Spectroscopic characterization of laser-produced atmospheric pressure helium microplasmas

Abstract

The temporal evolution of excitation temperature in a laser-produced atmospheric pressure helium plasma is measured and its electron density is explored using a number of different methods. These are the Stark broadening of the He lines at 4026, 4471 and 5015 Å, peak separation and peak intensity ratio of the allowed and forbidden components of the 4471 Å line and Stark broadening of the Hβ line in hydrogen–helium mixtures. The various results obtained from the helium lines agree within roughly an average factor of 2 at all times. Values obtained from peak separation values differ slightly from the Stark measurements by amounts that depend on the analysis method used. At early delay times, electron density values obtained from the Hβ line are considerably lower than those from helium lines. Electron density versus temperature curves and temporal evolution of the line intensities indicate that the laser-produced plasma is not in complete thermal equilibrium. Measured electron densities range from ∼1017 to slightly below 1015 cm−3. These are electron densities now being generated in electrically produced microplasmas where the use of spectroscopic techniques is also an important diagnostic tool.