Electron attachment to MoF6, ReF6, and WF6;: reaction of MoF6- with ReF6 and reaction of Ar+ with MoF6

Rate constants were measured for electron attachment to MoF6, ReF6, and WF6 in 133 Pa of helium gas using a flowing-afterglow Langmuir-probe apparatus. The experiment is a thorny one because the molecules tend to form oxide impurities on feedline surfaces and because of thermal decomposition of MoF6 on surfaces as the gas temperature is increased. The electron attachment rate constant for MoF6 is (2.3 +/- 0.8)x10(-9) cm(3) s(-1) at 297 K; only MoF6- is formed in the temperature range of 297-385 K. The rate constant increases with temperature up to the point where decomposition becomes apparent. Electron attachment to ReF6 occurs with a rate constant of (2.4 +/- 0.8)x10(-9) cm(3) s(-1) at 297 K; only ReF6- is produced. MoF6- reacts with ReF6 to form ReF6- on essentially every collision, showing definitively that the electron affinity of ReF6 is greater than that of MoF6. A rate constant of (5.0 +/- 1.3)x10(-10) cm(3) s(-1) was measured for this ion-molecule reaction at 304 K. The reverse reaction is not observed. The reaction of Ar+ with MoF6 was found to produce MoF5++F, with a rate constant of (1.8 +/- 0.5)x10(-9) cm(3) s(-1). WF6 attaches electrons so slowly at room temperature that the attachment rate was below detection level (<= 10(-12) cm(3) s(-1)). By 552 K, the attachment rate constant reaches a value of (2 +/- 1)x10(-10) cm(3) s(-1). (c) 2006 American Institute of Physics.