Electrostatic trapping of metastable NH molecules

We report on the Stark deceleration and electrostatic trapping of ${}^{14}\text{N}\text{H}$ $\left(a{}^1\Delta \right)$ radicals. In the trap, the molecules are excited on the spin-forbidden $A{}^3\Pi \leftarrow a{}^1\Delta$ transition and detected via their subsequent fluorescence to the $X{{}^3\Sigma }^{-}$ ground state. The $1∕e$ trapping time is $1.4\pm 0.1\text{s}$, from which a lower limit of 2.7 s for the radiative lifetime of the $a{}^1\Delta$, $v=0$, $J=2$ state is deduced. The spectral profile of the molecules in the trapping field is measured to probe their spatial distribution. Electrostatic trapping of metastable NH followed by optical pumping of the trapped molecules to the electronic ground state is an important step toward accumulation of these radicals in a magnetic trap.

Figure 1

Scheme of the experimental setup. NH molecules in the $a{}^1\Delta$ state are created by photodissociation of ${\text{HN}}_3$ seeded in Kr. The beam passes through a skimmer, a hexapole, and a Stark decelerator. The decelerated molecules are subsequently loaded into an electrostatic trap. The molecules are probed at the center of the trap by a far off-resonant laser-induced fluorescence detection scheme, that is shown in the inset. A photograph of the Stark decelerator is shown in the upper left corner.

Figure 2

The Stark shift of the metastable $a{}^1\Delta \left(v=0,J=2\right)$ state. In the upper panel the energy splitting (in ${\text{cm}}^{-1}$) in strong electric fields is shown, not resolving the hyperfine structure. The lower panel shows the energy splitting (in MHz) in low electric fields, including the hyperfine structure of ${}^{14}\text{N}\text{H}$.

Figure 3

A measured time-of-flight (TOF) profile of metastable NH molecules observed in a deceleration experiment (a), compared with the outcome of a numerical simulation (b). The molecules are decelerated from $520\text{m}∕\text{s}$ to a final velocity of $84\text{m}∕\text{s}$. The contribution of individual $M_J\Omega$ components to the simulated TOF-profile is shown in the lower part of the figure.

Figure 4

The arrival of undecelerated NH molecules in the trap center is followed by the steady signal of trapped NH molecules. The molecules are brought to a standstill 6 ms after their production. At that moment the trap is switched on. In the inset the population of electrostatically trapped $\text{NH}\left(a{}^1\Delta ,v=0,J=2\right)$ molecules is plotted as a function of time. The solid line is a single exponential fit to the data with a $1∕e$ lifetime of 1.4 s.

Figure 5

Spectral profiles obtained by scanning the detection laser frequency over the highest frequency component of the $P_2\left(2\right)$ transition, which excites the trapped NH molecules from the $a{}^1\Delta$ to the $A{}^3\Pi$ state. The field-free profile (I) is shown, scaled down by a factor 3, together with the profiles obtained while the shallow (II) or deep (III) trapping field is on. In the inset the result of numerical simulations is given, showing the number of trapped molecules as a function of their Stark shift and speed.