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With the improvement of acceleration techniques, the intrinsic emittance of the cathode has a strong impact on the final brightness of a free electron laser. The systematic studies presented in this paper demonstrate for the first time in a radiofrequency photocathode gun a reduction of the intrinsic emittance when tuning the laser photon energies close to the effective work function of copper. The intrinsic emittance was determined by measuring the core slice emittance as a function of the laser beam size at laser wavelengths between 260 and 275 nm. The results are consistent with the measured effective work function of the cathode. Slice emittance values normalized to the laser beam size reached values down to 500  nm/mm, close to that expected from theory. A 20% reduction of the intrinsic emittance was observed over the spectral range of the laser.