Effects of atmosphere on laser vaporization and excitation processes of solid samples

Effects of atmosphere on the laser vaporization and excitation processes were investigated with spectral measurements and with the direct measurement of vaporized weight of samples. The samples, metals and ceramics, were positioned in three different atmospheres, i.e. air, argon and helium, from atmospheric pressure to a pressure reduced to a few torr. The time-resolved emission intensities of the Fe I lines and the Al I lines of the Al alloy and Al metal samples were measured in two time windows, i.e. 0–1 μs and 1 μs-1 ms. The excitation temperatures and the electron number densities of the plasmas were also estimated. The emission spectra, the excitation temperatures, and the electron densities were shown to be appreciably influenced by the ambient atmosphere. The amount of sample vaporized which was measured directly with an electric micro-balance after irradiation by 500 laser shots changed considerably with the atmosphere, e.g. from 12 ng/pulse at atmospheric pressure to 330 ng/pulse at 10 torr in argon.

The results are discussed in the scope of the possibility of ambient gas breakdown before sample vaporization and a change in the laser radiation coupling to the solid surface. It is revealed that the control of the interaction between laser radiation and plasmas and the prevention of the preceding gas breakdown are important for effective laser vaporization and for the emission measurement of the plasmas. The sample characteristics also influence the initiation stage of the plasmas and the effects from the atmosphere.