Laser photodetachment measurements of the negative ion density in an oxygen currentless plasma

Laser photodetachment measurements of the negative ion density in an oxygen currentless plasma are reported. For this experiments a DC gas discharge tube with a coaxial nickel sectional cathode (diameter 5.10(-2) m) and a grid anode (diameter 2.10(-2) m and length 0.2 m) is used. The measurements are made in pure oxygen at gas pressure 109 Pa and discharge current 40-80 mk inside the anode region the reduced electrical field E/N-g is near zero (N-g is the gas density). The currentless plasma is formed by plasma particles moving from the negative glow into the cavity inside the anode. The negative charged particles dominantly are negative ions With Maxwellian energy distribution. The negative ion density is measured an the axis in the center of the discharge tube by measurement of the relative absorption of the laser power. A 5 mW 785 nm diode laser is used for the measurements. The obtained values of the oxygen negative ion density at gas pressure 109 Pa and 80 mA are similar to 10(13) cm(-3) and are in good agreement with the values obtained by second derivative Langmuir probe measurements.