RatiosRof the cross sections for associative to the sum of associative and Penning ionization have been measured using molecular beams of thermal energy (330deg;K) metastable atoms, A*, incident on a gas B confined in a collision chamber at low pressures, where A*is either 21,3Shelium or3P2,0neon and B is argon or krypton. By determiningRas a function of the collision chamber pressure in the range 0.05ndash;0.4 mgr; and by measuring the relative intensity of the secondary product ion,B2plus;, cross sectionssgr;Iandsgr;IIfor destruction of the associative ions,ABplus;plus;Brarr;Aplus;Bplus;plus;BandABplus;plus;Brarr;Aplus;B2plus;,have been determined. Extrapolation to zero pressure gives the true value ofRfor the primary processes. The effect of a change in the ratio of 21Ssol;23Shelium in the metastable atom beam has also been investigated by changing the exciting electron beam energy. In collisions of He*with both argon and krypton,Ris greater for 23Shelium than for 21Shelium by about 0.05. Within the experimental error,sgr;Iandsgr;IIfor destruction of ABplus;are independent of whether this ion is formed by collisions of 23Sor 21Shelium. The large size of the cross sectionssgr;I, which range from 210 Aring;2forNeArplus;sngbnd;Arto 470 Aring;2forHeKrplus;sngbnd;Kr, suggests that the associative ions are formed very close to the dissociation limit.
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