Broadening of He 1083nm, 388.8 nm, 501.6 nm, 587.5 nm, 667.8 nm, 706.5 nm, 728.1 nm lines and of H(α), H(β) and O 777 nm lines in atmospheric pressure helium

Abstract

Helium atoms in electronically excited states are produced in atmospheric pressure parallel plate DBD helium plasma. Pressure broadening coefficients of several helium lines and of O(777 nm), H(β) and H(α) lines are deduced from the analysis of the spectral profiles of those lines. Absorption spectroscopy on He(3S) metastable atoms with a diode laser is used for 1083 nm He line. The influence of the laser power density on recorded absorption spectral profile is analyzed. For the other lines, emission from the plasma is analyzed with a very high resolution 2 m focal length spectrometer, working in the 2nd, 3rd or 4th diffraction order of the 1200 g/mm grating. Instrumental width ΔI ranges between 2.2 and 9.8 pm, depending on the spectral line. Knowing with high precision ΔI and the Doppler widths of the lines and the fact that the resulting Gaussian width of each line remains, at least, 4 times smaller than its Lorentzian width, permits to deduce the pressure broadening coefficients of helium 388.8, 501.6, 587.5, 667.8, 706.5, 728.1 and 1083 nm lines and of 777 O line with less than 3 % uncertainty for most of them. Observation of the forbidden 2p 1P1 –4f 1F3 component in the blue wing of the allowed 492.19 nm He(2p 1P1 –4d 1D2) line, indicates the presence of a static electric field E ≈ 5 ± 1 kV/cm during the 0.4 µs light collection time from the DBD. It is concluded that in pulsed plasmas, like DBD and plasma-jet, before deducing the plasma density from the homogeneous widths of H(β) or H(α) lines, one should ascertain that the static E-field in the gas is small enough during the light collection period for not inducing significant splitting of these lines. This would induce an extra broadening of the line, leading to an overestimated electron density.