A simple technique of simultaneous co-evaporation has been used to form thin films of erbium-doped zinc sulphide which give good DCEL. For transitions from excited states to the

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ground state of the Er

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ion theoretical calculations of oscillator strengths have been compared with those determined experimentally from observations of spectral intensities of DCEL. The good agreement noted between experiment and theory gives further support for a previous finding that in rare-earth doped ZnS thin films the hot electrons exciting DCEL follow a Boltzmann distribution.