  We demonstrate the first application of a liquid-metal ion source for
collinear laser spectroscopy in proof-of-principle measurements on naturally
abundant In$^+$. The superior beam quality, i.e., the actively stabilized
current and energy of a beam with very low transverse emittance, allowed us to
perform precision spectroscopy on the $5s^2\;^1\mathrm{S}_0 \rightarrow
5s5p\;^3\mathrm{P}_1$ intercombination transition in $^{115}$In$^+$, which is
to our knowledge the slowest transition measured with collinear fluorescence
laser spectroscopy so far. By applying collinear and anticollinear
spectroscopy, we improved the center-of-gravity frequency
$\nu_\mathrm{cg}=1\,299\,617\,759.\,3\,(1.2)$ and the hyperfine constants
$A=6957.19\,(28)$\,MHz and $B=-443.7\,(2.4)$\,MHz by more than two orders of
magnitude. A similar accuracy was reached for $^{113}$In$^+$ in combination
with literature data and the isotope shift between both naturally abundant
isotopes was deduced to
$\nu(^{113}\mathrm{In})-\nu(^{115}\mathrm{In})=696.3\,(3.1)$\,MHz. Nuclear
alignment induced by optical pumping in a preparation section of the ion
beamline was demonstrated as a pump-and-probe approach to provide sharp
features on top of the Doppler broadened resonance profile.
