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International Symposium on Molecular Spectroscopy (ISMS)

SELECTIVE POPULATION OF MOLECULAR CORE NONPENETRATING RYDBERG STATES

David Grimes, Robert W. Field, Ethan Klein, Timothy Barnum, Yan Zhou

Published Jun 19, 2014• 0 citations
Publisher

Abstract

"Core nonpenetrating Rydberg states of molecules are a neglected state of matter. They could have a variety of uses, notably state-selective production of molecular ions. Due to the \textit{l}(\textit{l}+1)/r$^{2}$ centrifugal barrier that prevents the Rydberg electron in high-\textit{l} states from penetrating inside of the ion core, the electron is essentially uncoupled from the ion core, and the system becomes atom-like with long lifetimes, an ""almost good"" \textit{l} quantum number, and electronic transitions that follow $\Delta$J$^{+}$=0 and $\Delta$v$^{+}$=0 propensity rules. However, in most molecules access to these states, via a sequence of $\Delta$\textit{l}=+1 transitions from low-n*, low-\textit{l} states, is blocked by the necessity to traverse the ""zone of death,"" in which nonradiative decay mechanisms are prohibitively fast. We exploit Chirped Pulse millimeter-Wave (CPmmW) spectroscopy to efficiently excite Ca atoms and BaF molecules to core nonpenetrating states in the absence of nonradiative decay mechanisms. A universal method for preparing core nonpenetrating Rydberg states of molecules, which combines CPmmW spectroscopy with STImulated Raman Adiabatic Passage (STIRAP), will be discussed."

DOI
10.15278/isms.2014.re07

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Rotational-state-selective field ionization of triplet molecular Rydberg states of H2_{\mathrm{2}}2​
We apply the chirped-pulse millimeter-wave (CPmmW) technique to transitions between Rydberg states in calcium atoms. The unique feature of Rydberg-Rydberg transitions is that they have enormous electric dipole transition moments (~5 kiloDebye at n* ~ 40, where n* is the effective principal quantum number), so they interact strongly with the mm-wave radiation. After polarization by a mm-wave pulse in the 70-84 GHz frequency region, the excited transitions re-radiate free induction decay (FID) at their resonant frequencies, and the FID is heterodyne-detected by the CPmmW spectrometer. Data collection and averaging are performed in the time domain. The spectral resolution is ~100 kHz. Because of the large transition dipole moments, the available mm-wave power is sufficient to polarize the entire bandwidth of the spectrometer (12 GHz) in each pulse, and high-resolution survey spectra may be collected. Both absorptive and emissive transitions are observed, and they are distinguished by the phase of their FID relative to that of the excitation pulse. With the combination of the large transition dipole moments and direct monitoring of transitions, we observe dynamics, such as transient nutations from the interference of the excitation pulse with the polarization that it induces in the sample. Since the waveform produced by the mm-wave source may be precisely controlled, we can populate states with high angular momentum by a sequence of pulses while recording the results of these manipulations in the time domain. We also probe the superradiant decay of the Rydberg sample using photon echoes. The application of the CPmmW technique to transitions between Rydberg states of molecules is discussed.
The usual simplified view of Rydberg states of molecules as consisting of a single Rydberg electron loosely bound to a molecular ion core in a well-defined rotation-vibration state suggests an attractive possibility for state-selective production of molecular ions. A Rydberg electron excited above the energy of the ground state of the ion core will spontaneously autoionize, leaving behind a molecular ion. The autoionizing states are of strongly mixed character due to the ubiquitous nonadiabatic interactions between Rydberg series associated with different states of the ion core. Using our complete Multichannel Quantum Defect Theory (MQDT) fit model for CaF, we have predicted the locations and strengths of special autoionizing resonances that decay into a single rotation-vibration state of a molecular ion. Few molecules are as well characterized as CaF, nor as elegantly simple. We additionally describe the use of core nonpenetrating states as a general method to produce an ensemble of molecular ions in a single, selectable quantum state.
The population of a beam of lithium atoms has been transferred from a low angular momentum Rydberg state to a state with $|m|=n\ensuremath{-}1$, with nearly 100% efficiency. Selective field ionization is employed to detect the high ∣m∣|m|∣m∣ state. Transfer occurs by a series of adiabatic rapid passages; it can also occur by a single multiphoton process.
We have studied the nununu=1 Rydberg states of BaF in the energy region E=38800-39100 wn (n*=15-25) via optical-optical double resonance spectroscopy. Rydberg states excited above the first ionization potential spontaneously autoionize and 138^{138}138Ba19^{19}19F+^{+}+ ions are detected by TOF-MS. In addition, BaF possesses a particularly low ionization potential, which allows for the study of autoionization dynamics in the absence of predissociative decay. This work extends the assignments of core-penetrating Rydberg states of BaF (Jakubek and Field, 2000) for applications to state-selective ion production schemes. Polarization and Stark spectroscopy techniques will be discussed in the context of accurate and efficient assignment of spectra.