Doppler-free multi-photon ionization: a proposal for enhancing ion images

We report on the use of Doppler-free 2+1 resonance enhanced multi-photon ionization (REMPI) to probe H atoms resulting from HBr photolysis at 243 nm. The Doppler-free technique allows almost all H atoms to be ionized at a single probe wavelength, regardless of their lab-frame velocities. This technique yields a significant enhancement in ionization efficiency of the product while leaving the background ionization essentially unaffected. We point out the utility of this technique for the ion imaging of reaction products.