Crossed-beam measurements of absolute rate coefficients in associative ionisation collisions between Na*(np) and Na(3s) for 5⩽n⩽15

Associative ionisation in slow collisions between Na*(np) levels and the Na(3s) ground state is studied in a crossed-beam experiment. Excited levels are prepared by laser photoexcitation, and ionic species are analysed by time-of-flight mass separation. Rate coefficients for associative ionisation, measured for principal quantum numbers ranging from n=5 to 15, exhibit a pronounced on n with a maximum of approximately 3*10 -9 cm 3 s -1 near n=11. Comparisons are made with the theoretical model developed by Duman and Shmatov, and Mihajlov and Janev.