Laser-based VUV spectroscopy of doubly excited Ca I

We describe a new study of the odd-parity doubly excited spectrum of Ca I, in which a tunable coherent VUV source was used to scan over an energy range previously investigated by classical spectroscopy using synchrotron radiation. A vast improvement in the quality of the data, especially as regards the available spectral resolution, results from the use of a four-wave mixing source and enables the comparison between theory and experiment to be pushed further than ever before for doubly excited states in alkaline-earth spectra. Some remaining limitations of the theoretical computations are brought to light and discussed.