Anomalous Λ-doubling of the X2Π state of N Se

The rotational analysis of the A-X and A′-X systems of the ¹⁴N⁸⁰Se and ¹⁵N⁸⁰Se molecules has revealed an anomalous Λ-doubling of the X²Π state. This is attributed to an interaction between the nearly degenerate v + 1 and v levels of the $\text{X}{}^2\text{Π}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$ and $\text{X}{}^2\text{Π}{}_{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}$ substates. The s-uncoupling operator is shown to be responsible for the mixing. Due to the variation with r of the B rotational constant, the vibrational matrix element is non vanishing for Δv = 1. Its value obtained by a least squares fit of the observed term values is in excellent agreement with the calculated one.