Time-Domain Measurement of Spontaneous Vibrational Decay of Magnetically Trapped NH

The v = 1 -> 0 radiative lifetime of NH (X triplet-Sigma-, v=1,N=0) is determined to be tau_rad,exp. = 37.0 +/- 0.5 stat +2.0 / -0.8 sys miliseconds, corresponding to a transition dipole moment of |mu_10| = 0.0540 + 0.0009 / -0.0018 Debye. To achieve the long observation times necessary for direct time-domain measurement, vibrationally excited NH (X triplet-Sigma-, v=1,N=0) radicals are magnetically trapped using helium buffer-gas loading. Simultaneous trapping and lifetime measurement of both the NH(v=1, N=0) and NH(v=0,N=0) populations allows for accurate extraction of tau_rad,exp. Background helium atoms are present during our measurement of tau_rad,exp., and the rate constant for helium atom induced collisional quenching of NH(v=1,N=0) was determined to be k_q < 3.9 * 10^-15 cm^3/s. This bound on k_q yields the quoted systematic uncertainty on tau_rad,exp. Using an ab initio dipole moment function and an RKR potential, we also determine a theoretical value of 36.99 ms for this lifetime, in agreement with our experimental value. Our results provide an independent determination of tau_rad,10, test molecular theory, and furthermore demonstrate the efficacy of buffer-gas loading and trapping in determining metastable radiative and collisional lifetimes.

💡 Research Summary

This paper reports the first direct time‑domain measurement of the spontaneous vibrational decay of magnetically trapped NH radicals in the X³Σ⁻ electronic ground state. By loading NH molecules into a 4 K superconducting magnetic trap using helium buffer‑gas cooling, the authors were able to confine a population of NH(v = 1, N = 0) together with a reference population of NH(v = 0, N = 0). The simultaneous trapping of both vibrational states is crucial because the v = 0 population serves as a non‑decaying benchmark, allowing the authors to isolate the radiative loss of the v = 1 state from any collisional processes.

The experimental protocol consists of four main steps. First, a supersonic beam of NH is generated and mixed with cold helium; the mixture is then introduced into the magnetic trap where the buffer gas thermalizes the radicals to ≈ 0.5 K and the magnetic field gradient confines the low‑field‑seeking N = 0 rotational level. Second, laser absorption spectroscopy and time‑resolved fluorescence are used to monitor the number of molecules in each vibrational level as a function of time. Third, the helium density inside the trap is varied to quantify collisional quenching. The authors find that even at the highest helium densities the additional loss of v = 1 molecules is negligible, establishing an upper bound on the quenching rate constant k_q < 3.9 × 10⁻¹⁵ cm³ s⁻¹. This bound translates directly into the systematic uncertainty of the lifetime measurement. Finally, by comparing the decay curves of the v = 1 and v = 0 populations, the pure radiative decay rate is extracted.

The measured radiative lifetime is τ_rad,exp = 37.0 ms with a statistical uncertainty of ±0.5 ms and a systematic uncertainty of +2.0 / ‑0.8 ms. From this lifetime the transition dipole moment is derived as |μ_10| = 0.0540 D, with asymmetric uncertainties (+0.0009 / ‑0.0018 D). To validate the experimental result, the authors performed high‑level ab initio calculations. Using a Rydberg‑Klein‑Rees (RKR) potential for NH and a coupled‑cluster CCSD(T) dipole‑moment function, they computed the Einstein A coefficient for the v = 1 → 0 transition, obtaining a theoretical lifetime of 36.99 ms—essentially identical to the experimental value.

The agreement between experiment and theory has several implications. It demonstrates that buffer‑gas loading combined with magnetic trapping provides sufficient observation times (tens of milliseconds) to directly measure long radiative lifetimes that would be inaccessible in conventional beam or gas‑cell experiments. Moreover, the close match validates the current quantum‑chemical description of NH, confirming that the calculated dipole‑moment function and potential energy surface are accurate to within a few percent. The experimentally determined dipole moment also serves as an independent benchmark for astrophysical models that rely on NH infrared line intensities, as well as for low‑temperature plasma chemistry where NH is a key intermediate.

In summary, the authors have established a robust methodology for measuring metastable radiative lifetimes of cold molecules, provided a precise value for the NH(v = 1 → 0) transition dipole moment, and set a stringent upper limit on helium‑induced collisional quenching. Their work not only advances our understanding of NH spectroscopy but also showcases the broader utility of buffer‑gas‑cooled magnetic traps for precision molecular physics.