Predissociation measurements of the bond dissociation energies of EuO, TmO, and YbO

Abstract

The observation of a sharp predissociation threshold in the resonant two-photon ionization spectra of EuO, TmO, and YbO has been used to measure the bond dissociation energies of these species. The resulting values, D0(EuO) = 4.922(3) eV, D0(TmO) = 5.242(6) eV, and D0(YbO) = 4.083(3) eV, are in good agreement with previous values but are much more precise. In addition, the ionization energy of TmO was measured by the observation of a threshold for one-color two-photon ionization of this species, resulting in IE(TmO) = 6.56(2) eV. The observation of a sharp predissociation threshold for EuO was initially surprising because the half-filled 4f7 subshell of Eu in its ground state generates fewer potential energy curves than in the other molecules we have studied by this method. The observation of a sharp predissociation threshold in YbO was even more surprising, given that the ground state of Yb is nondegenerate (4f146s2, 1Sg) and the lowest excited state of Yb is over 2 eV higher in energy. It is suggested that these molecules possess a high density of electronic states at the energy of the ground separated atom limit because ion-pair states drop below the ground limit, providing a sufficient electronic state density to allow predissociation to set in at the thermochemical threshold.

Probing the Chemical Bond between Lanthanides and Carbon: CeC, PrC, NdC, LuC, and TmC2

“…The chemical bonding and thermochemical stability of MC 2 /LnC 2 species have long been compared to the bonding and thermochemical stability of MO/LnO species because there are four 2p electrons available in each ligand for bonding to a metal atom in these species. The idea of “pseudo-oxygen” character for the C 2 ligand when bonding with metal atoms is noted by the correlation of the −O and −C 2 electron affinities and the dipole moments and bond dissociation energies of the corresponding MO and MC 2 species. ,− The BDE of TmO has been measured previously by our group to be 5.242(6) eV and the BDE of TmC 2 is reported in this current work as 4.797(6) eV. It has been suggested that the TmO ground state correlates to the Tm + 4f 12 5d 1 6s 1 + O – 2p 5 , 2 P u separated ion limit .…”

“…Thus, the ion signal disappears when the molecule is excited to states at or above its ground separated atom limit. The use of spectroscopically measured predissociation thresholds to assign accurate BDEs to molecules with high electronic state densities has long been employed and discussed by our group. ,− The argument that sharp predissociation thresholds may be identified with thermochemical BDEs is strengthened by thermochemical cycles that demonstrate exceptional self-consistency when BDE values obtained from predissociation thresholds are employed in these cycles. , …”

“…For the BDE of LuC, a slightly larger ±30 cm –1 error limit was required to account for the broader appearance of the predissociation threshold. A more detailed explanation of the reasoning behind our assignment of error limits has been given previously. , …”

“…If the energy of these photons is resonant with a spectroscopic transition, the molecules are promoted into the excited state. In almost all our previous R2PI experiments, ,, the excited molecules are then irradiated by high-energy photons produced by a second, perpendicularly oriented ionization laser. In the experiments for CeC, PrC, NdC, and LuC, however, the molecules were ionized by absorption of a second photon from the same OPO laser pulse that initially excited them.…”

Early Transition Metals Strengthen the B2Bond in MB2Complexes

“…In previous work, − we have demonstrated that for small d- and f-block molecules, the high density of electronic states in the vicinity of the ground separated fragment limit allows the molecule to dissociate via spin–orbit and nonadiabatic couplings as soon as this limit is exceeded in energy, leading to a sharp drop in the ion signal obtained in a resonant two-photon ionization (R2PI) spectroscopy experiment. As illustrated in Figures and S1–S5, this method has allowed us to measure the predissociation thresholds of ScB 2 , TiB 2 , VB 2 , YB 2 , and MoB 2 .…”