Collisional moderation by foreign gases in the reaction of SiH3+ with C2H4

Collisional enhancement of the SiC₂H₇⁺ intensity in the reaction of SiH₃⁺ ions of 2.6 eV laboratory kinetic energy with C₂H₄ is due to moderation of the reactant-ion energy rather than removal of excess energy from the (SiC₂H₇⁺)* collision complex. The effectiveness of (CD₃)₄Si, CH₄, CO₂, N₂, D₂ and Ar as kinetic energy moderators was studied at a constant C₂H₄ pressure of 2.5 × 10⁻⁴ torr and at moderator pressures up to 5 × 10⁻³ torr. The most important property determining moderator efficiency is molecular complexity, i.e. number of vibrational degrees of freedom. The moderator (CD₃)₄Si is not efficient, despite its large number of vibrational degrees of freedom, because it undergoes a rapid reaction with SiH₃⁺ to produce (CD₃)₃Si⁺ ions.