Laser ablation at the solid-liquid interface: transient absorption of continuous spectral emission by ablated aluminium atoms

A light emission from aluminium atoms produced by a pulsed laser irradiation to the aluminium metal-water interface was studied by emission spectra and analysed on the basis of a rate model. The time dependence of the emission spectra was obtained from the 20-80 ns time range. Early in this time range the Al ( 2 P°- 2 S) transition appears as an absorption line in a continuous spectrum, suggesting that the optical thickness of the light-emitting region is considerably high and that aluminium atoms present in the light-emitting region are cold enough to absorb the continuous spectral emission. Later the emission surmounts the absorption, i.e. a positive emission peak over the continuous spectrum is observed. The time of the transition from the absorption regime to the emission regime depends on the irradiation laser fluence. These behaviours can be well described by the model calculation based on the kinetics of the two energy levels. By comparison between the model calculation and the experimental results, the time behaviour of the species in the light-emitting region is discussed.