Zero energy kinetic electron and mass analyzed threshold ionization spectroscopy of Na⋅(NH3)n (n=1, 2, and 4) complexes

We report spectroscopic measurements of Na⋅(NH3)n (n=1, 2, and 4) complexes using zero energy kinetic electron spectroscopy (ZEKE) and mass analyzed threshold ionization spectroscopy (MATI). Our MATI spectrum of Na⋅NH3 agrees with the ZEKE spectrum from a previous report, and the relative vibrational intensity distribution of the ν3 mode qualitatively agrees with the corresponding Franck–Condon factors. The ZEKE spectrum of Na⋅(NH3)2 shows a long vibrational progression superimposed on a continuous feature. Based on an intensity calculation, where the vibrational angular momentum of the cation and Franck–Condon factors are taken into account, we are able to reproduce the experimental spectrum and obtain new spectroscopic information. The ionization threshold is determined to be 28 727±30 cm−1, and the vibrational frequencies for the v16 intermolecular bending mode of the cation and the neutral ground state are 28.35 cm−1 and 25.1 cm−1, respectively. The ZEKE spectrum of Na⋅(NH3)4 is structureless, and we believe it is a superposition of two processes: a real ZEKE signal and a false ZEKE signal generated by trapped energetic electrons in the excitation region. We attribute the lack of structure in the ZEKE spectrum to the high internal temperature of the cluster and the high density of states at the ionization threshold.