Laser diagnostic density measurements of Hg(3P0,1,2) in a Hg-Ar discharge

Abstract

Two laser-based techniques are used to measure absolute and radial density distributions for the 6(3p0,1,2) states of mercury in the positive column of a low pressure Hg-Ar dc discharge. In the first method, a pulsed dye laser beam (20 ns, 0.2-cm–1 bandwidth), traversing the discharge axially, is scanned through one of the 6(3 p 0,1,2)→ 7(3S1) transitions, depending on the lower state of interest. The transmitted signal is detected with a calibrated photodiode and the power loss on resonance is plotted as a function of nonresonant power. At sufficiently high powers, this power loss approaches a constant saturated value which is directly proportional to the initial number density of absorbers in the lower state. Relative density measurements are made by setting the power in the saturated regime and measuring this power loss as a function of radial position. Results are given for the three 3P states at Hg pressures of 1 and 7 mTorr and discharge currents of 80 and 400 mA. Absolute densities are established using a well-known interferometric technique known as the hook method.1 Using the same laser as described above, the dye laser is tuned through the 3P1→3S1 transition of interest, and the spectral separation of the interference hooks is used to determine the absolute density.