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Optical pumping of molecules provides unique opportunities for control of chemical reactions at a wide range of rotational energies. This work reports a chemical reaction with extreme rotational excitation of a reactant and its kinetic characterization. We investigate the chemical reactivity for the hydrogen abstraction reaction SiO + + H 2  → SiOH + + H in an ion trap. The SiO + cations are prepared in a narrow rotational state distribution, including super-rotor states with rotational quantum number ( j ) as high as 170, using a broad-band optical pumping method. We show that the super-rotor states of SiO + substantially enhance the reaction rate, a trend reproduced by complementary theoretical studies. We reveal the mechanism for the rotational enhancement of the reactivity to be a strong coupling of the SiO + rotational mode with the reaction coordinate at the transition state on the dominant dynamical pathway. Control of chemical reactivity through excitation of rotational states is a relatively unexplored process that may play a role in interstellar chemistry. Here the authors show a marked acceleration of the hydrogen abstraction reaction between SiO + and H 2 by exciting super-rotor states of SiO + , in a joint experimental and theoretical study.

We describe a new instrument, the Argonne Auger Radioisotope Microscope (ARM), capable of characterizing the Auger electron (AE) emission of radionuclides, including candidates relevant in nuclear medicine. Our approach relies on event-by-event ion–electron coincidence, time-of-flight, and spatial readout measurement to determine correlated electron multiplicity and energy distributions of Auger decays. We present a proof-of-principle measurement with the ARM using x-ray photoionization of stable krypton beyond the K -edge and identify a bifurcation in the electron multiplicity distribution depending on the emission of K-LX electrons. Extension of the ARM to the characterization of radioactive sources of AE emissions is enabled by the combination of two recent developments: (1) cryogenic buffer gas beam technology to introduce Auger emitters into the detection region with well-defined initial conditions, and (2) large-area micro-channel plate detectors with multi-hit detection capabilities to simultaneously detect multiple electrons emitted in a single decay.

Improved optical control of molecular quantum states promises new applications including chemistry in the quantum regime, precision tests of fundamental physics, and quantum information processing. While much work has sought to prepare ground state molecules, excited states are also of interest. Here, we demonstrate a broadband optical approach to pump trapped SiO + molecules into pure super rotor ensembles maintained for many minutes. Super rotor ensembles pumped up to rotational state N  = 67, corresponding to the peak of a 9400 K distribution, had a narrow N spread comparable to that of a few-kelvin sample, and were used for spectroscopy of the previously unobserved C 2 Π state. Significant centrifugal distortion of super rotors pumped up to N  = 230 allowed probing electronic structure of SiO + stretched far from its equilibrium bond length. Optical pulses can be useful to create and control molecules in higher quantum states. Here the authors use optical pumping to create rotationally excited states of SiO + molecular ion into super rotor ensemble.

Molecular overtone transitions provide optical frequency transitions sensitive to variation in the proton-to-electron mass ratio ( μ ≡ m p / m e ). However, robust molecular state preparation presents a challenge critical for achieving high precision. Here, we characterize infrared and optical-frequency broadband laser cooling schemes for TeH + , a species with multiple electronic transitions amenable to sustained laser control. Using rate equations to simulate laser cooling population dynamics, we estimate the fractional sensitivity to μ attainable using TeH + . We find that laser cooling of TeH + can lead to significant improvements on current μ variation limits.

Abstract We describe a new instrument, the Argonne Auger Radioisotope Microscope (ARM), capable of characterizing the Auger electron (AE) emission of radionuclides, including candidates relevant in nuclear medicine. Our approach relies on event-by-event ion–electron coincidence, time-of-flight, and spatial readout measurement to determine correlated electron multiplicity and energy distributions of Auger decays. We present a proof-of-principle measurement with the ARM using x-ray photoionization of stable krypton beyond the K -edge and identify a bifurcation in the electron multiplicity distribution depending on the emission of K-LX electrons. Extension of the ARM to the characterization of radioactive sources of AE emissions is enabled by the combination of two recent developments: (1) cryogenic buffer gas beam technology to introduce Auger emitters into the detection region with well-defined initial conditions, and (2) large-area micro-channel plate detectors with multi-hit detection capabilities to simultaneously detect multiple electrons emitted in a single decay.

