Low-energy electrons can interact with molecules to cause vibrational excitation with large cross-sections due to the presence of resonances. This can be important for a number of reasons. Firstly these processes can absorb significant energy from the plasma electrons, affecting the electron energy distribution and potentially (via vibration-translation (VT) energy transfer) cause substantial gas heating. Secondly, the presence of vibrationally excited molecules may significant increase the rates of reactive collisional processes, including electron dissociative attachment and electron impact dissociation into neutral atoms. However, the cross-sections of these processes are often poorly known since they are extremely difficult to measure directly, and reliable theoretical calculations are only now appearing for simple diatomic molecules. We are studying discharges in pure O2 and pure Cl2 at low pressure, using a combined experimental and modelling approach. Notably we have measured the vibrational distributions for the first time, using high-sensitivity ultra-broadband ultraviolet absorption spectroscopy. In parallel we are modelling these systems using a detailed self-consistent 0D global model.