Comparing the charge transfer reactions of rotationally cold NH3 and ND3 with Ar

Capture theories are often used to predict the behavior of chemical reactions when there is limited experimental data available, allowing processes occurring in extreme environments such as the interstellar medium to be modeled. However, many capture theory predictions are yet to be experimentally validated. The accuracy with which capture theory methods can predict the effects of deuteration and rotational state distribution on reaction rate coefficients is an area of active investigation. In this work, the charge transfer reactions of Ar+ ions with rotationally cold ammonia isotopologues (NH3 and ND3) are investigated. Experimental reaction rate coefficients are significantly enhanced compared to previous measurements conducted with room temperature ammonia. Interesting isotope effects are observed, with the influence of the neutral ammonia reactant properties examined. Comparisons with adiabatic capture centrifugal sudden approximation calculations indicate that the different rotational populations of NH3 and ND3 and capture theory models based on long-range forces are insufficient to account for the observed isotope effect, pointing to the contribution of short-range forces. The results are discussed in the context of recent low-energy ion-molecule reaction studies and highlight the importance of explicitly considering short-range and long-range effects in ion-molecule collisions, with important implications for the modeling of astrochemical environments.