), obtained when crossing a beam of normal-H₂ from a first pulsed nozzle at ≃45 K propagating at velocity v₁ = 951 ms^-1 with a beam of 0.5% O₂ in Ne from a second pulsed nozzle at 300 K propagating at v₂ = 803 ms^-1; crossing angle χ was varied from 22.5° to 45°, resulting in a collision energy where μ stands for the reduced mass of the collision partners.