The Relative Intensities of Mercury Lines under Different Conditions of Excitation

Abstract

The relative intensities of the lines of the $2s-2p_i$ triplet have been measured by a photographic method with a calibrated tungsten lamp as a standard of comparison. The intensity ratios have been found to vary with conditions of excitation due to the absorption of the 5461 and 4047 lines by atoms in the metastable states $2p_1$ and $2p_3$. About three-quarters of the 5461 radiation and about one-third of the 4047 radiation were absorbed in a commercial mercury-filled tube with three electrodes when a high plate current was passed through the tube. A lesser absorption was found when an ordinary arc was used and when small plate currents were passed in the three-electrode tube. A tube specially built to excite mercury by electronic bombardment and provided with means for controlling the mercury pressure gave uniform results under various conditions of mercury pressure and plate current, and the double ratio of 8.5:3.3:1 was found for the intensities divided by the fourth powers of the corresponding frequencies. The ratio of 4078:4047 was found to vary with plate voltage in a manner similar to the variation of efficiency of ionization with electron velocity. It was attributed to the presence of two mechanisms of emission giving widely different intensity ratios, namely, excitation of the neutral atom to a higher energy level, and recombination of the ionized atom. A roughly quantitative survey of the intensities of the excitation and of the recombination spectra indicated that in comparison with atoms excited to a higher level by electronic impact as occurs in the arc, recombined atoms will prefer the singlet to the triplet system, while within each system their order of preference is $P$, $S$, $D$, and $p$, $s$, $d$, respectively. Also, within each state, a level at about the height of the $4S$ level is preferred to the others.