Spectroscopy and dynamics of jet-cooled hydrazines and ammonia. II. Electron-impact dissociative ionization

Abstract

Dissociative-ionization cross sections, fragment appearance potentials, and fragment kinetic energies were measured for electron-impact excitation of jet-cooled NH3, hydrazine (N2H4), and monomethyl hydrazine (MMH) over an energy range of 10–270 eV. A data base of 35 parent and fragment ions is reported. All measurements were made in a crossed electron–molecular beam apparatus using pulsed extraction and time-of-flight mass detection to ensure field-free excitation and high collection efficiency for energetic ions. Cross sections for NH3 ionization are in good agreement with previous measurements except for ions with high kinetic energy (KE). These discrepancies are attributed to instrument-dependent KE detection efficiencies in the previous results. Cross section data have not been previously reported for N2H4 and MMH. The measured cross sections for total ionization at 70 eV are 2.35 Å2 (NH3), 3.76 Å2 (N2H4), and 4.20 Å2 (MMH). KE distributions were measured by an ion deflection method and gave results consistent with time-of-flight peak-shape analysis. Mean KE values <εt≳ are reported for all fragment ions studied. For 170-eV excitation of NH3, <εt≳ varied from 0.026 eV (NH2+) to 1.4 eV (H+). The kinetic energies for N2H4 and MMH fragment ions at similar excitation energies are typically much lower than for fragment ions from NH3, conforming to statistical arguments based on density of internal states. High resolution mass spectra were recorded for MMH in order to distinguish different fragment ions of the same unit mass. Substantial rearrangement is evident for N2H4 and MMH dissociative ionization based on the appearance of ions such as NH3+ and NH4+ (the latter for MMH ionization only) and the magnitude of <εt≳ for certain ions. The role of electronic structure and geometry on dissociation is explored using a molecular orbital analysis to predict product correlations for the excited states of N2H4+.