The theories of absorption in mercury vapor of mercury resonance light of wave-length 2536.7A as formulated by Malinowski and H. A. Wilson are briefly reviewed. It is well established that the absorption coefficient for resonance radiation increases as the number of absorbing atoms decreases. This is probably due to a Doppler effect in the radiating vapor. To diminish the influence of the Doppler effect a jet of mercury vapor was used, and the resonance radiation coming from it was investigated. A series of measurements of the absorption coefficient of this light is given. The maximum atomic absorption coefficient observed is 10.22\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}13}$, nine times as large as any previously observed. It is shown that Malinowski's assumptions are not in accord with the observed effects.
The theories of absorption in mercury vapor of mercury resonance light of wave-length 2536.7A as formulated by Malinowski and H. A. Wilson are briefly reviewed. It is well established that the absorption coefficient for resonance radiation increases as the number of absorbing atoms decreases. This is probably due to a Doppler effect in the radiating vapor. To diminish the influence of the Doppler effect a jet of mercury vapor was used, and the resonance radiation coming from it was investigated. A series of measurements of the absorption coefficient of this light is given. The maximum atomic absorption coefficient observed is 10.22\ifmmode\times\else\texttimes\fi{}10\ensuremath−13, nine times as large as any previously observed. It is shown that Malinowski's assumptions are not in accord with the observed effects.
The absorption of the resonance radiation (2536A) of mercury was studied by dividing the radiation from a resonance lamp into two parts, each part going into a separate photo-electric cell. An absorption cell containing mercury vapor whose pressure could be controlled by cooling a side tube was placed in the path of one beam. The ratio of the amount of light passing into each beam could be varied by a mechanical device. From the setting of the mechanical device, giving a balance between the two photo-electric currents, the absorption of the mercury vapor could be inferred. The absorption per atom was calculated and found to decrease rapidly as the total amount of absorbing vapor was increased, thus showing that the resonance radiation is not homogeneous. The maximum atomic absorption coefficient was 11\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}14}$, which indicates that an atom can stop the radiation which falls upon an area one hundred times larger than its cross sectional area.
The radiation 4047, 2967 and 2752, all terminating on the 23P0 state of mercury, was passed through optically excited mercury vapor and the absorption of each wavelength for various lengths of excited vapor and for various pressures of admixed nitrogen was measured photometrically. With low dispersion the intensity of 2752 decreased nearly exponentially with increase in length of excited vapor giving a constant linear absorption coefficient. But the lines 4047 and 2967 were found not to obey the exponential law, the fractional decrease in intensity per unit length of path decreased rapidly as the total length of the absorbing vapor increased. With high dispersion the absorption of the five components of 4047 was strong and increased with increase of admixed nitrogen. The strongest component showed the strongest absorption and the amount of absorption decreased with decrease in intensity of components. The intensity of each component decreased nearly exponentially with increase in length of excited mercury vapor. Both components of 2967 showed strong absorption. None of the components of 4358 or 5461 showed measurable absorption.
Effect of a magnetic field on the polarization of resonance radiation of mercury and of sodium.---(1) Mercury $\ensuremath{\lambda}2536$. A beam of plane polarized monochromatic light was focussed on the bulb containing the vapor at low pressure (0\ifmmode^\circ\else\textdegree\fi{}C), and the polarization of the resonance radiation was determined photographically with the aid of a quartz wedge and a double image quartz prism. If the exciting beam is going east with the electric vector vertical, when the earth's field is carefully neutralized the polarization is 90 per cent in any horizontal direction and zero vertically; a field of only 2 gauss directed north reduces the polarization in its direction to nearly zero (the decrease with increasing field being exponential), changes the polarization east and also up to 60 percent, and increases the intensity directed upwards threefold. Similar effects are produced by fields in other directions. The polarization with zero field does not approach 100 per cent at low pressures, since 90 per cent was found also with the vapor at -50\ifmmode^\circ\else\textdegree\fi{}C, when the radiation first appeared. It was thought that the high value might be due to an orientation of the resonating atom by the field of the exciting light, but a beam of concentrated sunlight produced no effect. As found by Malinowski, a strong field of 10,000 gauss merely increased the intensity of the radiation about 10 per cent. This added light was found to be unpolarized. (2) Sodium D line. In the case of sodium vapor the tube was heated to 185\ifmmode^\circ\else\textdegree\fi{}C, the observations were visual, and as a source of unreversed light, the glow on the surface of a sodium glass vacuum tube carrying a discharge, was used. The effects observed are similar to those for mercury, but differ for some directions; fields forty times stronger were required, and the polarization in zero field was only 6 per cent, probably due to traces of hydrogen. This was increased by the field in some directions to 30 per cent.Effect of magnetic field on fluorescent light of iodine vapor and on white scattered light from mercury and ether vapors.---The polarization was found not to be affected.
