Dissociation energy of the CN molecule

Abstract

The perturbations observed on the vibrational levels of the B 2Σ state of CN must be caused by intersections with levels of a 2II state as established by the widening of the spin doublets of the B 2Σ state, i.e. by the simultaneous but inequal shifting of the spin-doublet components and by the decay of the perturbations with decreasing rotational quantum numbers, leaving the rotationless states unaffected. The perturbing state must be identical with the known A 2II state of CN, as proved by the observed positions of the perturbed places in the CN level scheme. In this way the vibrational set of the A 2II state becomes observable up to the 30th sublevel, where the vibrational convergence is already very marked. A short extrapolation fixes the convergence limit at about 60500 ± 1000 cm.-1 above the ground state X 2Σ (ν=o, J = o) of CN. A comparison of the positions of vibrational levels of the X 2Σ and A 2II states shows that the convergence limit of the former should lie in the neighbourhood of the latter; the B 2Σ state converges to about 65500 ± 1000 cm.-1 On this basis the correlation between C + N and CN levels is found as follows: A 2IIconvergence -> C (3P) + N (2P) = 60500 ± 1000 cm.-1 and B 2Σconvergence -> C (5S) + N (4S) = 65500 ± 1000 cm.-1 This could then be verified by the observation of intensity drops at energy values where the other C + N term combinations fall. Thus for the dissociation energy of the CN molecule into tetravalent C (5S) and trivalent N (4S) atoms one gets 8.09 e.V. in very good agreement with calculations made on the basis of CO (X 1Σ) + 11.06 e.V. -> C (5S) + O (3P).