Time Resolved Measurements of Velocity Distributions of Sputtered Barium in Fluorescent Lamps

Summary form only given. Velocity distributions of barium atoms near the electrode of a fluorescent lamp during cold (instant) start are measured by wavelength modulated laser absorption. Barium from the work function-lowering emitter material is ejected during cold start via sputtering, a consequence of energetic bombardment by ions from the discharge. Ions acquire bombardment energies of several hundred eV because of the correspondingly high cathode falls that exist during cold start, when the lamp is in a glow discharge state. The measurement technique employs a single longitudinal mode, external cavity diode laser (linewidth ~300 kHz) with wavelength that can be scanned over a 70 GHz range in the vicinity of the (6s5d)³D₃rarr(5d6p´)³D₃ ^o transition of barium (649.9 nm). The lower level of this transition is a metastable state 1.19 eV above the ground (¹S₀) state. Laser radiation is directed through the lamp in the electrode region and is subsequently detected by a photodiode. Specially constructed lamps without phosphor in the end regions are used. Comparisons of signal intensities with and without discharge provide the absorption, at each precise wavelength/frequency, due to the presence of barium in the (6s5d)³D₃ state. Absorption away from the natural line center is presumed due to the Doppler shift of the frequency relative to the moving barium atom. Although the maximum wavelength scan rate is 3 kHz, time resolved measurements of A.C. operated lamps driven at tens of kHz are possible by considering data at analogous points in successive phases of the discharge current/voltage. Typical measurements reveal velocity distributions that have high temperature thermal (Maxwellian) components plus non-thermal tails. The thermal component temperatures fluctuate from cathode to anode phase by about 200degK, with peaks of about 2200degK during the cathode phase. The no- -thermal tails, with observed energies as high as 7 eV, exist during the cathode phase and decay prior to the anode phase. Interpretations of these observations will be given in the presentation of this work.