One-photon mass-analyzed threshold ionization spectroscopy of 1- and 2-iodopropanes in vacuum ultraviolet

Abstract

One-photon mass-analyzed threshold ionization (MATI) spectroscopy of 1- and 2-iodopropanes has been studied using coherent vacuum ultraviolet (VUV) radiation generated by four-wave mixing in Kr gas. Accurate ionization energies to the lower and upper spin–orbit states of the molecular ions have been determined. These are 9.1755±0.0005 and 9.6903±0.0017 eV for the lower and upper spin–orbit states, respectively, of 2-iodopropane ion. For 1-iodopropane ion, gauche and trans peaks were resolved in the MATI spectra. Ionization energies to the lower spin–orbit states are 9.2567±0.0005 and 9.2718±0.0005, respectively, for the gauche and trans conformers. The corresponding values are 9.8332±0.0017 and 9.8466±0.0017 for the upper spin–orbit states. The pure ion beam of the gauche-only or that of trans-only could be selectively generated by tuning the VUV wavelength. Dissociation of 1- and 2-iodopropane ions, C3H7I+→C3H7++I, occurring in the ion core of highly excited Rydberg neutrals has been observed. Fragmentation thresholds for these reactions have been determined. This has led to an accurate potential energy diagram for the dissociation of the C3H7I+ system in the threshold region. The heat of formation at 0 K of 2-C3H7+ has been determined, 821.7±3.8 kJ mol−1, together with the proton affinity at 0 K of C3H6, 741.6±3.9 kJ mol−1. Plausible mechanisms for the production of 2-C3H7++I from 1-C3H7I+ have been proposed.