Measurements of thermal electron attachment rate coefficients to molecules using an electron swarm technique

An existing electron swarm apparatus has been redesigned and upgraded. In particular, the new design incorporates a novel planar radioactive foil to form an integral part of the drift tube, allowing us to overcome inherent problems present in our earlier system which used a cylindrical radioactive source. In addition to this, substantial upgrades have been made to improve the gating and amplification electronics and the data acquisition system. This has resulted in a much greater signal to noise ratio and improved accuracy. This paper describes the upgraded apparatus and its use in obtaining thermal ( 300 K) attachment rate coefficients to a number of molecules. The quality of the measurements and data are illustrated through the measurement of the thermal attachment rate coefficient for SF6 (k(th)(SF6) = (2.38 +/- 0.15) x 10(-7) cm(3) s(-1)). Thermal electron attachment rate coefficients for four other molecules are presented, namely for two derivatives of SF6, SF5CF3 and SF5Cl, and two perfluorocarbons, c-C4F8 and 2-C4F8.