Magnetic-field dependence of the cross section for mJ mixing in 2P1/2 Cs and Rb atoms.

Abstract

Transitions between the Zeeman substates of Cs 6 $^{2}\mathrm{P}_{1/2}$ and Rb 5 $^{2}$${\mathit{P}}_{1/2}$ atoms, induced in collisions with He atoms, have been investigated experimentally by methods of atomic fluorescence spectroscopy. The collision process has also been studied theoretically with time-dependent perturbation theory. Cs or Rb vapor contained in a quartz cell together with helium at low pressure was selectively excited by pulsed dye-laser radiation to the $^{2}$${\mathit{P}}_{1/2,\mathrm{\ensuremath{-}}1/2}$ Zeeman substate in a magnetic field ranging from 1.5 T to 7 T. The fluorescence spectrum, consisting of components emitted from the directly excited state and from the collisionally populated state, was resolved with a Fabry-P\'erot interferometer. Measurements of the relative intensities of the fluorescence components yielded the Zeeman mixing cross section for Cs-He and Rb-He collisions. It was found that the Cs 6 $^{2}$${\mathit{P}}_{1/2}$ cross section varied approximately as ${\mathit{B}}^{2}$ in the range 0B0.76 T, showed a much weaker field dependence in the range from 0.76 T to 5 T, where it peaked, and declined slightly at Bg5 T. The Rb 5 $^{2}$${\mathit{P}}_{1/2}$ cross section did not exhibit any variation with B. A theoretical calculation of the Cs cross section carried out using time-dependent perturbation theory explains the magnetic-field enhancement in terms of the minute magnetic-field-induced mixing of the ${\mathit{P}}_{1/2}$ and ${\mathit{P}}_{3/2}$ states and the resulting breaking of time-reversal symmetry. The results of the calculation are in good agreement with the experimental results at fields in the range 0B5 T.