Abstract As I have shown in numerous publications, the vapour of mercury in vacuo, even at temperatures below the solidifying point of the metal, when illuminated by the light of a water-cooled quartz mercury lamp, emits the monochromatic radiation of wave-length 2536. According to Bohr’s theory, this resonance radiation results from the fall of an electron from the 2p2 orbit, to which it has been raised from the IS orbit by the absorption of one quantum of 2536 radiation, back to the IS orbit, the orbit occupied by the electron when the atom is in its normal state. An atom in which the electron has been brought to the 2p2 orbit is in a condition to absorb the radiation of any other line produced by the fall of electrons from outer orbits to orbit 2p2, and Fuchtbauer has shown, in an extremely interesting and important paper (‘Physik. Zeit.,’ 1920), that a large number of the arc lines of mercury are radiated by cold mercury vapour illuminated by the mercury arc. He showed that, if the 2536 radiation was screened off by glass, none of the other lines appeared, and explained their presence as the result of the absorption by excited atoms with electrons at the 2p2 level, of the radiations terminating at this level, by which the electrons are carried out to other-orbits, from which they can fall back in various ways, giving rise to emission lines in the process. Though Fuchtbauer‘s experiment did not prove that this process actually took place, his explanation was the only one possible on modern theory, and the experiments which are to be considered in the present paper show that it was correct. Twenty years ago the most that could have been said would have been that the absorption of the 2536 radiation “shook” the atom in some way, causing it to emit its other frequencies as a sort of fluorescence, since Fuchtbauer’s experiment did not show that excitation by lines other than the 2536 line was necessary for the emission of the arc spectrum. To demonstrate experimentally that the step-by-step transfer of electrons from orbit to orbit actually takes place, it is necessary to illuminate the mercury vapour with various combinations of monochromatic light, and study the resultant emission spectrum.
The evidence in favor and against the assumption of Foote that every collision of a foreign gas molecule with an excited mercury atom is efficient in quenching the resonance radiation is discussed, and a new calculation of the efficiency of collisions is given, based on Stuart's measurements, which shows that the efficiency can be assumed to be equal to one in the case of CO, H2, and perhaps O2, but that it is undoubtedly smaller than one for H2O, N2, A, and He. CO has actually a greater quenching efficiency than H2. The radius of the excited mercury atom is calculated using an improved value for the amount of resonance-radiation re-absorbed in the resonance vessel and found to be ${r}_{{\mathrm{Hg}}^{\ensuremath{'}}}=2.91\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}8}$ cm, or 1.62 times larger than the radius of the normal atom for the case of H2, and ${r}_{{\mathrm{Hg}}^{\ensuremath{'}}}=5.5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}8}$ cm or three-fold normal for the case of CO. The apparent higher quenching efficiency of oxygen than hydrogen is explained by the partial oxidation of the mercury vapor and consequent decrease of the density of the last. It is shown that the life of metastable atoms increases with the admission of certain foreign gases into the fluorescence vessel.
Secondary radiation in cadmium vapor containing mercury as an impurity.---Attention is called to the points of difference between the conditions under which the two types of optically excited spectra of mercury vapor appear, namely resonance radiation and fluorescence. In the present experiments it is noted that when a bulb containing cadmium with a slight impurity of mercury is illuminated by light of 2288A wave-length the secondary radiation contains the mercury line 2536.7A in addition to the cadmium lines 2288A and 3261A provided the metal is distilling. As soon as the vapor is stagnant, however, only the cadmium 2288A line appears. The suggestion is made that the phenomenon is due to the presence in the distilling vapor of unstable HgCd molecules. In such a molecule the Cd atom may absorb the 2288A radiation and subsequently in the breakdown of the molecule may excite the Hg atom by a collision of the second kind.Polarization of the cadmium resonance, radiation in zero magnetic field.---The polarization of the cadmium resonance radiation (3261A) in zero magnetic field under certain conditions was found to be 35 percent. In order to explain this value by the kinetic theory, one must assign effective radii to the colliding atoms which are thirty times the radii effective in ordinary collisions.