Rotational (de)-excitation of C by collision with He at low temperature.

An appropriate estimation of the abundance of the observed C radical in the interstellar medium requires accurate radiative and collisional rate coefficients. We present the first two-dimensional potential energy surface (2D-PES) for the ground electronic state of the C(XΣ)-He(XS) van der Waals system, obtained using an explicitly correlated coupled-cluster method with single, double, and perturbative triple excitations (RCCSD(T)-F12). This PES is subsequently used in quantum close-coupling (CC) scattering calculations. Collisional excitation cross-sections of the rotational levels of C by He were calculated for energies up to 1500 cm using the standard (CC) method. The thermal dependence of the corresponding rate coefficients is given for the low and moderate temperature ≤ 300 K regime of interstellar molecular clouds. This is the first study on the collisional rate coefficients for this system and may have important implications for the astrophysical detection of C(XΣ) and modeling of carbon-rich media.