Abstract
Spectral lines of low-temperature nitrogen photoionized plasma were investigated. The photoionized plasma was created in the result of irradiation N2 gas using laser plasma EUV radiation pulses. The source was based on a 10J/10ns Nd:YAG (λ = 1064 nm) laser system and a gas puff target. The EUV radiation pulses were collected and focused using a grazing incidence multifoil EUV collector. The emission spectra were measured in the ultraviolet and visible (UV/Vis) range. It was found that the plasma emission lines in the lower region of the UV range are relativley weak. Nonetheless, a part of the spectra contains strong molecular band in the 300 - 430 nm originated from second positive and first negative systems band transitions of nitrogen. These molecular band transitions were identified using a code for study the diatomic molecules, LIFBASE. The vibrational band of Δv = 0 and ±1 transitions were significantly populated than of that with Δv = ±2 and 3 transitions. A comparison of the calculated and measured spectrum is presented. With an assumption of a local thermodynamic equilibrium (LTE), the vibrational temperature was determined from the integrated band intensities with the help of the Boltzmann plot method and compared to the temperature predicted by SPECAIR and LIFBASE simulations. A summary of the results and the variations in the vibrational temperatures was discussed.
Highlights
Photoionization of molecular nitrogen is performed and emission spectra in the result of the photoionization was measured
The plasma was produced by a laser-produced plasma (LPP) extreme ultraviolet (EUV) source
The plasma parameters and its condition can be studied based on these molecular band transitions by evaluating vibrational temperature with an assumption of the local thermodynamic equilibrium
Summary
Photoionization of molecular nitrogen is performed and emission spectra in the result of the photoionization was measured. The laser power density in such regime can be achieved an order of 1012 W/cm in the interaction region Such intensity range can be enough to laser-produced xenon plasma and the EUV radiation collected was used to ionize nitrogen gas up to first few ionization states. The plasma parameters and its condition can be studied based on these molecular band transitions by evaluating vibrational temperature with an assumption of the local thermodynamic equilibrium. If the plasma is far from LTE, determinations of vibrational temperature is more sophisticated unless the high-resolution spectra were used. In this case, the vibrational temperature inferred from the Boltzmann plot methods still give insight into the plasma condition. The 2PS of nitrogen molecule was widely used to study the gas temperatures in nitrogen and nitrogen-containing plasmas [6,7,8,9,10]
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