We have applied optical-optical-optical triple resonance spectroscopy to resolve a system of high Rydberg states in BH that serves quantitatively to characterize a fundamental example of electron-orbital-cation core rotational coupling. The third-color ionization-detected absorption spectrum originating from the photoselected 3s B1Sigma+ Rydberg state with vibrational and total angular momentum quantum numbers, v'=1 and N'=0 consists entirely of vibrationally autoionizing resonances for which final N=1 that converge in series to the BH+v+=1 rotational limits, N+=0, 1, and 2. For series with l=1 converging to N+=0 and 2, Rydberg orbital and rotational angular momenta couple to systematically perturb level energies and distribute lifetime in a well-isolated two-channel rotronic interaction that spans hundreds of wave numbers.

A simple two-channel quantum defect theory approach accounts for resonance positions in the np Rydberg series of (11)BH. The transition from Hund's case (b) to (d) in the interacting levels of this np series represents a fundamental example of electron orbital ⇔ cation core rotational coupling, and frame transformation theory offers a means to connect close-coupled electronically excited-state potentials and l-uncoupled Rydberg positions. This evolving interaction of the np Rydberg electron with the rotational and the vibrational motion of the (11)BH(+) core is formulated in terms of quantum defects, μ(λ)(v(+)).

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The ionization-detected multiresonant absorption spectrum of the high Rydberg states of 11 BH shows evidence of a valence electronic excitation of the X 2 ⌺ + 11 BH + core in which the principal and azimuthal quantum numbers of a bound Rydberg electron are conserved. This phenomenon, known as isolated core excitation ͑ICE͒, has been previously reported with Rydberg quantum state specificity only for atoms. In the present triple-resonant photoexcitation experiment, the third laser frequency happens to scan an energy interval of coincidental fourth-photon resonance with transitions in the A 2 ⌸-X 2 ⌺ + system of the 11 BH + ion core. For a certain range of principal quantum number n, we find that the initially prepared Rydberg electron remains unaffected while the core undergoes an adventitious, strongly allowed valence electron transition, taking the system from a Rydberg level that converges to the X 2 ⌺ + state of 11 BH + to a corresponding level converging to excited 11 BH + A 2 ⌸. We denote this absorption as ͉A 2 ⌸͉͘nl͘ ← ͉X 2 ⌺ + ͉͘nl͘. Using a simple mathematical model for which only transitions vertical in n and l are allowed, we show that the interval of principal quantum number over which the rate of adventitious ICE exceeds that of 11 BH * dissociation depends on the finite linewidths and positions of ͉A 2 ⌸͉͘nl͘ features and on the n-dependent predissociative lifetimes of 11 BH * Rydberg molecules.

Optical-optical-optical triple resonance spectroscopy isolates transitions to vibrationless Rydberg states of BH with principal quantum numbers from n=7 to 50. Corresponding resonances appear in the excitation spectrum of excited boron atoms produced by the dissociative relaxation of these states. The decay to neutral products occurs on a nanosecond time scale. Yet, corresponding resonances show Fano coupling widths that approach 1 cm-1. Above threshold, spontaneous ionization dominates, but line shapes match for resonances with the same electron orbital quantum numbers built on v+=0 and v+=1 cores. This striking feature-for-feature similarity in predissociation and autoionization line shapes affirms that inelastic electron-cation scattering pathways leading to electron ejection and dissociative recombination proceed through a common continuum.