The np Rydberg series of boron monohydride: l-uncoupling and its evolution for intermediate principal quantum numbers n = 4 to n = 11

Using optical-optical-optical triple-resonance spectroscopy, we assign rotational levels with N = 0-5 in the vibrationless, lower-n, p Rydberg states of (11)BH. We apply the Hill and Van Vleck formulation for energy levels with l = 1 in a Hund's case intermediate between (b) and (d) to gauge the energy separating (1)Π and (1)Σ(+) states with zero rotation for n = 4-11. This energy difference, A(l, ξ), represents the strength of the coupling, ξ, between the electron orbital angular momentum, l, and the internuclear axis, which determines the Λ-splitting constant, q(0). The np series exhibits a large q(0) that increases monotonically with n to reach a magnitude similar to the rotational constant, B(0), by n = 9. For higher principal quantum numbers, Λ ceases to be a good quantum number, and l-uncoupling becomes virtually complete for n > 10.