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Photo-excitation of long-lived transient intermediates in ultracold reactions


In many chemical reactions, the transformation from reactants to products is mediated by transient intermediate complexes. For gas-phase reactions involving molecules with a few atoms, these complexes typically live on the order of 10 ps or less before dissociating, and are therefore rarely influenced by external processes. Here, we demonstrate that the transient intermediate complex K2Rb2*, formed from collisions between ultracold KRb molecules, undergoes rapid photo-excitation in the presence of a continuous-wave laser source at 1,064 nm, a wavelength commonly used to confine ultracold molecules. These excitations are facilitated by the exceptionally long lifetime of the complex under ultracold conditions. Indeed, by monitoring the change in the complex population after the sudden removal of the excitation light, we directly measure the lifetime of the complex to be 360 ± 30 ns, in agreement with our calculations based on the Rice–Ramsperger–Kassel–Marcus (RRKM) statistical theory. Our results shed light on the origin of the two-body loss widely observed in ultracold molecule experiments. Additionally, the long complex lifetime, coupled with the observed photo-excitation pathway, opens up the possibility to spectroscopically probe the structure of the complex with high resolution, thus elucidating the reaction dynamics.

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Fig. 1: Ultracold reactions in an optical dipole trap.
Fig. 2: Trap light-induced excitation of the intermediate complex.
Fig. 3: Energies and rates of the electronic excitations of the complex.
Fig. 4: Lifetime of the intermediate complex.

Data availability

The data that support the findings of this study are available from the corresponding author upon reasonable request. Source data are provided with this paper.

Code availability

The computer codes used for theoretical calculations in this study are available from T.K. ( upon reasonable request.


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We thank L. Zhu for experimental assistance. This work is supported by DOE YIP and the David and Lucile Packard Foundation. M.A.N. is supported by an HQI postdoctoral fellowship. T.K. is supported by NWO Rubicon grant no. 019.172EN.007 and the NSF through ITAMP. H.G. acknowledges a MURI grant from ARO (W911NF-19-1-0283) and a Humboldt Research Award.

Author information

Authors and Affiliations



The experimental work and data analysis were carried out by Y.L., M.-G.H., M.A.N., D.D.G. and K.-K.N. Theoretical calculations were performed by T.K., and H.G. aided in the analysis of the results. All authors contributed to interpreting the results and writing the manuscript.

Corresponding author

Correspondence to Kang-Kuen Ni.

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Extended data

Extended Data Fig. 1 Continued formation of products at high ODT intensities.

Steady-state K2+ (red circles) and Rb2+ (blue circles) ion counts at ODT light intensities in the 11.3–46.2 kW/cm2 range, normalized by the number of experimental cycles (~ 80 for each data point). The error bars represent shot noise. The dashed lines indicate the levels to which the ion counts plateau, obtained by averaging, within each dataset, the values of the points at ODT intensities larger than 15 kW/cm2. (Inset) Timing schemes of the ODT (red and blue) and the pulsed UV ionization laser (purple) used for the measurements presented here. The red (blue) trace corresponds to a high (low) duty cycle modulation of the ODT. The instantaneous ODT intensity, I, is inversely proportional to the duty cycle, while the time-averaged ODT intensity, Iavg is constant for all measurements.

Source Data

Source data

Source Data Fig. 1

K2+, Rb2+ (Fig. 2a) and K2Rb2+ (Fig. 2b) ion counts as a function of ODT intensity.

Source Data Fig. 4

Fig. 4a: oscilloscope traces of the 1,064-nm and UV ionization light intensities for the complex lifetime measurement. Fig. 4b: K2Rb2+ ion counts as a function of the delay between ODT turn-off edge and the UV ionization pulse.

Source Data Extended Data Fig. 1

K2+ and Rb2+ ion counts as a function of ODT intensity.

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Liu, Y., Hu, MG., Nichols, M.A. et al. Photo-excitation of long-lived transient intermediates in ultracold reactions. Nat. Phys. 16, 1132–1136 (2020).

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