<br><br>
 It is very common to describe the strength of a laser field by its intensity, rather
 than using the electric-field amplitude. In atomic units (or, more precisely, in any
 Gaussian system of units), the relationship between instantaneous electric field
 and intensity is:
 <math> I(t) = \frac{c}{8\pi} E^2(t) </math>.
<br><br>
 It is common to read intensities in W/cm^2. The dimensions of intensities are
 [W]/(L^2T), where [W] are the dimensions of energy. The relevant conversion factors
 are:
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