Ion dissociation of hydrazoic acid investigated by synchrotron-radiation-based photoionization mass spectrometry.

We report mass resolved photoionization yield spectra of a molecular-beam cooled sample of HN3 using photoionization mass spectrometry based on high-resolution monochromatized synchrotron radiation. Spectra are reported at mass-to-charge ratios of 14(N+), 15(NH+), 29(N2H+), 42(N-3(+)), and 43(HN3+) in the region of each ionization threshold. The thresholds observed here are all lower than the previously reported ones obtained with electron impact ionization, which were the only ones available in the literature prior to this work. The appearance energies can be used to evaluate four key thermochemical quantities of relevance to HN3: D-0(H-N-3), D-0(N-N-2), D-0(N-H), and IE(NH). We observe the appearance energy of the parent ion (HN3+) to be 10.56 +/- 0.02 eV, somewhat below the reported ionization energies derived from photoelectron spectroscopy. Great care was taken to evaluate the importance of vibrational hot bands to the photoionization yield spectra. This experiment also provides a lower limit to the proton affinity of N-2 allowing us to bracket this quantity with improved certainty: 119.3 kcal/mol <= PA(N-2) <= 121.4 kcal/mol. We also derive an upper limit to the ionization energy of N2H (IE(N2H) <= 7.92 eV), a molecule that has yet to be observed. We hope that knowledge of this ionization energy might help in future attempts to detect this interesting radical. We also take this opportunity to review the status of the thermodynamics of many molecules and ions containing N and H within the context of these new results and make new recommendations. In particular, we recommend a new value for Delta H-f(0)degrees(HN3), nearly 5 kcal/mol larger than prior evaluations.