Experimental Configuration

        The experiment arrangement measures the interaction of cold C₆₀⁺ ions with an incident flux of neutral alkali atoms. The ions are stored within an rf Paul trap and collisionally relaxed by a cold He background gas. The trap is positioned within a UHV chamber evacuated to a base pressure of ~2x10^-10 Torr after bakeout. The trap is cooled to <80 K by contact with a copper baseplate of a liquid N₂ dewar and monitored by a thermocouple on the trap mount. Purified helium gas controlled by a leak valve is admitted into the trap directly through an aperture in the ring electrode after being cooled by passage through a copper maze.

        C₆₀ molecules emitted from an effusive oven operating at ~450⁰ C, enter the trap through an aperture in the ring electrode and are ionized by an electron beam pulse entering an aperture in the endcap electrode. The trapped ions are detected by resonant ejection of the cold ion cloud into the external dynode of an electron multiplier biased at -6 kV. The integrated ion signal of the electron multiplier is calibrated by comparison with in situ electronic detection. This provides the basis for absolute ion-number measurements and has a detection sensitivity of <5 ions. The number of trapped C₆₀⁺ ions was ~10³ for typical e-beam pulselengths of 100 ms.

        An effusive beam of alkali atoms emitted from a Knudsen cell enters the trap through a 1.0 mm aperture in the ring electrode. The alkali flux overfills the ion cloud diameter of <0.2 mm and then leaves the trap through an opposing 1 mm aperture. The measured C₆₀⁺ ion decay rate is proportional to the incident alkali flux. Cell temperatures of 60-350^° C produced beams of Cs, Rb, K, Na, and Li atoms incident on the ion cloud with a flux of ~10¹³ cm^-2/sec. A shutter controls the (0.1-10 sec) time interval during which C₆₀⁺ ions were exposed to the alkali flux. The relative collision energies in these experiments were set by the alkali oven temperatures, covering the range ~60 to 120 meV. In these experiments, relative charge-exchange rates were determined within an uncertainty of +/-23%; however, temperature gradients across the Knudsen cell limited the accuracy of absolute-rates to within a factor of 2-3.

The trap was sequenced through a set of stable operating points at which:

1. C₆₀⁺ ions are formed and efficiently trapped,
2. lighter mass fragments formed during ionization are eliminated,
3. the ion kinetic and vibrational energy are relaxed,
4. the alkali flux is admitted for a period of time and
5. the ions are resonantly ejected for detection.

        The C₆₀⁺ ions are ionized and stored for a period of 20 sec to allow cooling of the cloud by the He gas both before and after resonantly ejecting C_60-2n⁺ ion fragments. At the helium densities used in these measurements, the C₆₀⁺ ion makes ~10⁴ to 10⁵ collisions over 20 sec. This is adequate to thermalize both the translational and vibrational degrees of freedom. After the cold ion cloud is exposed to the alkali flux, the C₆₀⁺ ions and all heavier ions are resonantly ejected into the electron multiplier. Finally, the trap is flushed of any residual ions by applying 100 V dc to the ring electrode.