Velocity-modulation Fourier-transform spectroscopy of molecular ions

The first demonstration of the combination of two powerful techniques, Fourier-transform spectroscopy and velocity modulation, for the selective detection of molecular ions, is reported. This is also the first time that velocity modulation has been observed in emission. As an application of this we also reported the experimental measurement of the ratio of the dipole moment to the vibrational transition moment of a molecular ion, ${\mathrm{ArH}}^{+}$, by the Herman-Wallis effect.