Hyperfine-resolved optical spectroscopy of ultracold <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mmultiscripts> <mml:mi>Rb</mml:mi> <mml:mprescripts/> <mml:none/> <mml:mn>87</mml:mn> </mml:mmultiscripts> <mml:mmultiscripts> <mml:mi>Cs</mml:mi> <mml:mprescripts/> <mml:none/> <mml:mn>133</mml:mn> </mml:mmultiscripts> </mml:mrow> </mml:math> molecules: The <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mi mathvariant="normal">b</mml:mi> <mml:msup> <mml:mspace width="0.28em"/> <mml:mn>3</mml:mn> </mml:msup> <mml:msub> <mml:mi mathvariant="normal">Π</mml:mi> <mml:mn>0</mml:mn> </mml:msub> </mml:mrow> </mml:math> metastable state

Using an ultracold gas of <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"> <a:mrow> <a:mmultiscripts> <a:mi>Rb</a:mi> <a:mprescripts/> <a:none/> <a:mn>87</a:mn> </a:mmultiscripts> <a:mmultiscripts> <a:mi>Cs</a:mi> <a:mprescripts/> <a:none/> <a:mn>133</a:mn> </a:mmultiscripts> </a:mrow> </a:math> molecules, we perform hyperfine-resolved spectroscopy of transitions from the vibronic ground state to the lowest rovibrational states of the electronic state <b:math xmlns:b="http://www.w3.org/1998/Math/MathML"> <b:mrow> <b:mi mathvariant="normal">b</b:mi> <b:msup> <b:mspace width="0.28em"/> <b:mn>3</b:mn> </b:msup> <b:msub> <b:mi mathvariant="normal">Π</b:mi> <b:mn>0</b:mn> </b:msub> </b:mrow> </b:math> as a function of magnetic field. These transitions are spin forbidden, resulting in narrow linewidths, and feature near-diagonal Franck-Condon factors. We develop a model of the hyperfine and Zeeman structure that includes coupling between the <f:math xmlns:f="http://www.w3.org/1998/Math/MathML"> <f:msup> <f:mn>0</f:mn> <f:mo>+</f:mo> </f:msup> </f:math> and <g:math xmlns:g="http://www.w3.org/1998/Math/MathML"> <g:msup> <g:mn>0</g:mn> <g:mo>−</g:mo> </g:msup> </g:math> components of <h:math xmlns:h="http://www.w3.org/1998/Math/MathML"> <h:mrow> <h:mi mathvariant="normal">b</h:mi> <h:msup> <h:mspace width="0.28em"/> <h:mn>3</h:mn> </h:msup> <h:msub> <h:mi mathvariant="normal">Π</h:mi> <h:mn>0</h:mn> </h:msub> </h:mrow> </h:math> . We fit the spectra to obtain rotational and hyperfine coupling constants. We measure transition dipole moments associated with specific transitions by directly observing Rabi oscillations as a function of a resonant laser pulse duration. Using resonant <l:math xmlns:l="http://www.w3.org/1998/Math/MathML"> <l:mi>π</l:mi> </l:math> pulses, we prepare molecules in the electronically excited state and directly measure the spontaneous emission rate.