Ionic rare gas alkali excimers Rg+A were produced by exciting Rg/A mixtures in a buffer gas atmosphere by means of an ion or electron beam. As a representative candidate the formation of Kr+K decaying at 135 nm was investigated. The rate constant k(He) for the main reaction Kr++K→Kr+K was determined varying the buffer gas density [He] between 1017 and 1020 cm−3. Values between k(He)=1×10−11 and 5×10−10 cm3 s−1 were observed. At high buffer gas densities the quantum yield ηq for the emission of the 135 nm radiation depends on the ratio [K]/[Kr]. At the optimum value, [K]/[Kr]=0.06, an absolute yield of ηq=1.5% was obtained. Up to a deposited energy of 3 mJ/cm3 the quantum yield is independent on the excitation density. For neon as buffer gas the rate constants k(Ne) are smaller by about a factor of 3, while the efficiency is only slightly less than for He. Kr+K is split into five fine structure states. The quenching of these states by krypton and buffer gas causes the low quantum yield.