According to David van der Spoel: 

the field has the form of a gaussian laser pulse

E(t) = E0 exp ( -(t-t0)^2/(2 sigma^2) ) cos(omega (t-t0))

the four parameters are set like

E-x = 1 E0 0

E-xt = omega t0 sigma

In the special case that sigma = 0 the exponential term is disregarded

and only the cosine term is used.

More details in the attached paper.