ExoMol molecular line lists – XI. The spectrum of nitric acid

Abstract

Nitric acid is a possible biomarker in the atmospheres of exoplanets. An accurate line list of rotational and rotational-vibrational transitions is computed for nitric acid (HNO$_3$). This line list covers wavelengths longer than 1.42 $\mu$m (0 - 7000 cm$^{-1}$) and temperatures up to 500 K. The line list is computed using a hybrid variational -- perturbation theory and empirically tuned potential energy and dipole surfaces. It comprises almost 7 billion transitions involving rotations up to $J=100$. Comparisons with spectra from the HITRAN and PNNL databases demonstrate the accuracy of our calculations. Synthetic spectra of water - nitric acid mixturessuggest that nitric acid has features at 7.5 and 11.25 $\mu$m that are capable of providing a clear signature for HNO$_3$; the feature at 11.25 $\mu$m is particularly promising. Partition functions plus full line lists of transitions are made available in an electronic form as supplementary data to the article and at www.exomol.com.