Molecular Excitation in the Interstellar Medium: Recent Advances in Collisional, Radiative, and Chemical Processes

Collisional excitation is the basic process, and its efficiency relies on the composition of the medium providing the density of the main perturbers and the temperature, which establishes the degree of excitation and, consequently, drives the intensities of the radiated emission. Nevertheless, other excitation mechanisms can be at work such as radiative or chemical pumping. The computation of collisional inelastic rate coefficients usually takes place within the Born-Oppenheimer approximation for the separation of electronic and nuclear motions. Scattering cross sections are thus obtained by solving the motion of the nuclei on an electronic potential energy surface (PES) that is independent of the masses and spins of the nuclei. Recent studies have demonstrated that computational techniques employing advanced treatments for both the electronic and nuclear motion problems can rival experimental measurements, in terms of the achieved accuracy.