Measurement of electron density utilizing the H α-line from laser produced plasma in air

The electron density in a laser produced plasma experiment was measured utilizing the Stark broadening of the H _α-line at 656.27 nm. This line results from the interaction of the Nd:YAG laser at the fundamental wavelength of 1.06 μm with a plane solid aluminum target in a humid air. The measurements were repeated at several delay times (0-10 μs) and at a fixed gate time of 1 μs. The electron density from the optically thin Al II-line at 281.62 nm was measured in parallel from the same spectra. The electron density was found in the range from 10 ¹⁸ cm ^{- 3} down to 6 × 10 ¹⁶ cm ^{- 3} at longer delay time. The electron density from the H _α-line using the Griem's standard theory was compared with the predictions of other model due to Gigosos et al. The agreement between the measured electron density from both the H _α-line and the Al II-line would confirm the reliability of utilizing the H _α-line as an electron density standard reference line in LIBS experiments. Several important features characterize the H _α-line: it is a well isolated line, it gives large signal to background ratio, it lasts a long time after the termination of the laser (up to 10 μs), its Stark width is relatively large and does not exhibit self-absorption.