Resonant two-photon ionization (R2PI) spectroscopy was utilized to measure the bond dissociation energies (BDEs) of thirteen diatomic lanthanide chlorides by measuring their predissociation thresholds. These molecules retain an exceptionally high density of vibronic states near their ground separated atom limits because of the large number of states deriving from the open 4f subshell. This high density of vibronic states, combined with spin-orbit and nonadiabatic interactions, allows these molecules to rapidly find a pathway to molecular dissociation when excited at or above their ground separated atom limits. Even molecules with filled or half-filled 4f subshells (YbCl and EuCl, respectively) display sharp predissociation thresholds at the ground separated atom limit because of the high density of states arising from excited separated atom limits that drop below the ground separated atom limit. The R2PI spectra of the LnCl molecules display a dense manifold of vibronic states below the BDE. However, as soon as the bond dissociation energy is energetically achieved, the molecules dissociate before they can be ionized, causing a sharp drop in molecular ion signal. This predissociation threshold allows BDEs to be assigned as D₀(LaCl) = 5.134(16), D₀(CeCl) = ~5.04 eV, D₀(PrCl) = 4.60(13) eV, D₀(NdCl) = 4.634(90) eV, D₀(SmCl) = 4.452(14) eV, D₀(EuCl) = 4.383(9) eV, D₀(GdCl) = 4.916(11) eV, D₀(TbCl) = 4.975(3) eV, D₀(DyCl) = 4.282(3) eV, D₀(HoCl) = 4.236(5) eV, D₀(ErCl) = 4.634(8) eV, D₀(TmCl) = 4.057(9) eV, and D₀(YbCl) = 3.951(19) eV. The variation of LnCl BDEs across the lanthanide series are explained by correlating the adiabatic BDEs to the appropriate diabatic separated atom limit, allowing diabatic BDEs to be obtained. Insights into the LnCl periodic trends, their connections to ligand field theory, and their implications for chemical bonding will be presented.

Michael Morse

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