Dissociative recombination of N$_2$H$^+$: Isotopic effects

TL;DR

This study calculates dissociative recombination rates for N$_2$H$^+$ isotopologues to understand nitrogen isotope ratios in space, finding minimal effects on isotopic ratios from this process.

Contribution

It provides the first detailed quantum mechanical calculations of dissociative recombination cross sections for multiple N$_2$H$^+$ isotopologues, assessing their role in nitrogen fractionation.

Findings

Differences in recombination rates are below 1% for hydrogen isotopologues.

Deuterated isotopologue shows up to 30% difference, but not enough to explain observed ratios.

Recombination is unlikely to be the main cause of nitrogen isotope variations in space.

Abstract

The investigation of the isotopic ratio of interstellar nitrogen -- $^{14}$N versus $^{15}$N -- is done, for explaining its variations observed for N2H+ in different interstellar and Solar environments. The goal is to produce cross sections and rate coefficients for electron impact dissociative recombination for different isotopologues of N$_2$H$^+$, since it was envisioned as a novel source that can lead to nitrogen fractionation. We calculate dissociative recombination cross sections and rate coefficients using the normal mode approximation combined with the R-matrix theory and vibronic frame transformation within the multichannel quantum defect theory for eight isotopologues containing both $^{14}$N, $^{15}$N and H, D. Our calculations show that the relative differences respective to the main isotopologue ($^{14}$N$_2$H$^+$) is below 1% for the hydrogen containing isotopologues, but reaches almost 30% for the heaviest deuterated isotopologue, leading us to the conclusion that according to the present status of the theory, dissociative recombination is not responsible for the peculiar $^{14}$N/$^{15}$N isotopic ratios of N$_2$H$^+$ observed in the different interstellar molecular clouds.