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arXiv:1709.04527 · Physical Review Letters

Control of Ultracold Photodissociation with Magnetic Fields

Mickey McDonald, Iwona Majewska, C.-H. Lee, Stanimir Kondov, B. H. McGuyer, Robert Moszyński, Tanya Zelevinsky

Published Jan 16, 2018• 10 citations
arXivHTMLPDF

Abstract

Photodissociation of a molecule produces a spatial distribution of photofragments determined by the molecular structure and the characteristics of the dissociating light. Performing this basic reaction at ultracold temperatures allows its quantum mechanical features to dominate. In this regime, weak applied fields can be used to control the reaction. Here, we photodissociate ultracold diatomic strontium in magnetic fields below 10 G and observe striking changes in photofragment angular distributions. The observations are in excellent agreement with a multichannel quantum chemistry model that includes nonadiabatic effects and predicts strong mixing of partial waves in the photofragment energy continuum. The experiment is enabled by precise quantum-state control of the molecules.

DOI
10.1103/physrevlett.120.033201

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Photodissociation of a molecule produces a spatial distribution of photofragments determined by the molecular structure and the characteristics of the dissociating light. Performing this basic reaction at ultracold temperatures allows its quantum mechanical features to dominate. In this regime, weak applied fields can be used to control the reaction. Here, we photodissociate ultracold diatomic strontium in magnetic fields below 10 G and observe striking changes in photofragment angular distributions. The observations are in excellent agreement with a multichannel quantum chemistry model that includes nonadiabatic effects and predicts strong mixing of partial waves in the photofragment energy continuum. The experiment is enabled by precise quantum-state control of the molecules.
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Chemical reactions at temperatures near absolute zero require a full quantum description of the reaction pathways and enable enhanced control of the products via quantum state selection. Ultracold molecule experiments have provided initial insight into the quantum nature of basic chemical processes involving diatomic molecules, for example from studies of bimolecular reactions, but complete control over the reactants and products has remained elusive. The half-collision process of photodissociation is an indispensable tool in molecular physics and offers significantly more control than the reverse process of photoassociation. Here we reach a fully quantum regime with photodissociation of ultracold 88^{88}88Sr2_22​ molecules where the initial bound state of the molecule and the target continuum state of the fragments are strictly controlled. Detection of the photodissociation products via optical absorption imaging reveals the hallmarks of ultracold chemistry: resonant and nonresonant barrier tunneling, importance of quantum statistics, presence of forbidden reaction pathways, and matter wave interference of reaction products. In particular, this interference yields fragment angular distributions with a strong breaking of cylindrical symmetry, so far unobserved in photodissociation. We definitively show that the quasiclassical description of photodissociation fails in the ultracold regime. Instead, a quantum model accurately reproduces our results and yields new intuition.
At ultralow energies, atoms and molecules undergo collisions and reactions that are best described in terms of quantum mechanical wave functions. In contrast, at higher energies these processes can be understood quasiclassically. Here, we investigate the crossover from the quantum mechanical to the quasiclassical regime both experimentally and theoretically for photodissociation of ultracold diatomic strontium molecules. This basic reaction is carried out with a full control of quantum states for the molecules and their photofragments. The photofragment angular distributions are imaged, and calculated using a quantum mechanical model as well as the WKB and a semiclassical approximation that are explicitly compared across a range of photofragment energies. The reaction process is shown to converge to its high-energy (axial recoil) limit when the energy exceeds the height of any reaction barriers. This phenomenon is quantitatively investigated for two-channel photodissociation using intuitive parameters for the channel amplitude and phase. While the axial recoil limit is generally found to be well described by a commonly used quasiclassical model, we find that when the photofragments are identical particles, their bosonic or fermionic quantum statistics can cause this model to fail, requiring a quantum mechanical treatment even at high energies.
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This article reviews the current state of the art in the field of cold and ultracold molecules and demonstrates that chemical reactions, inelastic collisions and dissociation of molecules at subkelvin temperatures can be manipulated with external electric or magnetic fields. The creation of ultracold molecules may allow for spectroscopy measurements with extremely high precision and tests of fundamental symmetries of nature, quantum computation with molecules as qubits, and controlled chemistry. The probability of chemical reactions and collisional energy transfer can be very large at temperatures near zero kelvin. The collision energy of ultracold atoms and molecules is much smaller than perturbations due to interactions with external electric or magnetic fields available in the laboratory. External fields may therefore be used to induce dissociation of weakly bound molecules, stimulate forbidden electronic transitions, suppress the effect of centrifugal barriers in outgoing reaction channels or tune Feshbach resonances that enhance chemical reactivity.
We demonstrate detection of NaRb Feshbach molecules at high magnetic field by combining molecular photodissociation and absorption imaging of the photofragments. The photodissociation process is carried out via a spectroscopically selected hyperfine Zeeman level correlated with the Na (3P3/23{P}_{3/2}3P3/2​) +++ Rb (5S1/25{S}_{1/2}5S1/2​) asymptote which, following spontaneous emission and optical pumping, leads to ground-state atoms in a single level with near unity probability. Subsequent to the dissociation, the number of molecules is obtained by detecting the resultant 23Na^{23}\mathrm{Na}23Na and 87Rb^{87}\mathrm{Rb}87Rb atoms. We have also studied the heating effect caused by the photodissociation process and optimized the detection protocol for extracting the temperature of the molecular cloud. This method enables the insituin\phantom{\rule{0.16em}{0ex}}situinsitu detection of fast timescale collision dynamics between NaRb Feshbach molecules and will be a valuable capability in studying few-body physics involving molecules.