Generation of negative ions from SF6 gas by means of hot surface ionization

Sulfur hexafluoride (SF(6)) is a man-made compound with many industrial applications. This compound is also one of the most powerful greenhouse gases with a relatively long atmospheric lifetime. Therefore, it is important to investigate processes leading to SF(6) decomposition.

A magnetic sector mass spectrometer with a thermoemission gaseous ion source was used in this study. The filament temperature was changed and monitored pyrometrically during the course of the studies. In the hot surface ionization process, negative ions may be generated both by free electron attachment to a molecule and by thermal dissociation followed by electron capture to the one of the fragments formed.

Eight ion species: SF(5)(-), F(-), SF(6)(-), SF(4)(-), SF(3)(-), SF(2)(-), SF(-) and F(2)(-), with ion current intensities ratios of 1000:200:100:10:5:0.5:0.5:0.05, respectively, were detected. The filament temperature dependencies of the SF(5)(-), F(-), SF(6)(-), SF(4)(-) ion current intensities were measured. The optimal temperatures at which the maximum of the ion current intensity is observed were estimated in the 1830-2000 ± 10 °C range. The formation of F(2)(-) ions is probably disturbed by a dissociation process at high temperatures.

Negative surface ionization on the hot filament is a relatively simple and effective method for carrying out negative ion formation studies. Eight SF(6) decomposition channels leading to the formation of negative ions have been detected and analyzed using this technique.