Isolated core excitation ofB11H: Photoabsorption in competition with Rydberg predissociation

Abstract

The ionization-detected multiresonant absorption spectrum of the high Rydberg states of $^{11}\mathrm{B}\mathrm{H}$ shows evidence of a valence electronic excitation of the $X\phantom{\rule{0.2em}{0ex}}^{2}\ensuremath{\Sigma}^{+}\phantom{\rule{0.3em}{0ex}}^{11}\mathrm{B}\mathrm{H}^{+}$ core in which the principal and azimuthal quantum numbers of a bound Rydberg electron are conserved. This phenomenon, known as isolated core excitation (ICE), 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 $A\phantom{\rule{0.2em}{0ex}}^{2}\ensuremath{\Pi}\text{\ensuremath{-}}X\phantom{\rule{0.2em}{0ex}}^{2}\ensuremath{\Sigma}^{+}$ system of the $^{11}\mathrm{B}\mathrm{H}^{+}$ 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 $X\phantom{\rule{0.2em}{0ex}}^{2}\ensuremath{\Sigma}^{+}$ state of $^{11}\mathrm{B}\mathrm{H}^{+}$ to a corresponding level converging to excited $^{11}\mathrm{B}\mathrm{H}^{+}$ $A\phantom{\rule{0.2em}{0ex}}^{2}\ensuremath{\Pi}$. We denote this absorption as $\ensuremath{\mid}A\phantom{\rule{0.2em}{0ex}}^{2}\ensuremath{\Pi}⟩\ensuremath{\mid}nl⟩\ensuremath{\leftarrow}\ensuremath{\mid}X\phantom{\rule{0.2em}{0ex}}^{2}\ensuremath{\Sigma}^{+}⟩\ensuremath{\mid}nl⟩$. 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 $^{11}\mathrm{B}\mathrm{H}^{*}$ dissociation depends on the finite linewidths and positions of $\ensuremath{\mid}A\phantom{\rule{0.2em}{0ex}}^{2}\ensuremath{\Pi}⟩\ensuremath{\mid}nl⟩$ features and on the $n$-dependent predissociative lifetimes of $^{11}\mathrm{B}\mathrm{H}^{*}$ Rydberg molecules.