The Threshold Photoelectron Spectrum of NH3.

Using synchrotron radiation, we have investigated the threshold photoelectron spectrum of NH3 between 10-30 eV photon energy. In contrast to the He(I)-photoelectron spectrum, the threshold photoelectron spectrum shows three bands between 10 and 21 eV. The first band is dominated by the vibrational autoionization of the 5a1, the 6a1 and the 6pe Rydberg series. Between the first and second bands a new contribution, starting at 13.26 eV, is assigned to the autoionization of (a) member(s) of Rydberg series converging to the e-1 ionization continuum. Its (their) autoionization populates the upper vibrational levels of the NH3+ (X2A1) state. From about 14.5 eV the A2E band structures are identified. However, differences between the A2E band shapes in the He(I)- and the threshold photoelectron spectra suggest that very likely autoionization contributions are involved in the latter spectrum. These phenomena could play a role in the production of the NH2+ and NH+ fragment ions. The last very weak band, at about 25 eV in the photoelectron spectrum is assigned to the NH3+(B2A1) state.