Number of Ionization event estimation

For a single photon scattering/absorption event with a cross section ${sigma}$, which is $[L^2]$, or an area, the number of scattering events per pulse is:

$$ N = N_{gamma}rho_Asigma $$

Here

$$N_{gamma} = frac E {hbaromega}$$

is the number of photons in a pulse, and

$$rho_A = rho_{N_2}times L$$

is the areal density ($[L^{-2}]$) of target molecules. $ rho_{N_2}$ is the number density of molecules and $L$ is the length of the target chamber.