Predissociation thresholds have been observed in the resonant two-photon ionization spectra of TiSi, ZrSi, HfSi, VSi, NbSi, and TaSi. It is argued that because of the high density of electronic states at the ground separated atom limit in these molecules, the predissociation threshold in each case corresponds to the thermochemical bond dissociation energy. The resulting bond dissociation energies are D(TiSi) = 2.201(3) eV, D(ZrSi) = 2.950(3) eV, D(HfSi) = 2.871(3) eV, D(VSi) = 2.234(3) eV, D(NbSi) = 3.080(3) eV, and D(TaSi) = 2.999(3) eV. The enthalpies of formation were also calculated as ΔH°(TiSi(g)) = 705(19) kJ mol, ΔH°(ZrSi(g)) = 770(12) kJ mol, ΔH°(HfSi(g)) = 787(10) kJ mol, ΔH°(VSi(g)) = 743(11) kJ mol, ΔH°(NbSi(g)) = 879(11) kJ mol, and ΔH°(TaSi(g)) = 938(8) kJ mol. Using thermochemical cycles, ionization energies of IE(TiSi) = 6.49(17) eV and IE(VSi) = 6.61(15) eV and bond dissociation energies of the ZrSi and NbSi anions, D(Zr-Si) ≤ 3.149(15) eV, D(Zr-Si) ≤ 4.108(20) eV, D(Nb-Si) ≤ 3.525(31) eV, and D(Nb-Si) ≤ 4.017(39) eV, have also been obtained. Calculations on the possible low-lying electronic states of each species are also reported.

Predissociation thresholds have been observed in the resonant two-photon ionization spectra of TiSe, ZrSe, HfSe, VSe, NbSe, and TaSe. It is argued that the sharp onset of predissociation corresponds to the bond dissociation energy in each of these molecules due to their high density of states as the ground separated atom limit is approached. The bond dissociation energies obtained are D(TiSe) = 3.998(6) eV, D(ZrSe) = 4.902(3) eV, D(HfSe) = 5.154(4) eV, D(VSe) = 3.884(3) eV, D(NbSe) = 4.834(3) eV, and D(TaSe) = 4.705(3) eV. Using these dissociation energies, the enthalpies of formation were found to be ΔH(TiSe(g)) = 320.6 ± 16.8 kJ mol, ΔH(ZrSe(g)) = 371.1 ± 8.5 kJ mol, ΔH(HfSe(g)) = 356.1 ± 6.5 kJ mol, ΔH(VSe(g)) = 372.9 ± 8.1 kJ mol, ΔH(NbSe(g)) = 498.9 ± 8.1 kJ mol, and ΔH(TaSe(g)) = 562.9 ± 1.5 kJ mol. Comparisons are made to previous work, when available. Also reported are calculated ground state electronic configurations and terms, dipole moments, vibrational frequencies, bond lengths, and bond dissociation energies for each molecule. A strong correlation of the measured bond dissociation energy with the radial expectation value, ⟨r⟩nd, for the metal atom is found.

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