Patrick R. Stollenwerk

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$_2$ $\rightarrow$ 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 $\it{(j)}$ 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.

The 1+1 REMPI spectrum of SiO in the 210-220 nm range is recorded. Observed bands are assigned to the $A-X$ vibrational bands $(v``=0-3, v`=5-10)$ and a tentative assignment is given to the 2-photon transition from $X$ to the n=12-13 $[X^{2}Σ^{+},v^{+}=1]$ Rydberg states at 216-217 nm. We estimate the IP of SiO to be 11.59(1) eV. The SiO$^{+}$ cation has previously been identified as a molecular candidate amenable to laser control. Our work allows us to identify an efficient method for loading cold SiO$^{+}$ from an ablated sample of SiO into an ion trap via the $(5,0)$ $A-X$ band at 213.977 nm.

Molecules with deep vibrational potential wells provide optical intervals sensitive to variation in the proton-electron mass ratio ($μ$). On one hand, polar molecules are of interest since optical state preparation techniques have been demonstrated for such species. On the other hand, it might be assumed that polar species are unfavorable candidates, because typical molecule-frame dipole moments reduce vibrational state lifetimes and cause large polarizabilities and associated Stark shifts. Here, we consider single-photon spectroscopy on a vibrational overtone transition of the polar species TeH$^+$, which is of practical interest because its diagonal Franck-Condon factors should allow rapid state preparation by optical pumping. We point out that all but the ground rotational state obtains a vanishing low-frequency scalar polarizability from coupling with adjacent rotational states, because of a fortuitous relationship between rigid rotor spacings and dipole matrix elements. We project that for good choices of spectroscopy states, demonstrated levels of field control should make possible uncertainties of order $1 \times 10^{-18}$, similar to those of leading atomic ion clocks. The moderately long lived vibrational states of TeH$^+$ make possible a frequency uncertainty approaching $1 \times 10^{-17}$ with one day of averaging for a single trapped ion. Observation over one year could probe for variation of $μ$ with a sensitivity approaching the $1 \times 10^{-18}/\textrm{yr}$ level.

We perform ab-initio calculations needed to predict the cross-section of an experimentally accessible (1+1') resonance-enhanced multiphoton dissociation (REMPD) pathway in AlH+. Experimenting on AlH+ ions held in a radiofrequency Paul trap, we confirm dissociation via this channel with analysis performed using time-of-flight mass spectrometry. We demonstrate the use of REMPD for rotational state analysis, and we measure the rotational distribution of trapped AlH+ to be consistent with the expected thermal distribution. AlH+ is a particularly interesting species for ion trap work because of its electronic level structure, which makes it amenable to proposals for rotational optical pumping, direct Doppler cooling, and single-molecule fluorescence detection. Potential applications of trapped AlH+ include searches for time-varying constants, quantum information processing, and ultracold chemistry studies.