On the levels of a diatomic rigid polar molecule in a uniform electric field

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Abstract
Recurrence relations have been obtained which enable coefficients to be found in the perturbation theory series for the energy of a diatomic rigid molecule in a uniform electric field. The dependence of the series coefficients on quantum numbers, J, M is found up to the term of the order of lambda 6, where lambda =2 mu EI/h(cross)2. Computer calculations are made up to terms of the order of lambda 42. By means of numerical experiments a value of lambda 0 for each considered state is found such that, when lambda < lambda 0, the series is apparently converging. A comparison with exact numerical solutions and asymptotic expansion is given. In the cases considered the range of applicability of the perturbation theory series and the asymptotic expansion intersect and either one or the other expansion allows the energy to be calculated with an error of less than 1% for any value of the field at which the approximation of a rigid polar molecule is reasonable.