Excitation functions for the reactions of Ar$sup +$ with CH$sub 4$, CD$sub 4$, and CH$sub 2$D$sub 2$

Integral reaction cross sections as a function of initial translational energy (0.4--30 eV c.m.) are reported for isotopic variants of the exoergic ion- molecule reaction Ar$sup +$+CH$sub 4$ $Yields$ ArH$sup +$+CH$sub 3$. The excitation functions, which maximize at about 5 eV and decrease at lower collision energies, appear to possess translational energy thresholds at about 0.1 eV. At the higher energies there is a large isotope effect favoring abstraction of H over D. The observed threshold behavior, unusual for exoergic reactions of positive ions, is discussed in terms of the formation of an ArCH$sub 4$$sup +$ intermediate complex at low collision energies. (AIP)