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Skimmed supersonic beams provide intense, cold, collision-free samples ofatoms and molecules are one of the most widely used tools in atomic andmolecular laser spectroscopy. High-resolution optical spectra are typicallyrecorded in a perpendicular arrangement of laser and supersonic beams tominimize Doppler broadening. Typical Doppler widths are nevertheless limited totens of MHz by the residual transverse-velocity distribution in thegas-expansion cones. We present an imaging method to overcome this limitationwhich exploits the correlation between the positions of the atoms and moleculesin the supersonic expansion and their transverse velocities - and thus theirDoppler shifts. With the example of spectra of$(1\mathrm{s})(n\mathrm{p})\,^3\mathrm{P}_{0-2}\leftarrow(1\mathrm{s})(2\mathrm{s})\,^3\mathrm{S}_1$ transitions to high Rydberg statesof metastable triplet He, we demonstrate the suppression of the residualDoppler broadening and a reduction of the full linewidths at half maximum toonly about 1 MHz in the UV. Using a retro-reflection arrangement for the laserbeam and a cross-correlation method, we determine Doppler-free spectra withoutany signal loss from the selection, by imaging, of atoms within ultranarrowtransverse-velocity classes. As an illustration, we determine the ionizationenergy of triplet metastable He and confirm the significant discrepancy betweenrecent experimental (Clausen et al., Phys. Rev. Lett. 127 093001 (2021)) andhigh-level theoretical (Patk\'os et al., Phys. Rev. A 103 042809 (2021)) valuesof this quantity.

Imaging-assisted single-photon Doppler-free laser spectroscopy and the ionization energy of metastable triplet helium