Toward the detection of pure carbon clusters in the ISM

Rotational (de)-excitation of C5 by collision with He at low temperature

An appropriate estimation of the abundance of the observed C₅ 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 C₅(X¹Σ⁺)-He(X¹S) 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 C₅ by He were calculated for energies up to 1500 cm^-1 using the standard (CC) method. The thermal dependence of the corresponding rate coefficients is given for the low and moderate temperature T ≤ 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 C₅(X¹Σ⁺) and modeling of carbon-rich media.