Geometry changes in molecular photoionization: The NH3+ ((X)over-bar-(2) A(2)'')<-NH3(X(1)A(1)) transition

Single-photon threshold photoionization has been used to explore the ionization dynamics of the NH3+ ((X) over tilde (2)A(2)'')<--NH3 (X (1)A(1)) transition which involves a pyramidal-to-planar geometry change. Rotationally resolved threshold photoelectron spectra are presented for four vibrational levels of the (X) over tilde (2)A(2)'' cation state of NH3+ corresponding to excitation of even and odd quanta of the inversion mode, nu(2)(+). 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 Delta K=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. (C) 1996 American Institute of Physics.