A generalized ADK (Ammosov-Delone-Krainov) theory for ionization of open shell atoms is compared to ionization experiments performed on the transition metal atoms V, Ni, Pd, Ta, and Nb. Our theory is found to be in good agreement for V, Ni, Pd, and Ta, whereas conventional ADK theory overestimates ionization by orders of magnitude. The key to understanding the observed ionization reduction is the angular momentum barrier. Our analysis shows that the determination of the angular momentum barrier in open shell atoms is nontrivial. The Stark shift is identified as the second dominant effect responsible for ionization suppression. Finally, these two effects cannot explain the Nb data. An analysis of the electron spins suggests that Pauli blocking might be responsible for the suppression of tunneling in Nb.
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