Sympathetic Cooling of Complex Molecular Ions to Millikelvin Temperatures

Gas-phase singly protonated organic molecules of mass 410 Da (Alexa Fluor 350) have been cooled from ambient temperature to the hundred millikelvin range by Coulomb interaction with laser-cooled barium ions. The molecules were generated by an electrospray ionization source, transferred to and stored in a radio-frequency trap together with the atomic ions. Observations are well described by molecular dynamics simulations, which are used to determine the spatial distribution and thermal energy of the molecules. In one example, an ensemble of 830 laser-cooled ${}^{138}\mathrm{Ba}^{+}$ ions cooled 200 molecular ions to less than 115 mK. The demonstrated technique should allow a large variety of protonated molecules to be sympathetically cooled, including molecules of much higher mass, such as proteins.

Figure 1

(a) Simulated multispecies crystal (side- and cross-sectional views): laser-cooled ${}^{138}\mathrm{Ba}^{+}$ ions (blue) at 25 mK, sympathetically cooled barium isotopes (${}^{135–137}\mathrm{Ba}^{+}$, red), ${\mathrm{AF}}^{+}$ ions at 95 mK (green). Lasers propagate to the left. The ${\mathrm{Ba}}^{+}$ ensemble is $1660\mu \mathrm{m}$ long and $130\mu \mathrm{m}$ in diameter. The ${\mathrm{AF}}^{+}$ sheath is $300\mu \mathrm{m}$ in diameter. (b) CCD image of the barium crystal with ${\mathrm{AF}}^{+}$ present; six distinct layers are visible corresponding to three coaxial spheroidal shells (camera exposure time: 0.5 s). (c) Simulation of the crystal in (b). (d) Barium crystal after ejection of all ${\mathrm{AF}}^{+}$ ions. (e) Simulation of the crystal in (d), $T_{\mathrm{Ba},0}=20\mathrm{mK}$.

Figure 2

Mass scan of a Coulomb crystal containing barium and ${\mathrm{AF}}^{+}$ ions. Excitation of radial oscillations of (a) ${\mathrm{AF}}^{+}$ ions and (b) ${\mathrm{Ba}}^{+}$ ions. Calculated single-particle radial secular frequencies are 33.7 kHz and 102 kHz, respectively (respective axial frequencies are at 19.3 and 11.2 kHz).

Figure 3

Ions (of crystal in Fig. 1) detected on the CEM as the trap rf voltage is decreased. Heavier ${\mathrm{AF}}^{+}$ ions are ejected first (right-hand peak); inset (a) extraction of a different sample of non-laser-cooled barium and ${\mathrm{AF}}^{+}$ ions at $\sim 300\mathrm{K}$; inset (b) extraction of a laser-cooled cloud at a temperature of a few hundred mK for ${\mathrm{Ba}}^{+}$ and ${\mathrm{AF}}^{+}$ ions.