# ExoMol Line List XXI: Nitric Oxide (NO)

## TL;DR

This paper presents comprehensive and highly accurate line lists for nitric oxide isotopologues, combining empirical and high-level ab initio methods, significantly expanding the spectral data available for astrophysical and atmospheric research.

## Contribution

It introduces the most extensive and precise nitric oxide line lists to date, integrating empirical energy levels with ab initio calculations for six isotopologues.

## Key findings

- Line lists cover 0-40,000 cm^-1 with ~22,000 states and 2.3-2.6 million transitions.
- Excellent agreement with HITRAN and HITEMP absorption data at 296 K.
- Line lists outperform previous databases in coverage and accuracy.

## Abstract

Line lists for the ground electronic ground state for six major isotopologues of nitric oxide are presented. The line lists are constructed using empirical energy levels (and line positions) and high-level {\it ab inito} intensities. The energy levels were obtained using a combination of two approaches, from an effective Hamiltonian and from solving the rovibronic Schr\"{o}dinger equation variationally. The effective hamiltonian model was obtained through a fit to the experimental line positions of NO available in the literature for all six isotopologues using the programs SPFIT and SPCAT. The variational model was built through a least squares fit of the \textit{ab inito} potential and spin-orbit curves to the experimentally derived energies and experimental line positions of the main isotopologue only using the Duo program. The \textit{ab inito} potential energy, spin-orbit and dipole moment curves (PEC, SOC and DMC) are computed using high-level {\it ab inito} methods and the MARVEL method is used to obtain energies of NO from experimental transition frequencies. Each line list covers a wavenumber range from 0 - 40,000 \cm with approximately 22,000 rovibronic states and 2.3-2.6 million transitions extending to $J_{max} = 184.5$ and $v_{max} = 51$. Partition functions are also calculated up to a temperature of 5000 K. The calculated absorption line intensities at 296 K using these line lists show excellent agreement with those included in the HITRAN and HITEMP databases. The computed NO line lists are the most comprehensive to date, covering a wider wavenumber and temperature range compared to both the HITRAN and HITEMP databases. These line lists are also more accurate than those used in HITEMP. The full line lists are available from the CDS and ExoMol databases; data will also be available from CDMS.