PENNING IONIZATION OF NA, K, RB, AND CS BY HE(251-3) IN A STATIONARY AFTERGLOW

The rate of destruction of helium atoms in the metastable 2 3S1 state has been measured in a stationary helium afterglow containing either Na, K, Rb, or Cs. Any change in the destruction rate of He(23S) atoms when the alkali atom density is varied is attributed to Penning ionisation of the ground state alkali atoms. The measured rate coefficients in units of 10-10 cm3 s-1 are k(Na)=5.1+or-1.0, k(K)=7.7+or-1.5, k(Rb)=12.0+or-2.4, and k(Cs)=4.5+or-0.9. The corresponding Penning ionisation cross sections in units of 10-16 cm2 and for a temperature of about 350K are sigma (Na)=33+or-6, sigma (K)=55+or-10, sigma (Rb)=93+or-18, and sigma (Cs)=34+or-7. It is suggested that energy transfer from He(23S) to a core-excited autoionising state of K or Rb results in larger cross sections for these two alkalis.