Laser-Assisted Evaporative Cooling of Anions

We report the first cooling of atomic anions by laser radiation. ${\mathrm{O}}^{-}$ ions confined in a linear Paul trap were cooled by selectively photodetaching the hottest particles. For this purpose, anions with the highest total energy were illuminated with a 532 nm laser at their maximal radial excursion. Using laser-particle interaction, we realized a both colder and denser ion cloud, achieving a more than threefold temperature reduction from 1.15 to 0.33 eV. Compared with the interaction with a dilute buffer gas, the energy-selective addressing and removal of anions resulted in lower final temperatures, yet acted 10 times faster and preserved twice as large a fraction of ions in the final state. An ensemble of cold negative ions affords the ability to sympathetically cool any other negative ion species, enabling or facilitating a broad range of fundamental studies from interstellar chemistry to antimatter gravity. The technique can be extended to any negative ion species that can be neutralized via photodetachment.

Figure 1

Schematic drawing of the linear Paul trap. Ions enter the trap from the left and exit towards the MCP detector to the right.

Figure 2

Final anion temperature as a function of the lens (and hence laser beam) displacement from the trap axis after 12 s of laser illumination. Vertical dashed lines indicate the lens positions at which neither ion heating nor cooling occurs.

Figure 3

Number of ${\mathrm{O}}^{-}$ ions in the trap as a function of storage time. The measurements with the laser on were acquired with the laser beam positioned at $d=1.5\mathrm{mm}$ [Eq. (1)]. Datasets with laser on and off denoted with the same symbols were taken contemporaneously under otherwise identical conditions. Solid lines are fits to the data using the function of Eq. (2).

Figure 4

Temperature of ${\mathrm{O}}^{-}$ anions in the trap as a function of storage time. The measurements with the laser were acquired with $d=1.5\mathrm{mm}$ [Eq. (1)]. Solid lines are fits to the data using the function of Eq. (3).

Figure 5

Anion temperature as a function of the fraction of ions remaining in the trap. The number of remaining anions is normalized to the initial population before cooling. Error bars are comparable to the size of the data points. Solid lines are second-order polynomial fits and are meant to guide the eye.

Figure 6

Anion temperature as a function of storage time after the end of laser-induced cooling. Solid lines are simple exponential fits to the data.