Parallel Session: Processes, Contributed Talk (15min)

Slow-decay processes of long-lived Rydberg states of nitric oxide

M. H. Rayment1, S. D. Hogan1*
1Department of Physics and Astronomy, University College London, Gower Street, London, WC1E 6BT, United Kingdom

High Rydberg states of small molecules play an important role in electron-ion recombination in laboratory, atmospheric, and astrophysical plasmas [1-3]. However, until recently laboratory studies of the slow decay dynamics of molecules in these excited states have not been possible [4,5]. Here we describe a unique, state-of-the-art cryogenically cooled chip-based Rydberg-Stark decelerator, and the use of this device to electrostatically trap nitric oxide (NO) molecules for the first time [6]. Trapping the molecules, prepared in long-lived Rydberg states using resonance-enhanced two-colour two-photon excitation from the X 2Π1/2 ground state, for up to 1 ms enabled precise measurements of the excited state decay rates. These measurements were performed for molecules photoexcited to Rydberg states with principal quantum numbers, n, between 32 and 50, in Rydberg series converging to the N+ = 0, 1, and 2 rotational states of the v+ = 0 vibrational state of NO+. For the range of Rydberg states studied, decay time constants of ~300 μs were observed. However, counterintuitively, these decay times decreased as the value of n was increased, and therefore did not follow the n-scaling rules typical of high Rydberg states. Additionally, for some particular values of n deviations from this trend were seen. These observations were interpreted, with the aid of numerical calculations of the energy-level structure and lifetimes of the Rydberg states, to arise as a result of weak rotational and vibrational channel interactions. The vibrational channel interactions, between Rydberg states in the v+ = 0 vibrational series and nearby short-lived low-n states in the v+ = 1 series, resulted in contributions to the total excited-state decay rates on the order of 1 kHz. Rotational channel interactions, within the v+ = 0 series, were identified to specifically affect the decay time constants of some individual Rydberg states.

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[6] A. Deller, M. H. Rayment, S. D. Hogan, Phys. Rev. Lett., 2020, 125, 073201