Deuteron-to-Proton Mass Ratio from Simultaneous Measurement of the Cyclotron Frequencies of H_{2}^{+} and D^{+}.

By simultaneously measuring the cyclotron frequencies of an 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 H_{2}^{+} mass energy we have followed the decay of three 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 H_{2}^{+} rotational energy. Assuming the most probable assignment, we obtain a deuteron-to-proton mass ratio, m_{d}/m_{p}=1.999 007 501 272(9). Combined with the atomic mass of the deuteron [S. Rau et al., Nature (London) 585, 43 (2020).NATUAS0028-083610.1038/s41586-020-2628-7] we also obtain a new value for the atomic mass of the proton, m_{p}=1.007 276 466 574(10)  u.