Determination of helium number densities in high-frequency electrodeless plasma

The number densities of He first excited states 23S1, 21S0, 23P0,1,2, and 21P0 were measured by absorption and self-absorption methods. The emission lines in the range 290–730 nm were used to determine the number densities of He metastable and resonant states of inductively coupled high-frequency electrodeless lamps filled with helium at pressure 0.1 Torr. The obtained number densities of helium metastable levels 23S1 and 21S0 are 2×1012 and 3×1011 cm−3, respectively. For the 23P0,1,2 and 21P0 levels the determined number densities are ten times smaller than that of the metastable levels. Values of number densities from measurements using both methods are in good agreement.