Complex sentence: We have a collision between two dissimilar particles when a beam of fast particles is fired into a gas containing another type of particles, or two dissimilar molecules randomly collide in a gas mixture, and the above formula for scattering cross section has to be modified.
Split into simpler sentences: We have a collision between two dissimilar particles when a beam of fast particles is fired into a gas containing another type of particles, or two dissimilar molecules randomly collide in a gas mixture. For such cases, the above formula for scattering cross section has to be modified.