2 vanish, resulting in a reduced seven-dimensional model. Within the Condon approximation, the initial wave packet on the neutral PES is taken as the ground rovibrational state of the anion obtained by diagonalizing the seven-dimensional Hamiltonian on the anion PES. It is then propagated using the Chebyshev propagator, and the energy spectrum is extracted from a cosine Fourier transform of the Chebyshev autocorrelation function. The bound and resonance wavefunctions are obtained analogously from the Chebyshev wave packet. The details of the quantum dynamics calculations, along with a discussion of the internal coordinates and the vibrational modes of the neutral complex, are given in the Supplementary Information (SI).