Collision excitation of nitrous acid (HONO) by helium: isomerization effect

We generated new 3D-potential energy surfaces (3D-PESs) for the cis-HONO-He and trans-HONO-He weakly bound complexes along the intermonomer coordinates. We used the explicitly correlated Coupled Clusters with single, double, and perturbative triple excitations (CCSD(T)-F12) approach for the electronic structure computations, where the atoms were described using the aug-cc-pVTZ basis set. Then, we derived analytical forms for each PES. These PESs exhibit different shapes and present strong anisotropies. After quantum close-coupling scattering calculations for the lower rotational levels (up to 9(2,7)), and the coupled-states approximation for higher levels (up to 22(1,22)) using these PESs, we derived the collisional excitation cross sections of cis-HONO and trans-HONO by He for total energies 0.1 <= E <= 900 cm(-1) and the rate coefficients for kinetic temperatures T <= 100 K. Our work shows that the collision data of cis-HONO and trans-HONO are different mainly because of the different 3D-PESs since the rotational energy structures of both isomers are very similar. Also, computations show that the data of the non-detected cis-HONO are as large as those of the detected trans-HONO isomer. They confirm the large values for the detected transition 5(2,4) -> 4(1,3) of trans-HONO. Therefore, our work strongly suggests revisiting radiative transfer calculations to determine accurately the population of the rotational levels of these isomers. Our work should help astrophysicists for the detectability of such nitrogen oxide molecules and for the possible formation mechanisms and isomerization pathways specificities.