Determination of the dipole moment ofArH+from the rotational Zeeman effect by tunable far infrared laser spectroscopy

The first determination of the electric dipole moment of a molecular ion is reported. A tunable far-infrared laser was used to measure the Zeeman effect in low-J rotational transitions of ${\mathrm{ArH}}^{+}$ and ${\mathrm{ArD}}^{+}$, and the dipole was determined from the isotopic dependence of ${\mathrm{g}}_{\mathrm{r}}$. The result (μ=1.4±0.4 D) is in marginal agreement with ab initio calculations (2.2 D). It is shown that this method can ultimately provide molecular-ion dipole moments accurate to within a few percent. The adiabatic internuclear separations of both isotopes were also determined from the ${\mathrm{g}}_{\mathrm{r}}$ values.