Ground-State Pd Anions React with H₂ Much Faster than the Excited Pd Anions

Recent advances in experimental techniques have made it relatively easy to prepare reactant cations in well-defined states of electronic excitation. Extensive studies on the role of excited states in the cation-neutral reactions have contributed significantly to our understanding of reaction kinetics and dynamics. The excited states are often more reactive than the ground state. However, the reactions involving the excited atomic anion are very rare because the negative ions usually have no bound excited states. In the present work, we report the state-specific reaction of Pd anions with H₂. Surprisingly, we observed that the ground-state Pd anions react with H₂ 10 times faster than the excited Pd anions. The high-level calculations show that the difference is due to the reaction barrier.