Formation of He₄⁺ via electron impact of helium droplets

Electron impact ionization of superfluid helium droplets containing several thousand atoms produces a broad distribution of He_n⁺ ions that peaks at n = 2 and decreases monotonically toward larger n. In larger droplets (say 10⁵ or more atoms), however, the He₄⁺ signal intensity is anomalously large. We have studied the mechanism for the formation of He₄⁺ ions in large helium droplets by varying the duration of the electron impact excitation pulse. Droplets of different average sizes were generated using the expansion of helium at 20 bars and 9-20 K through a pulsed valve nozzle. The resulting ions were analyzed by time-of-flight mass spectroscopy (TOFMS) and quadrupole mass spectroscopy (QMS). The intensity distributions obtained with the TOFMS technique initially showed much smaller He₄⁺ signals than those obtained using QMS. However, we discovered that the intensity anomaly is associated with the duration of the electron bombardment pulse in the TOFMS instrument. Measurements with different electron bombardment pulse durations enabled us to discern a characteristic time of ∼10 μs for enhanced He₄⁺ production in large droplets under our experimental conditions. A qualitative model is presented in which metastables interact on droplet surfaces, yielding two He₂⁺ cores that share a Rydberg electron while minimizing repulsion between the cores. This is the He₄⁺(⁴A₂) state suggested by Knowles and Murrell.