Anion photoelectron spectroscopy of the hydrogen iodide anion, HI−

Among the hydrohalic acids, only hydrogen iodide (HI) forms its parent anion. HI- was identified mass spectrometrically, having been seen in three different anion-forming environments. In this work, HI-/DI- anions were produced by Rydberg electron transfer using electronically excited K**(8d) atoms as HI/DI collision partners, after which their anion photoelectron spectra (aPES) were measured using both magnetic bottle and velocity map imaging electron energy analyzers. The electron affinities (EA) of HI and DI were determined to be EA(HI) = 0.051 ± 0.015 eV and EA(DI) = 0.028 ± 0.020 eV, while the bond dissociation energies (D0) of their respective anions were determined to be D0(HI-) = 0.045 ± 0.015 eV and D0(DI-) = 0.063 ± 0.020 eV. Analysis of the vibrational structure in their aPES revealed that these anions possess double-well potentials, with their vibrational wavefunctions ranging over the two wells via tunneling. Franck-Condon analysis identified the bond lengths at the two minima of the double-well potential to be ∼2.03 and ∼2.37 Å. Thus, HI- and DI- each exist in two forms. In addition to these valence-bound HI- and DI- anions, the existence of non-valence-bound HI- (and perhaps DI-) species was implied by weak but persistent shoulder-like photoelectron signals at extremely low electron binding energies. Comparison of the anion photoelectron spectrum of HI- with those of alkali halide anions, MX-, supported the covalent character of the H-I chemical bond.