Laser absorption measurements of atomic metastable densities

Helium triplet metastable densities have been calculated from measured absorption of resonant, CW, UV laser radiation on the 2³S-5 ³P transition at 294.511 nm. Gas temperatures have also been deduced by measuring the Doppler width of this transition. A 3 mm bore capillary tube filled to pressures in the range 1-8 Torr and run at currents in the range 2.5-80 mA was studied. With constant discharge current (60 mA) at a maximum value for the triplet metastable density of 11*10¹² cm ^-3 at about 2 Torr was obtained. The metastable densities show the expected saturation behaviour, saturating at currents above 40 mA. Experimental values are used in conjunction with the rate equation describing the metastable level population in order to estimate the rate coefficient for loss of metastables through electron collisions. Mention is made of the use of the visible laser radiation for measuring the UV laser wavelength in a Kowalski-type wavemeter.