Details

Autor(en) / Beteiligte

• Fine, Jordan
• Verma, Deepak
• Jones, Curtis F
• Wittig, Curt
• Vilesov, Andrey F

Titel

Formation of He 4 + via electron impact of helium droplets

Ist Teil von

• The Journal of chemical physics, 2018-01, Vol.148 (4), p.044302

Ort / Verlag

United States

Erscheinungsjahr

2018

Link zum Volltext

Quelle

American Institute of Physics (AIP) Journals

Beschreibungen/Notizen

• Electron impact ionization of superfluid helium droplets containing several thousand atoms produces a broad distribution of He ions that peaks at n = 2 and decreases monotonically toward larger n. In larger droplets (say 10 or more atoms), however, the He signal intensity is anomalously large. We have studied the mechanism for the formation of He 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 He 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 He production in large droplets under our experimental conditions. A qualitative model is presented in which metastables interact on droplet surfaces, yielding two He cores that share a Rydberg electron while minimizing repulsion between the cores. This is the He ( A ) state suggested by Knowles and Murrell.