Predicting line intensities of the HCNH+ isotopologues: first hyperfine-resolved spectroscopic and collisional data

ABSTRACT Iminomethylium, HCNH$^+$, is a ubiquitous molecular cation in the interstellar medium (ISM). Although isotopologues of HCNH$^+$ precursors are known to exist, no isotopically substituted derivative of HCNH$^+$ has been identified in space, primarily due to the absence of hyperfine-resolved spectral data. In this work, we present the first hyperfine-resolved spectroscopic data for DCNH$^+$, H$^{13}$CNH$^+$, HC$^{15}$NH$^+$, and HCND$^+$. In addition, we provide hyperfine-resolved collisional rate coefficients with H$_2$ and Einstein coefficients that enable non-LTE predictions of line intensities in cold molecular clouds. Radiative-transfer calculations tailored to TMC-1 indicate that HCND$^+$ is the most detectable isotopologue, with the j = 2–1 rotational line expected at $\sim$7–40 mK (while the j = 1–0 line is blocked by atmospheric O$_2$), whereas H$^{13}$CNH$^+$ and HC$^{15}$NH$^+$ yield mK-level emission for the j = 1–0 and 2–1 lines. On the other hand, DCNH$^+$ lines are intrinsically weak owing to its nearly-zero dipole moment. The frequencies, hyperfine patterns, and radiative and collisional coefficients reported here remove the main barrier to targeted searches of HCNH$^+$ isotopologues and enable robust non-LTE determinations of isotopic ratios in the cold ISM.