Discrete–continuum and discrete–discrete interactions in the autoionization spectrum of ¹¹BH

Abstract

We have applied triple-resonant optical photoionization spectroscopy to characterize the lineshapes of autoionizing Rydberg resonances in ¹¹BH converging to the X²Σ⁺ ground state of ¹¹BH⁺ in vibrational levels from v ⁺ = 0 through 4. Third photon transitions originating from the B¹Σ⁺ v′ = 0, 1, 2 or 3 states of ¹¹BH reach high Rydberg resonances in Δv = 0 (below vertical threshold) and Δv = 1 (above vertical threshold) transitions. Observed spectra display contributions from discrete–continuum and discrete–discrete transition moments, where intensities are governed by Franck–Condon factors and the discrete–continuum mixing of final states. The spectra show Fano line profiles attributable to the phase interference between competing photoionization/photodissociation transition moments. Two fitting parameters, q and Γ, which characterize asymmetric spectral lineshapes in terms of an interaction between a single discrete state and single continuum, serve well to model the photoionization cross-section. Quantitative trends in these parameters yield insight into the discrete–continuum coupling of final states, driven by differences between the electronic potentials of the gateway B¹Σ⁺, 3pπ D ¹Π and E¹Σ⁺ states of ¹¹BH and that of the X²Σ⁺ ground state of the cation.

Keywords:

• Autoionization
• Vibronic coupling
• Lineshapes
• Rydberg states
• MQDT

Acknowledgements

This work was supported by the Natural Sciences and Engineering Research Council of Canada (NSERC).