Polarisation of the 61S0 - 61P1 and 61S0 - 63P1 lines of mercury, excited by electron impact

Abstract

The polarisation of the 61S0 – 61P1 (1850 A) and the 61S0 – 63P1 (2537 A) lines of mercury, excited by ow energy electron impact, have been measured. In both cases, the emitted radiation was observed in a direction perpendicular to the electron beam. In the first instance, the polarisation of the 2537 A line was measured with an energy resolution of 300 meV, in an experiment where the mercury was in the form of a vapour. The results from this gave a first indication that resonances in the excitation of the line are responsible for departures of the polarisation from the theory of McConnell and Moiseiwitsch. A crossed-beam apparatus was assembled, and the polarisation of the 1850 A line was measured, again under low resolution. Agreement with the theory in this case is reasonable, apart from in the region with a few eV of threshold, where the measured polarisation is rather lower than that predicted. To examine more carefully the resonances observed in the 2537 A line, an electron monochromator was built, and the polarisation of the line was measured under high resolution. Data are presented with an energy resolution of 140 meV and 100 meV. A total of six features are observed, of which resonances at 4.92 and 5.50 eV are particularly well marked. The observed polarisation of the resonance at 4.92 eV appears to be in agreement with the hypothesis that 4P3/2 negative ion state of mercury is being formed at that energy.