Transport properties of Si ions moving through He gas

Abstract

In this study, we revisit the mobility and diffusion coefficients of Si⁺ ions in their ground ²P state moving in helium gas using quantum–mechanical transport cross sections. We have built the Si⁺-He potential from reliable and very recent energy points. These data points used in our construction are smoothly connected to adequate long- and short-range shapes, and calculations of classical cross section coefficients are also calculated. For the first time, we report transport coefficients for Si⁺(²P) across a wide range of reduced fields (10–1000 Td) and temperatures (100–500 K), achieving uncertainties of < 3% (low T) and < 5% (high T). While prior theoretical studies (Tuttle et al. in Mol Phys 115:437, 2017; Davies et al. in Phys Chem Chem Phys 24:7144, 2022) could not resolve whether the ²P_3/2 or ²P state aligns with experimental mobility data (Fahey et al. in J Chem Phys 75:669, 1981), our results demonstrate that neither the ²P ground state nor its substates reproduce Fahey’s measurements within experimental uncertainty (± 5%). The discrepancy (8–12% at E/N < 50 Td) arises from quantum resonances in the momentum-transfer cross section (see Fig. 7, inset), which are absent in classical models. We corroborate Viehland’s hypothesis (Viehland et al. in Int J Ion Mobil Spectrom 20:95, 2017) that the low-pressure conditions (0.25–0.45 Torr) in Fahey’s experiment suppressed collisional excitation, favoring a non-statistical population of spin–orbit states. These findings challenge assumptions about ion–neutral interactions at low energies and underscore the need for state-resolved experiments.

Graphical abstract