High-precision microwave spectroscopic measurements and analysis of rotational energy level transitions in the ground vibronic state of the open-shell barium monofluoride (BaF) molecule are reported with the purpose of contributing to studies of physics beyond the Standard Model. BaF is currently among the key candidate molecules being examined in the search for a measurable electron electric-dipole moment, $e\text{EDM}$, as well as the nuclear anapole moment. Employing Fourier-transform microwave spectroscopy, these pure rotational transition frequencies for the ${}^{138}\mathrm{Ba}{}^{19}\mathrm{F}, {}^{137}\mathrm{Ba}{}^{19}\mathrm{F}, {}^{136}\mathrm{Ba}{}^{19}\mathrm{F}, {}^{135}\mathrm{Ba}{}^{19}\mathrm{F}$, and ${}^{134}\mathrm{Ba}{}^{19}\mathrm{F}$ isotopologues are analyzed here in a combined global fit with previous microwave data sets for ${}^{138}\mathrm{Ba}{}^{19}\mathrm{F}$ ($v=0–4$), ${}^{137}\mathrm{Ba}{}^{19}\mathrm{F}$, and ${}^{136}\mathrm{Ba}{}^{19}\mathrm{F}$ using the program spfit. As a result, hyperfine parameters are significantly improved and we observe a distinctive structure in a Born-Oppenheimer breakdown (BOB) analysis of the primary rotational constant. This can be understood using the nuclear field shifts due to the known isotopic variation in the size of barium nuclei and in combination with the smaller linear mass-dependent BOB terms.

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• Atomic & molecular structure
• Fine & hyperfine structure
• Molecular spectra
• Rotational states
• Vibrational states
• Molecules
• Laser ablation
• Microwave techniques
• RF, microwave, & terahertz sources