Laser cycling of resonances can remove entropy from a system via spontaneously emitted photons, with electronic resonances providing the fastest cooling timescales because of their rapid spontaneous relaxation. Although atoms are routinely laser-cooled, even simple molecules pose two interrelated challenges for cooling: every populated rotational-vibrational state requires a different laser frequency, and electronic relaxation generally excites vibrations. Here we cool trapped AlH þ molecules to their ground rotational-vibrational quantum state using an electronically exciting broadband laser to simultaneously drive cooling resonances from many different rotational levels. Undesired vibrational excitation is avoided because of vibrational-electronic decoupling in AlH þ . We demonstrate rotational cooling on the 140(20) ms timescale from room temperature to 3:8 þ 0:9 À 0:3 K, with the groundstate population increasing from B3 to 95:4 þ 1:3 À 2:1 % . This cooling technique could be applied to several other neutral and charged molecular species useful for quantum information processing, ultracold chemistry applications and precision tests of fundamental symmetries.

We perform ab initio calculations needed to predict the cross-section of an experimentally accessible (1 + 1 ′ ) resonance-enhanced multiphoton dissociation (REMPD) pathway in AlH + . Experimenting on AlH + ions held in a radiofrequency Paul trap, we confirm dissociation via this channel with analysis performed using time-of-flight mass spectrometry. We demonstrate the use of REMPD for rotational state analysis, and we measure the rotational distribution of trapped AlH + to be consistent with the expected thermal distribution. AlH + is a particularly interesting species for ion trap work because of its electronic level structure, which makes it amenable to proposals for rotational optical pumping, direct Doppler cooling, and single-molecule fluorescence detection. Potential applications of trapped AlH + include searches for time-varying constants, quantum information processing, and ultracold chemistry studies.