Photoabsorption of the molecular IH cation at the iodine 3d absorption edge

Yields of atomic iodine Iq+ (q >= 2) fragments resulting from photoexcitation and photoionization of the target ions IH+ and I+ have been measured in the photon-energy range 610-680 eV, which comprises the thresholds for iodine 3d ionization. The measured ion-yield spectra show two strong and broad resonance features due to the excitation of the 3d(3/2,5/2) electrons into epsilon f states rather similar for both parent ions. In the 3d pre-edge range, excitations into (np pi)-like orbitals and into an additional sigma* orbital are found for IH+, which have been identified by comparison of the atomic I+ and molecular IH+ data and with the help of (time-dependent) density functional theory (DFT) and atomic Hartree-Fock calculations. The (5p pi) orbital is almost atomlike, whereas all other resonances of the IH+ primary ion show a more pronounced molecular character, which is deduced from the chemical shifts of the resonances and the theoretical analysis.