Spectroscopy and dynamics of jet-cooled hydrazines and ammonia. I. Single-photon absorption and ionization spectra

Electronic-state properties of hydrazine (N2H4), monomethyl hydrazine (MMH), unsymmetrical dimethyl hydrazine (UDMH), and ammonia were investigated by single-photon vacuum ultraviolet (vuv) absorption and photoionization spectroscopy in a molecular-beam apparatus. The photoionization spectrum of NH3 shows a sharp threshold at a value of 10.16 eV in excellent agreement with previous measurements. The ionization thresholds of the hydrazines are very broad reflecting the large (nearly 2 eV) structural relaxation energy for conversion from a C2 pyramidal gauche neutral geometry to a D2h planar ion geometry. The measured threshold photoionization potentials are 8.32 eV (N2H4), 8.05 eV (MMH), and 7.87 eV (UDMH). The vuv absorption spectra of the hydrazines are also broad, which is to be contrasted with the well-resolved spectrum of NH3. The band shapes can be attributed to planarization of the Rydberg excited states in analogy to the ion structure. A molecular-orbital model is developed to provide understanding of the excited states in terms of their symmetries, energy ordering, and selection rules, as a function of geometry.