Dynamics of predissociation and autoionization in the Rydberg states of boron monohydride

Cesar Ricardo Viteri, Purdue University

Abstract

Optical-optical-optical triple resonance spectroscopy is applied to isolate single rovibronic Rydberg states of the 11BH molecule. The first step transfers population from the X1Σ+ state with isotopic purity to the first excited singlet state, A 1Π. A second pulse promotes this population to the N' = 0 rotational level of v' = 0, 1, 2 or 3 in the B 1Σ+ state. This optically selected gateway state serves as an originating point for transitions driven by a third laser which scans below and above vertical ionization thresholds. Final Rydberg states confined by selection rules to angular momentum N = 1 display a complete picture of predissociating and vibrational autoionizing structure with core vibrations of v+ = 0 through v+ = 2 and v+ = 1 through v+ = 4 respectively. At low principal quantum number, n, large Λ-type doubling suggests the beginning of l-uncoupling. At higher n, the Rydberg systems quantitatively characterize a fundamental example of electron orbital [special characters omitted] cation core rotational coupling. Transition from Hund's case (b) to (d) of two interacting np series is analyzed using frame transformation theory. Vibrationless Rydberg states decay to B and H neutral products on a nanosecond time scale. For a certain range of third-photon excitation frequencies, an initially prepared Rydberg electron acts as a spectator of a strongly allowed A2Π←X2Σ + valence electronic absorption by the ion, constituting the very first molecular example of an Isolated-Core-Excitation. Above the adiabatic ionization threshold, the rate of electron loss exceeds neutral fragmentation by only an order of magnitude. Lineshapes match for resonances with the same electron orbital quantum numbers built on v+ = 0 and v+ = 1 cores. Feature-for-feature similarity in predissociation and autoionization lineshapes suggests that inelastic electron-cation scattering pathways leading to electron ejection and dissociative recombination proceed through a common continuum. Autoionization spectra display discrete-continuum and discrete-discrete transitions where intensities are governed by Franck-Condon factors and continuum-discrete mixing of final states. The precise pattern of high-Rydberg states observed reflect differences between the electronic potentials of the gateway B1Σ + state of 11BH and the X2Σ + ground state of the cation.

Degree

Ph.D.

Advisors

Grant, Purdue University.

Subject Area

Chemistry

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