Nondestructive Identification of Cold and Extremely Localized Single Molecular Ions

We demonstrate a simple and nondestructive method for identification of a single molecular ion sympathetically cooled by a single laser cooled atomic ion in a linear Paul trap. The technique is based on a precise nondestructive determination of the molecular ion mass through a measurement of the eigenfrequency of a common motional mode of the two ions. The demonstrated mass resolution is sufficiently high that molecular ion mass doublets can potentially be distinguished from each other. The obtained results represent an important step towards single molecule gas phase chemical physics.

Figure 1

Sketch of the experimental setup. Cooling of the ${}^{40}\mathrm{C}\mathrm{a}^{+}$ ion [left (blue) dot] is provided by three laser beams as shown, while the molecular ion [right (red) dot] will be cooled sympathetically. See text for details.

Figure 2

Images of fluorescence from ${}^{40}\mathrm{C}\mathrm{a}^{+}$ ions. (a) Two ${}^{40}\mathrm{C}\mathrm{a}^{+}$ ions at thermal equilibrium in the trap. (b) The same two ${}^{40}\mathrm{C}\mathrm{a}^{+}$ ions as in (a), but with a modulated voltage applied to the trap electrodes at a frequency near the resonance frequency ${\nu }_1=98.7\mathrm{k}\mathrm{H}z$. (c) The same situation as in (a), but with one of the ${}^{40}\mathrm{C}\mathrm{a}^{+}$ ions and presumably a nonfluorescing ${}^{40}\mathrm{C}\mathrm{a}{{}^{16}\mathrm{O}}^{+}$ ion. (d) Image of the ${}^{40}\mathrm{C}\mathrm{a}^{+}$ ion in (c), but when a modulated force at a frequency close to the resonance frequency ${\nu }_{-}=89.4\mathrm{k}\mathrm{H}z$ is present. In all the experiments the radial trap frequencies were 380 kHz and the exposure time of the CCD chip was 100 ms.

Figure 3

The position resolved fluorescence along the trap axis as a function of the drive frequency of the intensity modulated laser beam. (a) Two ${}^{40}\mathrm{C}\mathrm{a}^{+}$ ions. (b) One ${}^{40}\mathrm{C}\mathrm{a}^{+}$ ion and one ${}^{42}\mathrm{C}\mathrm{a}^{+}$ ion, where only the ${}^{40}\mathrm{C}\mathrm{a}^{+}$ ion is fluorescing. Each gray scale (false-colored) contour plot is composed of axial projections of the fluorescence intensities in gated images recorded during the frequency scans [scan rates: (a) $2\mathrm{H}\mathrm{z}/\mathrm{\text{image}}$, (b) $8\mathrm{H}\mathrm{z}/\mathrm{\text{image}}$]. Dashed lines indicate equilibrium positions of the ions in the absence of modulation. The dark gray (red) areas near the dashed lines corresponds to high intensity, while the dark (blue) areas are low intensity regions. The radial trap frequencies were 400 kHz in all experiments.

Figure 4

Resonance frequencies obtained for three combinations of calcium isotopes. Each data point represents the resonance frequency found from fits to data series as those shown in Fig. 3. The stated resonance frequencies are the statistical average values of the data points. The horizontal position of the data points reflects the time order of the experiments.