where σ and δ i are, respectively, the crosssections and ith transition strengths. Similar to Eqn. 1, a line-broadening function g is applied. Cross sections are usually given in the cgs Unit System, that being cm 4 •s photon and cm 6 •s 2 photon 2 for 2PA and 3PA respectively. For the particular case of 2PA, cross sections are typically given in Göppert-Mayer units (GM), with 1 GM = 10 -50 cm 4 •s photon . When appropriately scaled, Eqns. 2 and 3 yield the TPA and 3PA spectra, respectively, in GM and cm 6 •s 2 photon 2 units, as: