AbstractA small tubular reactor having an inner diameter of 1–2 mm andused as the source in a molecular beam apparatus is described in detail. This arrangement allows the study of fast reactions with reaction times smaller than 1 msec. The preexplosive reaction phase between F2and H2and CH4, respectively, is investigated to find out the initiation reactions. In the F2/H2reaction, initiation is brought about by heterogeneous generation of F atoms or some other surface reaction. Evidence is also obtained for chain branching reactions. In the F2/CH4case the dominant initiation reaction is the homogeneous reaction CH4+ F2→ CH3+ HF + F. The rate constant for the reaction between 300 and 400 K is 1012.3±0.3exp[−47 ± 8 kJ/mol/RT] cm3/mol sec. The analysis of the experimental data also yields the rate constant for the propagation reaction CH3+ F2→ CH3F + F, which is 1012.3±0.3exp[−4.6 ±2.1 kJ/mol/RT