The excited state properties of dyes become important under intense illumination of a short laser pulse. The pulse induces a significant population in an excited state which will have different absorptive properties, this causes a change in absorption of the incident pulse within its duration. An increase in absorption arises from molecules in an excited state having an absorption greater than the corresponding ground state.
The z-scan technique has been used to measure the induced absorption of a carbocyanine dye 1,1′,3,3,3′,3′- Hexamethylindtricarbocyanine Iodide (HITCI) for laser pulses of different wavelengths. An optical parametric oscillator pumped by the third harmonic of a Q-switched Nd:YAG laser was used. Z-scans were taken at a number of different wavelengths between 460 and 630 nm. Data is presented which shows the spectral region where induced absorption occurs. An eight level rate equation model is used to predict the non-linear behaviour and to fit to the experimental data, good correlation is found between the model and data.