Quantum-state-specific reaction rate measurements for the photo-induced reaction Ca⁺ + O₂ → CaO⁺ + O

ABSTRACT

Atoms and molecules often react at different rates depending on their internal quantum states. Thus, controlling which internal states are populated can be used to manipulate the reactivity and can lead to a more detailed understanding of reaction mechanisms. We demonstrate this control of reactions by studying the electronically excited state reaction ${\mathrm{C}\mathrm{a}}^{+}+{\mathrm{O}}_2\to {\mathrm{C}\mathrm{a}\mathrm{O}}^{+}+\mathrm{O}$. This reaction is exothermic only if Ca${}^{+}$ is in one of its excited electronic states. Using laser-cooling and electrodynamic trapping, we cool and trap Ca${}^{+}$ at millikelvin temperatures for several minutes. We can then change the fraction of time they spend in each of the two excited states by adjusting the detunings of the cooling lasers. This allows us to disentangle the reactions that begin with Ca${}^{+}$ in the ²P${}_{1/2}$-state from the ones where Ca${}^{+}$ is in the ²D${}_{3/2}$-state. Using time-of-flight mass spectrometry, we determine independent reaction rate constants for Ca${}^{+}$ in both electronically excited states.

GRAPHICAL ABSTRACT

Keywords:

• Linear ion trap
• Coulomb crystals
• chemical reactions
• photo-assisted reaction

Disclosure statement

No potential conflict of interest was reported by the authors.

ORCID

Philipp C. Schmid http://orcid.org/0000-0002-1644-0495

Thanh L. Nguyen http://orcid.org/0000-0002-7794-9439

John F. Stanton http://orcid.org/0000-0003-2345-9781

Notes

1 ${}^{40}$Ca${}^{+}$ has a second possible cooling scheme, which uses the ${}^2$S${}_{1/2}$–${}^2$P${}_{3/2}$ transition for cooling and the ${}^2$P${}_{3/2}$–${}^2$D${}_{5/2}$ transition for repumping. The two transition wavelengths – 393 nm and 854 nm – are not within the bandwidth of our laser systems. We therefore neglect these electronic states for the scope of this work.

Additional information

Funding

This work was supported by the National Science Foundation (Division of Physics PHY-1734006, CHE-1664325, and Division of Chemistry CHE-1464997), Air Force Office of Scientific Research (AFOSR) (FA9550-16-1-0117), and National Institute of Standards and Technology (NIST).