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Effect of kinetic energy on the doping efficiency of cesium cations into superfluid helium droplets

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  • We present an experimental investigation of the effect of kinetic energy on the ion doping efficiency of superfluid helium droplets using cesium cations from a thermionic emission source. The kinetic energy of Cs⁺ is controlled by the bias voltage of a collection grid collinearly arranged with the droplet beam. Efficient doping from ions with kinetic energies from 20 eV up to 480 V has been observed in different sized helium droplets. The relative ion doping efficiency is determined by both the kinetic energy of the ions and the average size of the droplet beam. At a fixed source temperature, the number of doped droplets increases with increasing grid voltage, while the relative ion doping efficiency decreases. This result implies that not all ions are captured upon encountering with a sufficiently large droplet, a deviation from the near unity doping efficiency for closed shell neutral molecules. We propose that this drop in ion doping efficiency with kinetic energy is related to the limited deceleration rate inside a helium droplet. When the source temperature changes from 14 K to 17 K, the relative ion doping efficiency decreases rapidly, perhaps due to the lack of viable sized droplets. The size distribution of the Cs⁺-doped droplet beam can be measured by deflection and by energy filtering. The observed doped droplet size is about 5 × 10⁶ helium atoms when the source temperature is between 14 K and 17 K.
  • Article Copyright 2015 American Institute of Physics. This article may be downloaded for personal use only. Any other use requires prior permission of the author and the American Institute of Physics.,The following article appeared in Journal of Chemical Physics and may be found at: http://scitation.aip.org/content/aip/journal/jcp
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  • Chen, L., Zhang, J., Freund, W. M., & Kong, W. (2015). Effect of kinetic energy on the doping efficiency of cesium cations into superfluid helium droplets. The Journal of Chemical Physics, 143(4), 044310. doi:10.1063/1.4927471
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  • 143
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  • 4
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  • This work is supported by the National Institute of General Medical Sciences (Grant Nos. 1RC1GM092054-01 and 1R01GM101392-01A1) from the National Institutes of Health. Additional support from the Murdock Charitable Trust, the Oregon Nanoscience and Microtechnologies Institute, and the Environmental Health Science Center at Oregon State University funded by the National Institute of Environmental Health Sciences (Grant No. ES000240) are also deeply appreciated.
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