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Physical Review

Absorption and Resonance of the Helium Infrared Lines

M. R. Wehr

Published Mar 1, 1932• 1 citation
Publisher

Abstract

The absorption of helium excited at low voltages for its own lines, the $2^{3}S\ensuremath{-}2^{3}P$ at 10830A and the $2^{1}S\ensuremath{-}2^{1}P$ at 20582A, is determined. The percentage absorption of the $2^{3}S\ensuremath{-}2^{3}P$ line increases when either the exciting current or the pressure in the absorbing tube is increased, and also when either the current or the pressure in the source tube is decreased. When the light to be absorbed is taken end on from a Geissler tube source, the percentage absorption is much less than when the light is taken off at right angles to the capillary. The changes with source tube conditions are ascribed to the corresponding variations of the shape and width of the emission line. Observable absorption begins when the exciting voltage is 25 volts. No absorption is detected at 19.8 volts, the excitation potential of the $2^{3}S$ state. The 10830A line shows strong resonance which is directly proportional to the percentage absorption. The absorption for the 20582A line is very small. It shows no detectable resonance.

DOI
10.1103/physrev.39.796

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Mean Lives of Mercury Lines λ2537 and λ1849

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The mean life of the mercury line $\ensuremath{\lambda}2537$ was measured by the alternating voltage method previously described by Webb. The radiation was excited in a quartz tube by impact with electrons from a hot cathode. The photoelectric system was contained in a separate tube also of quartz. The photoelectric surface was zinc evaporated on a nickel plate. Measurements of the apparent life were made with diminishing mercury vapor pressures until further reduction of pressure produced no change in the measured rate of decay of the radiation. At these low pressures (below 3\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}4}$ mm, for the geometry used) the absorption and reemission of the radiation was negligible and the true life of a single excitation process was measured. This was found to be 1.08\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}7}$ sec. with an estimated precision of one percent.Absorption coefficient of $\ensuremath{\lambda}1849$. An absorption cell with a one mm thick absorbing layer of mercury vapor was mounted so as to move in and out between the excitation and photoelectric tubes used in the above measurements. The absorption of $\ensuremath{\lambda}1849$ was measured at pressures such that the absorption of $\ensuremath{\lambda}2537$ was negligible. From this the atomic absorption coefficient for the main component of the line was found to be 6.2\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}11}$ from which the mean life of the $2^{1}P_{1}$ state was calculated to be 2.9\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}10}$ sec. with an estimated precision of 25 percent.

