Amorphous solid water (ASW) has an important role in surface reaction on interstellar grain. Researchers have used ASW as an ice dust analog produced by various methods such as vapor depositions. The structure of ASW is known to depend on the formation method and temperature. However, orientation on H2O in ice bulk, which may affect reactions on ice, is still unclear. In previous works, ASW was found to be electrically biased when it was deposited on surfaces. In this work, it was first observed that the surface voltage of vapor-deposited ASW on more volatile molecular solids like CO inverts when underlayer solids sublimate and that the inversion depends on deposition methods. Our experiment apparatus consists of a vacuum chamber, He refrigerator, a Fourier transform infrared spectroscopy, quadrupole mass spectrometer and a Kelvin probe to monitor the surface voltage. The ASW was produced by the two methods: vapor deposition through a capillary plate (45 degrees normal to substrate) or background deposition, over the pre-condensed CO on an Au substrate at 10 K. When CO sublimates at ~35 K, surface voltage of the sample rapidly increases from -1.3V to 3.8V. Surprisingly, not only the value of surface voltage inverts, but also the absolute value increases. And it decreases to zero with crystallization of ice. This phenomenon occurs in case of capillary plate deposition. In background deposition, this rapidly increasing cannot be observed. We will discuss this Phenomenon.

Inversion of surface voltage on H2O films affecting sublimation of CO underlayer

2019 Fall National Meeting

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