Geometry changes in molecular photoionization: The NH₃⁺ (X̃ ²A″₂)←NH₃ (X ¹A₁) transition

Single-photon threshold photoionization has been used to explore the ionization dynamics of the NH₃⁺ (X̃ ²A″₂)← NH₃ (X ¹A₁) transition which involves a pyramidal-to-planar geometry change. Rotationally resolved threshold photoelectron spectra are presented for four vibrational levels of the X̃ 2A″₂ cation state of NH₃⁺ corresponding to excitation of even and odd quanta of the inversion mode, v₂⁺. The rotational state distributions exhibit strong symmetry effects imposed by the nuclear spin statistics and vibronic parity of the neutral and cation out-of-plane bend levels. The observed rotational structure is consistent with dominant ΔK=0 transitions and parity assignments for specific transitions show that both l=even and l=odd photoelectron final states are excited. These observations indicate that the photoelectron experiences a non-planar (pyramidal) cation potential. These results are discussed in light of previous rotationally resolved measurements on NH₃ using multiphoton ionization and other systems which undergo bend excitation following ionization.