By simultaneously measuring the cyclotron frequencies of an ${\mathrm{H}}_2^{+}$ ion and a deuteron in a coupled magnetron orbit we have made an extended series of measurements of their cyclotron frequency ratio. From the observed changes in ${\mathrm{H}}_2^{+}$ mass energy we have followed the decay of three ${\mathrm{H}}_2^{+}$ ions to the vibrational ground state. We are able to assign some of our measured ratios to specific rovibrational levels, hence reducing uncertainty due to ${\mathrm{H}}_2^{+}$ rotational energy. Assuming the most probable assignment, we obtain a deuteron-to-proton mass ratio, $m_d/m_p=1.999007501272(9)$. Combined with the atomic mass of the deuteron [S. Rau et al., Nature (London) 585, 43 (2020).] we also obtain a new value for the atomic mass of the proton, $m_p=1.007276466574(10)\mathrm{u}$.

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• Atomic & molecular structure
• Cooling & trapping
• Penning traps