Chirped magneto-optical trap of molybdenum

We have directly loaded a cryogenic beam of molybdenum atoms into a magneto-optical trap. By chirping the detuning of the trapping lasers, we were able to enhance the number of atoms loaded into the trap by more than a factor of two. We optimize the trapped samples for atom number, temperature, and lifetime by varying the laser-cooling parameters. The laser-cooled molybdenum atoms are subsequently transferred to a magnetic trap where we achieve vacuum-limited lifetimes of 100,ms. Comparison of magneto-optical trap and magnetic trap lifetimes allow us to extract partial decay rates of the $\text{z},^7\text{P}^\text{o}_4\rightarrow\text{a},^5\text{D}_4$ and $\text{z},^7\text{P}^\text{o}_4\rightarrow\text{a},^5\text{D}_3$ transitions. We also provide measurements of the isotope shifts for the $\text{a},^7\text{S}_3\rightarrow \text{z},^7\text{P}^\text{o}_4$ transition with up to four times better precision than previously reported. Finally, we discuss the prospect of applying the chriped magneto-optical trap to produce laser-cooled samples of MgF.

💡 Research Summary

This paper presents the first successful demonstration of a magneto-optical trap (MOT) for molybdenum (Mo) atoms, employing an innovative frequency-chirping technique to enhance loading efficiency from a cryogenic beam source.

The research was conducted by a team at the National Institute of Standards and Technology (NIST). The core achievement is the direct loading of a molybdenum atomic beam into a MOT without an intermediate slowing stage. The atomic source is a two-stage cryogenic buffer gas beam (CBGB), where a pure Mo target is ablated by a pulsed laser and the released atoms are cooled by a ~3 K helium buffer gas. This method produces a slow atomic beam with a most probable velocity of about 105 m/s, a significant reduction compared to conventional high-temperature ovens.

The MOT operates on the cycling transition at 380 nm ((a,^7S_3\rightarrow z,^7P^\text{o}_4)). A key challenge is that the excited state can decay into metastable (a,^5D) levels, primarily (a,^5D_4) and (a,^5D_3). A repump laser at 715 nm addresses leaks to the (a,^5D_4) state, substantially improving MOT lifetime and atom number.

The central innovation is the implementation of a “chirped” MOT. During the loading phase, the detuning of the trapping lasers is linearly ramped over a duration of 3.5 ms, synchronized with the arrival of the atomic beam. This chirp effectively increases the velocity range of atoms that can be captured by the MOT. Through optimization of the chirp amplitude, final detuning, and magnetic field gradient, the team increased the number of trapped (^{98})Mo atoms by a factor of 2.4 compared to a static MOT, capturing up to 24,000 atoms. The paper provides a kinematic analysis, showing that the chirp compensates for the technically limited magnetic field gradient in their setup, bringing the effective capture velocity closer to the scattering-rate-limited value.

The properties of the trapped samples were thoroughly characterized. The temperature of atoms in the MOT was measured via time-of-flight expansion. The MOT lifetime was found to be highly dependent on the presence of the repump laser. Subsequently, the laser-cooled atoms were transferred to a pure magnetic trap, where a vacuum-limited lifetime of 100 ms was achieved. By comparing the decay rates in the MOT and the magnetic trap, the researchers extracted partial decay rates for the leak transitions (z,^7P^\text{o}_4\rightarrow a,^5D_4) and (z,^7P^\text{o}_4\rightarrow a,^5D_3).

Furthermore, the cold and slow nature of the CBGB and MOT samples enabled high-precision spectroscopy. The isotope shifts for the cooling transition were measured for all five stable bosonic isotopes with up to four times better precision than previous reports.

Finally, the paper discusses the prospect of applying this chirped MOT technique directly to molecular species like MgF produced from a similar CBGB source, suggesting a pathway to simplify the apparatus for laser cooling and trapping molecules.

In conclusion, this work establishes a robust platform for laser cooling and trapping molybdenum using a chirped MOT loaded directly from a cryogenic beam. It overcomes traditional challenges associated with transition metals, delivers new precision spectroscopic data, and opens avenues for extending the method to other atoms and molecules.