Fine-structure changing collisions in ⁸⁷Rb upon D₂ excitation in the hyperfine Paschen-Back regime

Abstract We investigate fine structure changing collisions in 87 Rb vapour upon D 2 excitation in a thermal vapour at 75 ∘ C; the atoms are placed in a 0.6 T axial magnetic field in order to gain access to the hyperfine Pashen-Back regime. Following optical excitation on the D 2 line, the exothermic transfer 5P <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msub> <mml:mrow/> <mml:mrow> <mml:mn>3</mml:mn> <mml:mo>/</mml:mo> <mml:mn>2</mml:mn> </mml:mrow> </mml:msub> </mml:mrow> </mml:math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mo>→</mml:mo> </mml:math> 5P <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msub> <mml:mrow/> <mml:mrow> <mml:mn>1</mml:mn> <mml:mo>/</mml:mo> <mml:mn>2</mml:mn> </mml:mrow> </mml:msub> </mml:mrow> </mml:math> occurs as a consequence of buffer-gas collisions; the 87 Rb subsequently emits a photon on the D 1 transition. We employ single-photon counting apparatus to monitor the D 1 fluorescence, with an etalon filter to provide high spectral resolution. By studying the D 1 fluorescence when the D 2 excitation laser is scanned, we see that during the collisional transfer process the <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msubsup> <mml:mi>m</mml:mi> <mml:mrow> <mml:mi>J</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>′</mml:mi> </mml:mrow> </mml:msubsup> </mml:mrow> </mml:math> quantum number of the atom changes, but the nuclear spin projection quantum number, <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msubsup> <mml:mi>m</mml:mi> <mml:mrow> <mml:mi>I</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>′</mml:mi> </mml:mrow> </mml:msubsup> </mml:mrow> </mml:math> , is conserved. A simple kinematic model incorporating a coefficient of restitution in the collision accounted for the change in velocity distribution of atoms undergoing collisions, and the resulting fluorescence lineshape. The experiment is conducted with a nominally ‘buffer-gas free’ vapour cell; our results show that fine structure changing collisions are important with such media, and point out possible implications for quantum-optics experiments in thermal vapours producing entangled photon pairs with the double ladder configuration, and solar physics magneto-optical filters.