Physical Review•Jun 1, 1932•17 citations
The absorption of helium excited at low voltages for its own lines, the 23S\ensuremath−23P2^{3}S\ensuremath{-}2^{3}P23S\ensuremath−23P at 10830A and the 21S\ensuremath−21P2^{1}S\ensuremath{-}2^{1}P21S\ensuremath−21P at 20582A, is determined. The percentage absorption of the 23S\ensuremath−23P2^{3}S\ensuremath{-}2^{3}P23S\ensuremath−23P line increases when either the exciting current or the pressure in the absorbing tube is increased, and also when either the current or the pressure in the source tube is decreased. When the light to be absorbed is taken end on from a Geissler tube source, the percentage absorption is much less than when the light is taken off at right angles to the capillary. The changes with source tube conditions are ascribed to the corresponding variations of the shape and width of the emission line. Observable absorption begins when the exciting voltage is 25 volts. No absorption is detected at 19.8 volts, the excitation potential of the 23S2^{3}S23S state. The 10830A line shows strong resonance which is directly proportional to the percentage absorption. The absorption for the 20582A line is very small. It shows no detectable resonance.
Abstract By means of a direct recording Fabry-Perot interferometer, the contours of several singlet lines of helium were studied over a wide range of pressure and temperature. The source was a microwave discharge cooled with liquid nitrogen or liquid helium. Analysis of the tracings allowed the Doppler and Lorentz components to be separated quantitatively. A detailed study was carried out for the line λ 7281 Å (21P - 31S). By extrapolation of the Lorentz width to zero pressure the radiation width of the level 2 was determined and led to an ƒ-value 0·377 + 0·035 of the resonance line λ 584 Å (11S— 21P) The resonance broadening is found to be independent of the speed of the atoms, over a range of a factor five in the speed, in agreement with theories of optical collision by Weisskopf and others. The absolute value of the resonance broadening constant is compared with various theories. The strong line λ 6678 Å (21P -31D) showed self-absorption due to imprisonment of resonance radiation.
Dissociated iodine vapor at 1100\ifmmode^\circ\else\textdegree\fi{} to 1500\ifmmode^\circ\else\textdegree\fi{}C in a quartz tube failed to show any resonance fluorescence of the 2062 line in a 78 minute exposure in the same apparatus in which mercury vapor gave a detectable fluorescence of the 2537 line in a 1 second exposure. This fact and the narrowness of the 2062 line in absorption, seem to indicate that the lower level involved in the transition producing this line is not the normal state of the atom but is a slightly higher one. A calculation based on the observed line width shows that this level is probably at least 0.545 volt above the normal level and suggests that there may be a resonance line of atomic iodine at about 1800A.
By direct measurement of the contours of the line wings, the resonance broadening of the sodium DDD lines in absorption was determined for vapor pressures of sodium ranging from ${10}^{\ensuremath{-}3}$ to 70 mm Hg with negligible amount of foreign gases. Over nearly the entire range, the half-intensity width varied linearly with the density of the absorbing atoms; i.e., $\frac{\ensuremath{\Delta}{\ensuremath{\nu}}_{\frac{1}{2}}}{N}=0.80\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}7}$ ${\mathrm{sec}.}^{\ensuremath{-}1}$ ${\mathrm{cm}}^{\ensuremath{-}3}$---the constant being 1.6 times that predicted by theory but much less than the corresponding experimental results for potassium. Above 5 mm pressure, $\ensuremath{\Delta}{\ensuremath{\nu}}_{\frac{1}{2}}$ increased slightly faster than NNN; moreover, the contour for the absorption coefficient was more adequately represented by an inverse cube contour than by the inverse square. Somewhat in contradiction to theory, the relative width, $\frac{\ensuremath{\Delta}{\ensuremath{\nu}}_{\frac{1}{2},8}}{\ensuremath{\Delta}{\ensuremath{\nu}}_{\frac{1}{2},l}}$ was 1.12. No definite evidence of van der Waals force was observed at the higher pressures; likewise no appreciable shifts or asymmetries. The natural width measured was consistent with previous results.
Lineshapes associated with the absorption and emission spectra of neutral atoms in liquid helium and cold helium gas are discussed. A neutral atom in liquid helium forms a microcavity due to a strong short-range repulsive force between the helium and impurity atoms, producing very broad and largely blueshifted absorption bands and relatively narrow emission lines with much smaller shifts. A broad and weak emission band observed by exciting the D2 line of Ag atoms in liquid helium has confirmed the hypothesis that quenching of D2 emission lines for heavy group 1a and 1b atoms are due to the formation of exciplex. In cold helium gas, sudden decrease of the emission lifetimes below 5.2 K by the excitation of the D2 line of Ag atoms is attributed to the condensation of He atoms on the metastable 2D5/2 state. Very narrow absorption lines of Eu innershell electron transitions have revealed that superfluid helium behaves as a static and highly homogeneous low temperature matrix.
A mixture of potassium vapor and mercury vapor contained in a tube fashioned after the Cario-Lochte-Holtgreven sodium vapor lamp was radiated by light from a water-cooled mercury arc. The resulting fluorescence radiation contained potassium lines as well as two of the three known K2{\mathrm{K}}_{2}K2​ absorption bands and a new band with maximum at 5721A. On exposure of the mixture to light from a hot mercury arc or to light having all wave-lengths shorter than 2800A filtered out, the fluorescence was eliminated. This indicated that the excitation was caused by absorption of 2536A by the mercury. Absorption of the 2536A mercury line raises the mercury atom to the 63P16^{3}P_{1}63P1​ state, having an energy of 4.86 volts. The ionization potential of potassium is 4.32 volts; thus in a collision of the second kind, the excited mercury atom has enough energy to ionize the potassium atom. There was no evidence of a potassium emission from an energy state in the continuum equal in energy to the Hg 63P16^{3}P_{1}63P1​ or 63P06^{3}P_{0}63P0​ states. There were, however, two strong lines in fluorescence which could not be ascribed to the usual K energy levels nor to any probable impurity. To test whether or not ionization of potassium actually takes place, a filament was introduced into the tube to supply electrons. A large increase in intensity of the fluorescence of potassium lines was obtained with the hot filament. It was furthermore found that the intensity increased with filament temperature.
The mean life of the mercury line $\ensuremath{\lambda}2537$ was measured by the alternating voltage method previously described by Webb. The radiation was excited in a quartz tube by impact with electrons from a hot cathode. The photoelectric system was contained in a separate tube also of quartz. The photoelectric surface was zinc evaporated on a nickel plate. Measurements of the apparent life were made with diminishing mercury vapor pressures until further reduction of pressure produced no change in the measured rate of decay of the radiation. At these low pressures (below 3\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}4}$ mm, for the geometry used) the absorption and reemission of the radiation was negligible and the true life of a single excitation process was measured. This was found to be 1.08\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}7}$ sec. with an estimated precision of one percent.Absorption coefficient of $\ensuremath{\lambda}1849$. An absorption cell with a one mm thick absorbing layer of mercury vapor was mounted so as to move in and out between the excitation and photoelectric tubes used in the above measurements. The absorption of $\ensuremath{\lambda}1849$ was measured at pressures such that the absorption of $\ensuremath{\lambda}2537$ was negligible. From this the atomic absorption coefficient for the main component of the line was found to be 6.2\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}11}$ from which the mean life of the 21P12^{1}P_{1}21P1​ state was calculated to be 2.9\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}10}$ sec. with an estimated precision of 25 percent.