Coupling of Electron Orbital Motion with Rotation in the High Rydberg States of BH

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 $3s$ $B{}^1\Sigma ^{+}$ Rydberg state with vibrational and total angular momentum quantum numbers, $v^{\prime }=1$ and $N^{\prime }=0$ consists entirely of vibrationally autoionizing resonances for which final $N=1$ that converge in series to the ${\mathrm{BH}}^{+}{v}^{+}=1$ 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.

Figure 1

Ionization-detected absorption spectrum of transitions in BH that originate from the $3s$ $B{}^1\Sigma ^{+}$ Rydberg level with vibrational and total angular momentum quantum numbers, $v^{\prime }=1$ and $N^{\prime }=0$. Ladders labeled by principal quantum numbers mark unperturbed Rydberg series of constant quantum defects, $\delta =0.526$, 0.907, and 0.529, converging, respectively, to ${\mathrm{BH}}^{+}$ $v^{+}=1$ rotational states $N^{+}=0$, 1, and 2 at third photon energies of 26 920.0, 26 944.7, and $26994.2{\mathrm{cm}}^{-1}$.

Figure 2

(a) Top. Lu-Fano plot of quantum defects derived from the experimental positions of resonances assigned to BH series converging to ${\mathrm{BH}}^{+}$ $v^{+}=1$ limits, $N^{+}=0$ and 2 (dark points). The light points show a reference plot of defect pairs derived from positions in unperturbed Rydberg series converging to $N^{+}=0$ and 2, pictured as ladders in Fig. 1. The hyperbola is drawn from Eq. (5) for $\xi =0.36$. (b) Bottom. Plot of the ratio of fragmentation channel amplitudes in the region of the avoided crossing, as determined by Eq. (6) for the coupling parameter, $\xi =0.36$, derived from the fit in (a).