B 3 Π 0+u —X 1 ∑+g system of 127 I 2 : rotational analysis and long-range potential in the B 3 Π 0+u state

The results of detailed rotational and term-value analyses involving some 7000 lines with 58 levels in B3Π0+u(4 ⩽v⩽ 77) and 6 levels in X1∑+g(0 ⩽v⩽ 5) are given. For the B3Π0+u state the rotational constants Bv can be represented by Bv= [graphic omitted]cn(v+½)n, but neither the vibrational levels nor the centrifugal stretching constants follow simple polynomials in (v+½). At high v, ([graphic omitted])10/7 is linear in Ev and a short extrapolation leads to a value of the limit, 20 043.208 ± 0.033 cm–1, relative to X1∑+g(v= 0, J= 0). The highest bound vibrational level has v= 87. An RKR potential energy curve has been calculated for the B3Π0+u state, and a consistent value of the limit, 20 043.220 ± 0.015 cm–1, is obtained from a fit to an attractive part of the curve near the limit, assuming a potential V(r)=–C5/r5–C6/r6–C8/r8. Values of the long-range interaction constants have been obtained: the term in r–6 is not negligible even at internuclear distances as large as 20 Å. The ground-state dissociation energy, D0(I2) is found to be 12 440.1 cm–1, corresponding to 35.568 kcal mol–1 or 148.815 kJ mol–1.