Symmetry in Alkali Halide Polyatomics: LiKF₂, LiCsF₂, & Li₂FCl

With the current interest in lithium chemistry and the extension of parity nonconservation (PNC) searches to small polar polyatomic molecules, 20 new high precision Fourier transform microwave (FTMW) transitions have been measured in Bilbao for the mixed alkali halide dimer LiKF₂ and progress has been made analyzing our existing ⁶LiCsF₂ and ⁶Li₂FCl Hannover data sets.  This set includes 3 new lines for the isotopologue ⁶Li⁴¹KF₂ (~7% abundance using enriched ⁶Li) and 17 new lines for ⁷Li³⁹KF₂. Large inertial defects are consistent with the floppy out-of-plane motion for these ionically bound molecules, and intensity modulation is observed for the identical ⁶Li bosons in ⁶Li₂FCl and the identical ¹⁹F fermions in all LiKF₂ and LiCsF₂ isotopologues. The C2v symmetry of these kite-shaped planar rings is analogous to the important identical fermion structure of the 2 H atoms in PNC candidate YbNH₂.  Further studies using lithium in natural abundance would thus complete the set by offering identical lithium fermions in ⁷Li₂FCl and broken symmetry for ⁶Li⁷LiFCl.  Work is continuing to crack the congested hyperfine structure in ⁶LiCsF₂ with the help of ab initio theory support, and we are only a couple of transitions away from being able to determine a precise structure for Li₂FCl.