# Irradiance and electric field strength

## Main Question or Discussion Point

Dear physicists,
I am desperate. I am using a QM code to propagate a wavefunction of a system that experienced an electric field pulse. However, in literature all values for the electric field strength are given in V/m but the program asks for W/cm^2! I am kind of very confused here and did not find any answer that might relief me from confusion... Does anyone know how this can be done?

## Answers and Replies

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Well, finally with the help of my flatmate I figured it out. I post it just in case anybody else like me has this problem....
One has to solve the integral

$$\frac{1}{\Delta T} \int \frac{1}{2} \epsilon_{0} |E|^{2} d^{3}r$$

$$\Delta T$$ is the duration time of the pulse, E the electric field strength in V/m and $$\epsilon_{0}$$ is the permittivity of vacuum. r is the length of the simulation box where the molecule is located in. This gives me the power of the field in W. Finally, divide this by the area of the side perpendicular to the axis along which the pulse enters the simulation box and W/m^2 results.