Anomalous Decline of Molecular Ion Mobility in Cooled Helium Gas

Abstract

We present a first successful theoretical account of the ion mobilities of and in helium gas at 4.3 K. Measured mobilities of various molecular ions at low effective temperatures reportedly tend to values smaller than their polarization limits, with the exception of N(2)(+). The present theoretical results obtained by the classical trajectory calculations agree with the experimental ones very well, and make it definitive that the anomalous decline of molecular ion mobility is caused by a Feshbach-like resonance due to the anisotropic interaction potential between a molecular ion and a helium atom. The mechanism thus revealed is supported by quantitative quantum mechanical calculations. The process appears very similar to that of laser cooling.