Single and double photoionisation of Ca atoms between 35 and 42 nm

Photoionisation of Ca atoms has been investigated by means of a time-of-flight mass spectrometer coupled with a source of monochromatised synchrotron radiation and an atomic Ca beam. Relative photoionisation cross sections (q⁺ for single photoionisation and q²⁺ for double photoionisation, as well as the ratio R=q²⁺/q⁺) have been measured in the wavelength range 35-42 nm with a 0.15 nm bandwidth. Comparison with the photoabsorption data of Mansfield and Newsom (1977) shows a good correspondence in the peak structure between the absorption resonance and the Ca²⁺ yield for proton energies above the lowest 3p ionisation limit. The variation of R with wavelength exhibits a step behaviour which correlates with some of the 3p ionisation limits.