Giant spin-orbit interactions in argon photoionization

We have measured the relative cross sections for quartet and doublet satellite states of Ar{sup +} formed in photoionization. Quartet states are dipole forbidden in the nonrelativistic limit and result from spin-orbit interactions in the continuum [H. W. Van der Hart and C. H. Greene, J. Phys. B 32, 4029 (1999)]. Because of this, the cross sections for their formation are generally thought to be smaller than the dipole-allowed doublet states. We find that the cross sections for the 3p{sup 4}[{sup 3}P]4p {sup 4}D{sub 1/2}{sup o} and 3p{sup 4}[{sup 3}P]4p {sup 4}D{sub 3/2}{sup o} quartet states over certain energy regions are 16 to 30 times larger than doublet states. This counterintuitive result remains to be theoretically explained.