Experimental study reveals state-selective excitation in jet-cooled ammonia via IR–UV ion gain spectroscopy, indicating a route to access previously unreachable molecular states.
We present the results of a double-resonance IR–UV spectroscopic experiment focusing on the effect of exciting different rovibrational levels of the so-called ’umbrella’ mode ( ν 2 ) in the X ̃ ground electronic state when probing the spectral range of the B ̃ ( 2 4 ) transition. First, we observe that implementing a background-free IR ion-gain detection scheme ensures improved sensitivity of IR vibrational spectroscopy of the ground state in comparison to the IR ion-dip method. Secondly, we demonstrate that through the combined spectral resolution of the Free-Electron Laser FELIX and the state-selectivity of a Resonance-Enhanced MultiPhoton Ionisation (REMPI) scheme we are able to identify two-photon transitions from the X ̃ ( 2 1 ) state into the C ̃ ′ ( 0 ) and B ̃ ( 2 2 , 3 1 ) states. These findings will facilitate assignment of hot-band contributions in REMPI spectra of ammonia as well as open up a route for a controlled population of states that are inaccessible from the ground state directly.
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Duda et al. (2026) studied this question.
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