AbstractThe non-linear optical properties of chloro-aluminium phthalocyanine and 1,3,3,1′,3′,3′- hexamethylindotricarbocyanine have been studied as model systems using z-scan and time resolved experimental techniques. Excited state lifetimes and absorption cross sections have been measured as well as the spectral response of the induced absorption. Excited state lifetimes are critical in determining the performance of a reverse saturable absorber. Even though a first excited state absorption cross section may be large compared with the ground state absorption cross section the lifetimes and transition probabilities of higher lying states can lead to constraints upon how effective organic dyes are as optical limiting materials.Both novel saturation of the induced absorption at high laser fluences and intermolecular quantum beats in 1,3,3, 1′,3′,3′-hexamethylindotricarbocyanine are demonstrated experimentally and accounted for theoretically. Quantum chemical calculations have been developed to propose a probable inter-molecular mechanism for the observed quantum beats.