1 Aarhus University2 Department of Physics and Astronomy, Science and Technology, Aarhus University3 Department of Physics and Astronomy, Faculty of Science, Aarhus University, Aarhus University4 Department of Chemistry, Science and Technology, Aarhus University5 Max-Planck-Institut für Kernphysik6 Nicolaus Copernicus University7 Department of Physics and Astronomy, Science and Technology, Aarhus University8 Department of Chemistry, Science and Technology, Aarhus University
We present a method to measure the decay rate of the first excited vibrational state of polar molecular ions that are part of a Coulomb crystal in a cryogenic linear Paul trap. Specifically, we have monitored the decay of the |ν=1,J=1⟩X towards the |ν=0,J=0⟩X level in MgH+ by saturated laser excitation of the |ν=0,J=2⟩X-|ν=1,J=1⟩X transition followed by state selective resonance enhanced two-photon dissociation out of the |ν=0,J=2⟩X level. The experimentally observed rate of 6.32(0.69) s-1 is in excellent agreement with the theory value of 6.13(0.03) s-1 (this Letter). The technique enables the determination of decay rates, and thus absorption strengths, with an accuracy at the few percent level.