Isolated core excitation of $^{11}\mathrm{B}\mathrm{H}$: Photoabsorption in competition with Rydberg predissociation

Isolated core excitation of ${}^{11}B H$ : Photoabsorption in competition with Rydberg predissociation

C. Ricardo Viteri, Andrew T. Gilkison, Scott J. Rixon, and Edward R. Grant

Phys. Rev. A 75, 013410 – Published 11 January 2007

Abstract

The ionization-detected multiresonant absorption spectrum of the high Rydberg states of ${}^{11}B H$ shows evidence of a valence electronic excitation of the $X {}^{2}Σ^{+} {}^{11}B H^{+}$ 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 $A {}^{2}Π − X {}^{2}Σ^{+}$ system of the ${}^{11}B H^{+}$ 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 $X {}^{2}Σ^{+}$ state of ${}^{11}B H^{+}$ to a corresponding level converging to excited ${}^{11}B H^{+}$ $A {}^{2}Π$ . We denote this absorption as $∣ A {}^{2}Π ⟩ ∣ n l ⟩ \leftarrow ∣ X {}^{2}Σ^{+} ⟩ ∣ n l ⟩$ . 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 ${}^{11}B H^{*}$ dissociation depends on the finite linewidths and positions of $∣ A {}^{2}Π ⟩ ∣ n l ⟩$ features and on the $n$ -dependent predissociative lifetimes of ${}^{11}B H^{*}$ Rydberg molecules.

Received 21 August 2006

DOI:https://doi.org/10.1103/PhysRevA.75.013410

Authors & Affiliations

C. Ricardo Viteri and Andrew T. Gilkison^*

Department of Chemistry, Purdue University, West Lafayette, Indiana 47907, USA

Scott J. Rixon and Edward R. Grant^†

Department of Chemistry, University of British Columbia, Vancouver, British Columbia, Canada V6 T 1Z3

^*Present address: Schering-Plough, 2000 Galloping Hill Road, Kenilworth, NJ 07033-0530.
^†Author to whom correspondence should be addressed. Electronic address: edgrant@chem.ubc.ca

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Vol. 75, Iss. 1 — January 2007

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Figure 1
Energy-level diagram showing the pulse sequence ( $ω_{1}$ and $ω_{2}$ ) by which vibrationally excited ${}^{11}B H$ molecules are promoted first to a selected rovibrational level of the $A {}^{1}Π$ valence state, then to the gateway $B {}^{1}Σ^{+}$ , $ν^{'} = 0$ , $N^{'} = 0$ Rydberg state. A third counter-propagating laser, $ω_{3}$ , accesses the $∣ X {}^{2}Σ^{+}, ν^{+} = 0, N^{+} ⟩ ∣ n l ⟩$ Rydberg states with $N^{+} = 0$ , 1 or 2; these have energy $E_{N +}^{n}$ and rotational ionization thresholds $V_{ion} (N^{+})$ . Such a state can follow one of two paths. It can dissociate to form ${}^{11}B^{*}$ , which ionizes following one-photon absorption in the still-present $ω_{2}$ and $ω_{3}$ laser fields. Alternatively, when laser frequency $ω_{3}$ coincides with a transition energy $T_{A X} (N^{+}, N_{A}^{+})$ of the $A {}^{2}Π − X {}^{2}Σ^{+}$ (0,0) band of ${}^{11}B H^{+}$ , the Rydberg state $∣ X {}^{2}Σ^{+}, ν^{+} = 0, N^{+} ⟩ ∣ n l ⟩$ can absorb a photon. In the simplest form of this interaction, the initially prepared high-Rydberg electron acts as a spectator to an allowed $A {}^{2}Π \leftarrow X {}^{2}Σ^{+}$ valence electronic transition in the ion core, which occurs with an on-resonance energy $E_{A} (n, N^{+}, N_{A}^{+})$ detuned from $E_{N^{+}}^{n}$ by a small amount as per Eq. (6).Reuse & Permissions
Figure 2
Competition between light absorption and neutral fragmentation in Rydberg states of ${}^{11}B H$ . The interval of $n$ for which the rate of photoabsorption, $Φ σ_{A X}$ , exceeds the predissociation rate, $k (n)$ , depends on the finite linewidths and positions of $∣ A {}^{2}Π ⟩ ∣ n l ⟩$ Rydberg features.Reuse & Permissions
Figure 3
Ionization-detected absorption structure observed at (a) $11 amu$ $({}^{11}B^{+})$ and (b) $12 amu$ $({}^{11}B H^{+})$ for transitions originating from the $B {}^{1}Σ^{+}$ , $ν^{'} = 0$ , $N^{'} = 0$ state of ${}^{11}B H$ converging to thresholds associated with the $X {}^{2}Σ^{+} ν^{+} = 0$ state of ${}^{11}B H^{+}$ . Ladders labeled by principal quantum numbers mark Rydberg resonances converging to rotational states $N^{+} = 0$ , 1 and 2. The ion signal detected at $12 amu$ shows the distribution of ${}^{11}B H^{+}$ features centered around $n = 17 - 18$ as core $A {}^{2}Π \leftarrow X {}^{2}Σ^{+}$ transitions. The simulated spectrum in trace (c) is based on a mathematical model where the isolated-core excitations are vertical in principal quantum number $n$ and orbital angular momentum $l$ .Reuse & Permissions
Figure 4
Comparison of line shapes and intensities distributions of predissociating Rydberg series converging to $ν^{+} = 0$ (top, present work) with autoionizing Rydberg series converging to $ν^{+} = 1$ (bottom, from Ref. 24). Only ${}^{11}B H^{*}$ Rydberg molecules which dissociate during the $10 ns$ that the laser fields persist are resonantly detected in the ${}^{11}B^{+}$ mass channel. In the region of high- $n$ Rydberg states $(n = 27 - 38)$ , the intensities of resonances detected through the dissociation channel decreases systematically with increasing principal quantum number. Taking these relative intensities, we use Eq. (2) to estimate predissociation constants $k (n)$ as plotted in the inset. The solid curve is obtained by fitting these 12 data points to Eq. (3).Reuse & Permissions
Figure 5
Nine possible pathways for the $A {}^{2}Π \leftarrow X {}^{2}Σ^{+}$ (0,0) valence electronic promotion of the ion core, based on the three populated levels, $N^{+} = 0$ , $1$ , and $2$ , of the $X {}^{2}Σ^{+}$ ground state (transition energies are listed in Table ).Reuse & Permissions

Physical Review A

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Isolated core excitation of ${}^{11}B H$ : Photoabsorption in competition with Rydberg predissociation

C. Ricardo Viteri, Andrew T. Gilkison, Scott J. Rixon, and Edward R. Grant

Phys. Rev. A 75, 013410 – Published 11 January 2007

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Physical Review A

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Isolated core excitation of B11H: Photoabsorption in competition with Rydberg predissociation

C. Ricardo Viteri, Andrew T. Gilkison, Scott J. Rixon, and Edward R. Grant

Phys. Rev. A 75, 013410 – Published 11 January 2007

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Isolated core excitation of ${}^{11}B H$ : Photoabsorption in competition with Rydberg predissociation