Collisional Excitation and Weak Maser Action of Interstellar Methanimine

The inelastic cross sections and rate coefficients for the rotational excitation of methanimine (CH₂NH) by cold H₂ have been determined quantum mechanically based on a new highly correlated five-dimensional potential energy surface. This surface was fitted to more than 60 000 ab initio points with a root-mean-square error of ∼1-2 cm^-1 in the region of the potential well whose depth is -374.0 cm^-1. The rotationally inelastic rate coefficients have been combined with spectroscopic data to generate the nonequilibrium spectrum of CH₂NH toward the interstellar molecular cloud Sgr B2(N). The transition 1₁₀ → 1₁₁ at 5.29 GHz is found to be inverted, and the predicted weak maser emission spectrum is in excellent agreement with new observations performed with the 100 m Green Bank Telescope.