A microwave‐discharge uv light source has been improved to yield significant photon fluxes at 26.9 and 40.81 eV. In order to optimize the 26.9‐eV (NeII) and 40.81‐eV (HeII) radiation, the discharge was operated at ∼2.5 Pa (0.019 Torr) in an external constant magnetic field of ∼0.070 T (700 G), which, together with the oscillating electric field of the cavity, produces electron cyclotron resonance. When the discharge conditions were optimized for production of 40.81‐eV photons, features near the Fermi energy in the photoemission distribution from W(100) for 40.81‐eV photons are approximately 6% as intense as the corresponding features in the distribution for 21.22‐eV photons. We estimate that under these conditions the flux of 40.81‐eV photons is roughly 50% of the flux of 21.22‐eV photons. Photoemission energy distributions with hν=16.85, 21.22, 26.9, and 40.81 eV have been measured for saturated exposures of CO on W(100) at a temperature of ∼80 K. The variation in these data with photon energy is important for making orbital assignments to the energy levels of adsorbed molecular CO.