Luminescence of Oxygen Atoms Stimulated by Metastable Helium at Cryogenic Temperatures

We present investigations of the afterglow of oxygen-helium gas mixtures at cryogenic temperatures. The cooling of a helium jet containing trace amounts of oxygen after passing through a radio frequency discharge zone led to the observation of strong emissions from atomic oxygen. The effect results from the increasing efficiency of energy transfer from metastable helium atoms and molecules to oxygen impurities in the cold dense helium vapor. This effect might find an application for the detection of small quantities of the impurities in helium gas.

Figure 1

(a) Scheme of the setup for the spectroscopic study of the afterglow in cold helium vapor: 1, helium dewar; 2, quartz tube; 3, space filled with liquid nitrogen; 4, electrodes for radio frequency discharge; 5, system for registration of optical spectra; 6, gas jet; 7, lens; 8, quartz beaker; 9, superfluid helium in the beaker; 10, level of liquid helium in the main helium bath; and 11, fountain pump. (b) Photo of the jet when liquid helium is absent from the beaker. (c) Photo of the jet when the beaker is filled with liquid helium. The arrow shows the level of He II in the beaker. Note that the jet creates a crater in the helium surface, as evidenced by the cone of green light emanating from the vapor below the indicated helium level.

Figure 2

(a) Spectrum of the helium jet with traces of oxygen, when the beaker is empty (red curve) and after it is filled with He II (black curve). Light was collected at the top of the beaker. (b) Spectrum of the atomic oxygen lines near 543.6 nm. (c) Spectrum of the ${\mathrm{He}}_2$ band near 639.6 nm. (d) Spectrum of the oxygen lines near 777.4 nm.

Figure 3

Dynamics of the intensities of atomic oxygen (green curve: 1, $\lambda =543.6\mathrm{nm}$; 2, $\lambda =777.4\mathrm{nm}$; 4, $\lambda =615.7\mathrm{nm}$; 5, $\lambda =533.0\mathrm{nm}$) and helium (red curve: 6, $\lambda =587.6\mathrm{nm}$; 7, $\lambda =706.5\mathrm{nm}$) atomic lines and also molecular helium bands (blue curve: 3, $\lambda =639.6\mathrm{nm}$) during the filling of He II in the beaker. (The arrow at $t=227\mathrm{s}$ shows the moment when helium starts to fill the beaker.)

Figure 4

(a) Comparison of the spectra of helium jets with the 1 ppm trace of oxygen (black curve) and the spectra of helium jets after purification of the helium gas in the trap cooled to liquid helium temperature (red curve). (b) Comparison of the spectra of helium jets with the 1 ppm trace of the oxygen (black curve) and the spectra of helium jets with the 25 ppm trace of the oxygen (red curve).

Figure 5

Energy transitions between quantum states of molecular and atomic helium, and atomic oxygen with a wavelength of emitted light.