Study of ionizing collisions involving excited states in a potassium-rubidium mixture at thermal energy

This study concerns mainly ionising collisions involving excited states in a saturated mixture of K-Rb vapours, at thermal energy. The experimental method consists into continuous resonant two steps laser excitation of the atoms (n ≤ 10) and mass spectrometry of ion currents. Radiative and collisional relaxation of the atoms create a complex medium. The most efficient collisional processes are Penning ionisation and Hornbeck-Molnar ionisation. In the heteronuclear system Rb(n1) + K(4P), the following exit channels may be operative: Rb(n1) + K(4P) → Rb+ + e- + K Rb(n1) + K(4p) → K+ + e- + Rb Rb(n1) + K(4P) → KRb+ + e-. The measurements show that the first channel has an average cross section ∼ 10-13 cm2. Those of the other channels are at least three orders of magnitude smaller and thus comparatively negligible. The data obtained from 5D to 10S allow to conclude that the flux in the entrance channel ionises at large separation between Rb(n1) and K(4P). The process of ionisation is dominated by polarisation forces, exchange forces being negligible. In the present mixture, Hornbeck-Molnar ionisation leads to homonuclear molecular ions K2+, Rb2+ as well as the heteronuclear one KRb+. We have measured the rate coefficients for the systems: K(n1) + Rb → KRb+ + e-Rb(n1) + K → KRb+ + e-. The rate coefficients increase with the excitation energy of the level n1; they do not exhibit fundamental differences with those measured in pure alkali vapours