Fluorescent emission of the mercury line 2537 A at pressures between 10-3 and 10-1 mm
Peter Pringsheim, Olive D. Saltmarsh
Abstract In his first papers on the resonance radiation of mercury and sodium Wood describes how the character of the emission changes with increasing pressure of the fluorescent vapour. At low pressure (p = 10-3 mm) the emission comes wholly from a narrow well-defined beam corresponding to the path of the primary light (beam fluorescence). With increasing density of the vapour the beam becomes diffuse until the luminosity fills the whole volume (volume fluorescence) and finally at pressures of about 0·1 mm it withdraws entirely to the spot of entrance of the primary light (surface fluorescence). In all three cases the absorption and emission of light are due to normal atoms; the phenomenon is that of normal or true resonance radiation. For the D-lines of sodium, Lord Rayleigh showed as early as 1919 that, even at pressures at which the true resonance radiation is concentrated on the surface, an emission can in addition be observed within the volume along the path of the primary light, if the D-lines in the spectrum of the exciting source are not too narrow. This new fluorescence beam which can only be excited by the outer parts of the D-lines is partially polarized. Later Jablonsky and Pringsheim found that though the normal resonance radiation observed on the surface is unpolarized under these conditions, the degree of polarization of the new fluorescence beam can be as high as 30% and is practically unaltered by the action of a magnetic field. For the mercury resonance line 2537 A it was Rayleigh who again studied the phenomena at higher pressures but with his main interest concentrated on the duration of the processes and on the appearance of the well-known mercury bands. In his paper, the continuous change of the fluorescence in the vapour volume is studied from room temperature up to 200°C. He used as a source of excitation a “not fully cooled” mercury lamp, giving a somewhat broadened and partially self-reversed line at 2537 A, which produces the effect called by Rayleigh “core effect”. The latter is in Rayleigh’s terminology the fluorescence of mercury vapour produced by the more central part of the 2537 A line while the very different fluorescence emission which is excited by a much broadened and self-reversed line is called by him the wing effect. The core effect always shows a line which, with the available dispersion, seems to coincide with the resonance line itself; when the pressure increases, a number of bands appear which have no importance for the present paper. Since in saturated vapour at 100° C the normal resonance radiation has withdrawn to the surface, it is improbable that the core effect is produced by the most central region of the primary line by which the normal resonance radiation at the lowest pressures is excited.
Proceedings of the Royal Society of London A Mathematical and Physical Sciences•Mar 2, 1936•0 citations