Optical Mass Spectrometry of Cold ${\mathrm{RaOH}}^{+}$ and ${{\mathrm{RaOCH}}_3}^{+}$

We present an all-optical mass spectrometry technique to identify trapped ions. The new method uses laser-cooled ions to determine the mass of a cotrapped dark ion with a sub-dalton resolution within a few seconds. We apply the method to identify the first controlled synthesis of cold, trapped ${\mathrm{RaOH}}^{+}$ and ${{\mathrm{RaOCH}}_3}^{+}$. These molecules are promising for their sensitivity to time and parity violations that could constrain sources of new physics beyond the standard model. The nondestructive nature of the mass spectrometry technique may help identify molecular ions or highly charged ions prior to optical spectroscopy. Unlike previous mass spectrometry techniques for small ion crystals that rely on scanning, the method uses a Fourier transform that is inherently broadband and comparatively fast. The technique’s speed provides new opportunities for studying state-resolved chemical reactions in ion traps.

Figure 1

(a) ${\mathrm{Ra}}^{+}$ energy levels and transitions used in this work. ${\mathrm{\Delta }}_{\mathrm{SP}}$ (${\mathrm{\Delta }}_{\mathrm{DP}}$) is the detuning, and ${\mathrm{\Omega }}_{\mathrm{SP}}$ (${\mathrm{\Omega }}_{\mathrm{DP}}$) is the Rabi frequency of the 468 (1079) nm light. (b) The $S_{1/2}$ to $P_{1/2}$ spectrum with ${\mathrm{\Delta }}_{\mathrm{SP}}$ and ${\mathrm{\Delta }}_{\mathrm{DP}}$ set to amplify ion motion. The local slope at ${\mathrm{\Delta }}_{\mathrm{SP}}$ (red dashed line) is negative, while the global slope (blue dashed line) is positive. (c) OMS geometry with two ${\mathrm{Ra}}^{+}$ and one ${{\mathrm{RaOCH}}_3}^{+}$ shown between two radial trap electrodes, as well as the relative orientation of the cooling and repump light and the magnetic field.

Figure 2

$P_{1/2}$ state population as a function of 468 nm detuning. A fit of the spectrum to a numerical solution of the $\mathrm{\Lambda }$-level system that accounts for all Zeeman levels [38, 39] gives ${\mathrm{\Delta }}_{\mathrm{DP}}/2\pi =-10\mathrm{MHz}$, ${\mathrm{\Omega }}_{\mathrm{SP}}/2\pi =19\mathrm{MHz}$, and ${\mathrm{\Omega }}_{\mathrm{DP}}/2\pi =13\mathrm{MHz}$. The blue line at ${\mathrm{\Delta }}_{\mathrm{SP}}/2\pi =-22\mathrm{MHz}$ is the detuning of the 468 nm light used for CPT amplification. The $P_{1/2}$ state population is not suppressed to zero at ${\mathrm{\Delta }}_{\mathrm{SP}}={\mathrm{\Delta }}_{\mathrm{DP}}$ due to the finite linewidths of both the 468 and 1079 nm lasers ($\sim 3\mathrm{MHz}$).

Figure 3

Fourier transformed PMT counts for Coulomb crystals where two ${\mathrm{Ra}}^{+}$ surround a third ion, labeled in the legend, in a linear chain. The Fourier amplitudes are normalized by their backgrounds for clarity. The dashed vertical lines show the calculated center ion masses. The peak amplitudes vary due to drift in the power and frequency of the 468 and 1079 nm lasers over the span of several days during which the measurements were taken.

Figure 4

Fourier spectra of the fluorescent light from ${{}^{88}\mathrm{Sr}}^{+}$, which is in a crystal with a second ${{}^{88}\mathrm{Sr}}^{+}$, ${}^{87}{\mathrm{Sr}}^{+}$, ${}^{86}{\mathrm{Sr}}^{+}$, or ${}^{84}{\mathrm{Sr}}^{+}$ ion. The calculated dark ion masses for the above crystals are shown as dashed vertical lines in the plot [30].