Vacuum ultraviolet pulsed field ionization study of ND3: Accurate thermochemistry for the ND2-ND2+ and ND3-ND3+ system

The dissociation of energy-selected ND3+ to form ND2++D near its threshold has been investigated using the pulsed field ionization-photoelectron (PFI-PE)-photoion coincidence method. The breakdown curves for ND3+ and ND2+ give a value of 15.891±0.001 eV for the 0 K dissociation threshold or appearance energy (AE) for ND2+ from ND3. We have also measured the PFI-PE vibrational bands for ND3+(X-tilde;v2+=0, 1, 2, and 3), revealing partially resolved rotational structures. The simulation of these bands yields precise ionization energies (IEs) for ND3+ X-tilde(0,v2+=0-3,0,0)<-ND3 X-tilde(0,0,0,0). Using the 0 K AE (ND2+) and IE(ND3)=10.200±0.001 eV determined in the present study, together with the known 0 K bond dissociation energy for ND3 [D0(D-ND2)=4.7126±0.0025 eV], we have determined the D0(ND2+-D), IE(ND2), and 0 K heat of formation for ND2+ to be 5.691±0.001 eV, 11.1784±0.0025 eV, and 1261.82±0.4 kJ/mol, respectively. The PFI-PE spectrum is found to exhibit a steplike feature near the AE(ND2+), indicating that the dissociation of excited ND3+ at energies slightly above the dissociation threshold is prompt, occurring in the time scale ≤10-7 s, as observed for the NH3 system. The available energetic data for the NH2-NH2+ and NH3-NH3+ system are found to be in excellent accord with those for the ND2-ND2+ and ND3-ND3+ system after taking into account the zero-point vibrational energy corrections. This finding indicates that the thermochemical data for these two systems are reliable with well-founded error limits