At low electron energies, lower than about 10eV, electron collision cross sections for all molecules cannot determine the exact values. We can determine the same cross sections for atoms, if we have no concentration of dimmers of atom.
In the experiments of electron and photon collisions with molecules, many molecules are rotationally excited [M(r)], and some molecules are vibrationally excited [M (v)]. Except the light molecules, such as H₂ and N₂, there are no concentration of ground state molecules [M (g)]. This is the initial condition of all experiments of electron and photon collisions. The concentration of M(r) and M(v) depends on gas temperature. We can see a great many − more than one thousand − papers on temperature dependence of photon and electron collision cross section data for many molecules. The cross section values for M(r) and M(v) are always different. All molecules are mixture of M(r₁) + M(r₂) + … + M(v₁) + M(v₂) + ….
Probably, differences of cross section data of M(r₁) and M(r₂) are not so large, but differences of M(r₁) and M(v₁) are large. In triatomic molecules, such as CO₂ and N₂O, there are some clear and concrete data of large difference of cross sections of M(r) and M(v). This large change are occurred by the deformation of molecules by vibrational excitation. Some linear molecules change to bending molecules, and bending molecules have the dipole moment. This phenomena occur by gas temperature, and this is the initial condition of the collision experiments of molecules.
Many scientists belong to photon collision community know well these facts by great many complicated experimental data. But many electron collision group persons do not recognize this simple conclusion. One example will be discussed here. DCS values of elastic electron collision cross section for molecule are measured at 2eV and 300K, for example. And the measured data are compared with theoretically calculated values of DCS. Experimental data are not coincide with theoretical values for many molecules. The theoretical values are calculated by assuming all molecules are in the ground state, M(g). But DCS values obtained by experiments are weighted mean values of DCS of M(r) and M(v) molecules, determined by the concentration at 300K.