Techniques for achieving complete quantum control over atoms have been developed and perfected over the past four decades with great success. This work has led to multiple Nobel prizes and has been the catalyst for rapid advances in a broad array of research fields. A natural progression forward is to develop control over molecules. Compared to atoms, molecules have an increased complexity in their internal structure due to their additional degrees of freedom of rotations and vibrations. Control over these additional degrees of freedom would allow for the study of chemistry with unprecedented detail, the study of new many-body effects, and more precise tests of fundamental physics. The work in this thesis represents a step forward in the goal of having complete quantum control over a molecule. We demonstrate rotational cooling on the silicon monoxide cation (SiO$^+$) via optical pumping with a spectrally pulse-shaped broadband laser. Cooling is achieved on a 100 ms time scale and attains a ground state population of 94(3)\\% ($T=0.53(6)$ K). I also describe a novel approach to pulse shaping for populating arbitrary rovibrational states of molecules with diagonal Franck-Condon Factors (FCFs). This technique is demonstrated on SiO$^+$ and is used to achieve steady state preparation of so-called molecular super-rotors. We demonstrate a narrow rotational population distribution ($\\Delta N=3$) around arbitrary targeted rotational states between $N=0$ and $N=65$. Control is accomplished through asymmetric pumping of transitions that add and remove angular momentum such that population is stochastically driven into a target rotational state. Furthermore, preliminary results demonstrating selective population of the first excited vibrational state are also presented. The full preparation process for the control experiments of SiO$^+$ is also described. We characterize the photoionization spectrum of neutral SiO for the purpose of loading an ion trap, we measure the SiO$^+$ optical branching ratios and lifetimes of states relevant to laser control, and we characterize the dissociating $X^2\\Sigma^+\\rightarrow C^2\\Pi$ transition for state readout. Finally, we characterize the reaction rate of trapped SiO$^+$ with the background UHV environment. These steps described for SiO$^+$ are crucial for demonstrating control, and such a process could be generalized to other molecules of interest. In the remaining portions of the thesis I detail a proposal for optically pumping another molecule with diagonal FCFs, TeH$^+$, for the purpose of detecting a variation in the proton-to-electron mass ratio.

We demonstrate excitation of metastable krypton and xenon beams using a vacuum ultraviolet lamp and directly compare the performance of this method to metastable excitation based on a radiofrequency-driven plasma discharge. In our apparatus, lamp-based metastable excitation outperforms the plasma discharge across a wide range of beam flux values relevant for Atom Trap Trace Analysis (ATTA). Moreover, we do not observe significant degradation in lamp performance after over 160 hours of operation. We find that lamp-based excitation is particularly advantageous at the smallest and largest beam fluxes tested, demonstrating the utility of this approach both for improving krypton ATTA and for enabling the detection of radioactive xenon isotopes using ATTA. Finally, we demonstrate an additional enhancement to lamp-based metastable excitation efficiency and stability by applying an external magnetic field.

We demonstrate rotational cooling of the silicon monoxide cation via optical pumping by a spectrally filtered broadband laser. Compared with diatomic hydrides, SiO\\+ is more challenging to cool because of its smaller rotational interval. However, the rotational level spacing and large dipole moment of SiO\\+ allows direct manipulation by microwaves, and the absence of hyperfine structure in its dominant isotopologue greatly reduces demands for pure quantum state preparation. These features make \\(^{28}\\)Si\\(^{16}\\)O\\+ a good candidate for future applications such as quantum information processing. Cooling to the ground rotational state is achieved on a 100 ms time scale and attains a population of 94(3)\\%, with an equivalent temperature \\(T=0.53(6)\\) K. We also describe a novel spectral-filtering approach to cool into arbitrary rotational states and use it to demonstrate a narrow rotational population distribution (\\(N\\pm1\\)) around a selected state.

Optical pumping of molecules provides unique opportunities for the control of chemical reactions at a wide range of rotational energies. Chemical reactivity for the hydrogen abstraction reaction SiO\\(^+\\) + H\\(_2\\) \\(\\rightarrow\\) SiOH\\(^+\\) + H is investigated in an ion trap. The SiO\\(^+\\) cation is prepared with a narrow rotational state distribution, including super-rotor states with rotational quantum number \\(\\it{(j)}\\) as high as 170 using a broad-band optical pumping method. The super-rotor states of SiO\\(^+\\) are shown to substantially enhance the reaction rate, a trend reproduced by complementary theoretical studies. The mechanism for the rotational enhancement of the reactivity is revealed to be a strong coupling of the SiO\\(^+\\) rotational mode with the reaction coordinate at the transition state on the dominant dynamical pathway. This work reports for the first time a chemical reaction with extreme rotational excitation of a reactant and its kinetic characterization.

Improved optical control of molecular quantum states promises new applications including chemistry in the quantum regime, precision tests of fundamental physics, and quantum information processing. While much work has sought to prepare ground state molecules, excited states are also of interest. We demonstrate a broadband optical approach to pump trapped SiO\\(^+\\) molecules into pure super rotor ensembles maintained for many minutes. Super rotor ensembles pumped up to rotational state \\(N=67\\), corresponding to the peak of a 9400 K distribution, had a narrow \\(N\\) spread comparable to that of a few-kelvin sample, and were used for spectroscopy of the previously unobserved C\\(^2\\Pi\\) state. Significant centrifugal distortion of super rotors pumped up to \\(N=230\\) allowed probing electronic structure of SiO\\(^+\\) stretched far from its equilibrium bond length.