Diode laser absorption measurements at the Hα-transition in laser induced plasmas on different targets

The diode laser atomic absorption spectroscopy (DLAAS) technique has been utilized to assess the degree of optical opacity of plasma at the wavelength of the Hα-line. The plasma is produced at atmospheric conditions by focusing a 6 ns Nd:YAG laser pulse at 1.064 μm on different solid target materials including aluminum, iron and titanium as major elements as well as flat pieces of plastic and wood characterized by a high content of hydrogen. The optical depth was investigated as a function of delay times ranging from 0 to 5 μs, and at laser fluences ranging from 7 to 19 J/cm2, all at a fixed gate time of 1 μs. The results show that the plasma associated with metallic targets is almost optically thin at the Hα-line over all fluences and at delay times ≥ 1 μs, but rather thick for hydrogen-rich targets (plastic and wood) over all delay times and fluences.