Formation of He 4 + via electron impact of helium droplets.

Electron impact ionization of superfluid helium droplets containing several thousand atoms produces a broad distribution of Hen + 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 + (4 A2 ) state suggested by Knowles and Murrell.