Determination of the Dissociation Energy of the Cr₂Molecule

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 Cr 2 (g) was determined in the temperature range between 1542 and 1819 K to be magnified image The enthalpies of sublimation and dissociation for Cr 2 (g) essentially resulted from eight independent runs according to a second‐law evaluation as Δ sub H = 612 ± 4 kJ mol −1 , Δ sub H = 653 ± 4 kj mol −1 , and Δ sub H = 652 ± 4 kJ mol −1 as well as Δ sub H = 150 ± 5 kJ mol −1 , Δ sub H = 142 ± 5 kj mol −1 , and Δ sub H = 139 ± 5 kj mol −1. The binding energy for Cr 2 (g), D e = 142 ± 5 kJ mol −1 , was determined. An evaluation of the enthalpy cf dissociation for Cr 2 (g) according to the third‐law method shows indirectly the existence of low‐lying electronic states for this dimer. The binding energy of Cr 2 (g) obtained here is compared with those predicted by numerous theoretical computations using different methods. The ionization potential of Cr 2 + was determined by ionization efficiency measurements to be 6.4 ± 0.2 V.