Hyperfine Structure of Thallium Bromide

The hyperfine structure of the $\text{J\hspace{0.17em}=\hspace{0.17em}2}$ and $\text{J\hspace{0.17em}=\hspace{0.17em}3}$ rotational states of TlBr has been measured with a molecular‐beam electric resonance spectrometer. Hyperfine transition frequencies were measured under conditions of very weak electric and magnetic fields. The linewidth was 500 Hz. The hyperfine interaction constants have been determined for ²⁰⁵Tl⁷⁹Br, ²⁰³Tl⁷⁹Br, ²⁰⁵Tl⁸¹Br, and ²⁰³Tl⁸¹Br in the first five vibrational states for $\text{J\hspace{0.17em}=\hspace{0.17em}2}$ . In addition, the interaction constants for $\text{J\hspace{0.17em}=\hspace{0.17em}3, υ\hspace{0.17em}=\hspace{0.17em}0}$ were determined for Tl⁷⁹Br and Tl⁸¹Br, but it was not possible to resolve the effect of the two thallium isotopes in the $\text{J\hspace{0.17em}=\hspace{0.17em}3}$ state. The spectra measured are well described by a hyperfine Hamiltonian containing the bromine quadrupole interaction, the spin–rotation interactions of both the thallium and the bromine nuclei, and both the scalar and tensor parts of the spin–spin interaction between the nuclei. The dependence of the magnetic hfs constants on vibrational state and on isotopic composition shows good agreement with theory. The magnetic octupole interaction of the bromine nucleus in TlBr is negligibly small, and we find no evidence for nuclear polarization or pseudoquadrupole effects.