where c 1 and c 2 are constants. The values c 1 = 3 and c 1 = 1 are widely quoted, but more sophisticated models of the HHG process afford c 1 = 3.17 and c 2 = 1.32, or else c 1 = 3.34 and c 2 = 1.83. Defining a phase θ = ω 0 t, with an initial (t = t ini ) value θ ini and a recombination (t = t rec ) value θ rec , then the pair of values θ ini + nπ and θ rec + nπ also satisfy z(θ) = 0, for integer values of n. The displacement z and velocity ż both change sign every half cycle, z(θ) = -z(θ -π). As a result, the harmonic field E HHG (t) can be expressed as 2