Determination of the Dissociation Energy of the Cr2 Molecule

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Abstract
A Knudsen cell — mass spectrometer system has been developed allowing for the determination of extremely small partial pressures. This instrument and a second different Knudsen cell — mass spectrometer system were used to study the thermochemistry of the vapour over chromium. The results obtained by the two instruments agreed excellently. The partial pressure of Cr2(g) was determined in the temperature range between 1542 and 1819 K to be magnified image The enthalpies of sublimation and dissociation for Cr2(g) essentially resulted from eight independent runs according to a second‐law evaluation as ΔsubH = 612 ± 4 kJ mol−1, ΔsubH = 653 ± 4 kj mol−1, and ΔsubH = 652 ± 4 kJ mol−1 as well as ΔsubH = 150 ± 5 kJ mol−1, ΔsubH = 142 ± 5 kj mol−1, and ΔsubH = 139 ± 5 kj mol−1. The binding energy for Cr2(g), De = 142 ± 5 kJ mol−1, was determined. An evaluation of the enthalpy cf dissociation for Cr2(g) according to the third‐law method shows indirectly the existence of low‐lying electronic states for this dimer. The binding energy of Cr2(g) obtained here is compared with those predicted by numerous theoretical computations using different methods. The ionization potential of Cr2+ was determined by ionization efficiency measurements to be 6.4 ± 0.2 V.