Spin-controlled atom-ion chemistry

Abstract

Quantum control of chemical reactions is an important goal in chemistry and physics. Ultracold chemical reactions are often controlled by preparing the reactants in specific quantum states. Here we demonstrate spin-controlled atom-ion inelastic (spin-exchange) processes and chemical (charge-exchange) reactions in an ultracold Rb-Sr⁺ mixture. The ion's spin state is controlled by the atomic hyperfine spin state via spin-exchange collisions, which polarize the ion's spin parallel to the atomic spin. We achieve ~ 90% spin polarization due to the absence of strong spin-relaxation channel. Charge-exchange collisions involving electron transfer are only allowed for (RbSr)⁺ colliding in the singlet manifold. Initializing the atoms in various spin states affects the overlap of the collision wave function with the singlet molecular manifold and therefore also the reaction rate. Our observations agree with theoretical predictions.

Fig. 1

Energy levels diagrams. a, b Level structure of the ⁸⁸Sr⁺ electronic ground state and hyperfine structure of the ⁸⁷Rb. The Zeeman splitting is for B = 3 G. c Pictorial representation of potential energy curves of the (RbSr)⁺ complex. The experimental entrance channel (Sr⁺(5s)+Rb(5s)) is not the absolute ground state of the system which allows for radiative charge-exchange processes (curly lines). During a collision, the atomic asymptotic state (Sr⁺(5s)+Rb(5s)) splits into a superposition of singlet (¹Σ⁺, blue solid line) and triplet (³Σ⁺, red dashed line) states. Only radiative charge exchange from the singlet state is allowed (blue curly line), as the molecular ground state of the system (Sr(¹S)+Rb⁺) is also a singlet state (¹Σ⁺, solid black line). A pictorial representation of spin-exchange collision is also shown

Fig. 2

Collisional pumping of the spin of the ion. Sr⁺ ion spin projection on the ${∣\downarrow ⟩}_{{\mathrm{Sr}}^{+}}$ state, P(↓), as function of number of Langevin collisions (time). In blue (red) the ion is prepared in the ${∣\uparrow ⟩}_{{\mathrm{Sr}}^{+}}$ $\left({∣\downarrow ⟩}_{{\mathrm{Sr}}^{+}}\right)$ spin-state. The atoms are prepared in state ${∣1,-1⟩}_{\mathrm{Rb}}$ (a) or ${∣1,0⟩}_{\mathrm{Rb}}$ (b). Insets show energetically allowed and forbidden spin-exchange processes. Error bars represent 1 SD

Fig. 3

Charge-exchange control. a Charge-exchange rate for different initial hyperfine states of Rb atoms. b Density dependence of the charge-exchange rate averaged for ${∣1,0⟩}_{\mathrm{Rb}}$ and ${∣1,-1⟩}_{\mathrm{Rb}}$. From a fit to a power-law we estimate the charge-exchange scaling on the density to be k_CE ∝ ρ^0.94(8). Error bars represent 1 SD