Photoionization cross sections of ultracold Sr in P and S states at 390 nm and the resulting blue-detuned magic wavelength optical lattice clock constraints.

We present the measurements of the photoionization cross sections of the excited P and S states of ultracold Sr atoms at 389.889 nm wavelength, which is the magic wavelength of the S-P clock transition. The photoionization cross section of the P state is determined from the measured ionization rates of Sr in the magneto-optical trap in the P state to be 2.