Magneto-Optical Trapping of a Metal Hydride Molecule

Abstract

We demonstrate a three-dimensional magneto-optical trap (MOT) of a metal hydride molecule, CaH. We are able to scatter $\sim$$10^{4}$ photons with vibrational loss covered up to vibrational quantum number $ν=2$. This allows us to laser slow the molecular beam near zero velocity with a "white-light" technique and subsequently load it into a radio-frequency MOT. The MOT contains 230(40) molecules, limited by beam source characteristics and predissociative loss of CaH. The temperature of the MOT is below one millikelvin. The predissociative loss mechanism could, in turn, facilitate controlled dissociation of the molecule, offering a possible route to optical trapping of hydrogen atoms for precision spectroscopy.

Figures (5)

• Figure 1: (a) Schematic of the experiment. CaH molecules are generated from a CBGB source operating at $\sim$$6$ K. The molecules are subsequently laser slowed and trapped in the MOT region $73$ cm away from the cell exit. (b) Relevant energy level structure for laser slowing. The deceleration force is supplied by the frequency-broadened main cycling laser addressing the $A^{2}\Pi_{1/2}(\nu'=0,\ J'=1/2,\ +)\leftarrow X^{2}\Sigma^{+}(\nu=0,\ N=1,\ -)$ transition at $695$ nm. Vibrational repumping covers up to ($\nu=2$) for both laser slowing and the MOT. (c) Relevant energy level structure for the MOT. The lasers address the same transition as the main cycling laser of slowing but with a distinct laser frequency for each hyperfine state. The $A^2\Pi_{1/2}(\nu'=0,\ J'=1/2,\ +)$ excited state has a $\delta_A\approx18$ MHz hyperfine splitting. The MOT is optimal with a global detuning of $\delta\approx-5$ MHz.
• Figure 2: Characterization of laser slowing. (a) Laser-induced fluorescence (LIF) of an unperturbed molecular beam subtracted from the LIF of a molecular beam subject to laser slowing. The solid black line marks ballistic propagation from the cell and serves as a guide to the eye. (b) Time-integrated LIF for unperturbed and the slowed beams as a function of forward velocity. Error bars represent 1-$\sigma$ uncertainties. The inset shows fluorescence for detection at zero velocity. The results indicate efficient laser slowing, with molecules decelerated below the MOT capture velocity ($\lesssim$$10$ m$/$s).
• Figure 3: CaH MOT measurements. (a) LIF detected with a PMT under 3 configurations: slowing only (gray), MOT (red), and antiMOT (blue). The presence of molecules in the MOT after the molecular beam has fully traversed the region demonstrates trapping. (b) LIF of the antiMOT phase subtracted from that of the MOT phase. $1/e$ lifetime can be extracted from an exponential fit to the tail of the trace. (c) $1/e$ MOT lifetime as a function of the laser power; $\sim$$30$ ms is achieved with a few milliwatts. The solid line is a guide to the eye. (d) Camera images of the MOT and antiMOT, integrating from $25$ ms to $55$ ms after ablation. The images are smoothed with a Gaussian filter of $0.6$ mm standard deviation.
• Figure 4: MOT trapping and cooling force measurement. MOT displacement as a function of time after an applied push by the main cycling laser for slowing. (ii)-(v) are sample MOT images at $3.5$ ms, $9$ ms, $13$ ms, and $16$ ms after the push. (i) shows the unperturbed MOT, with its fitted center position marked by the gray square point and the corresponding shaded region. The images are smoothed with a Gaussian filter of $0.6$ mm standard deviation and normalized to the same scale for better visualization. Error bars represent 1-$\sigma$ uncertainties. The fit of the displacements versus time yields a trapping frequency of $\omega=2\pi\times48(3)$ Hz and a damping constant of $\beta=510(110)$ s$^{-1}$.
• Figure 5: MOT size and temperature measurements. Geometric mean MOT size as a function of the MOT laser power per beam. The MOT size increases with higher power due to sub-Doppler heating. The inset shows the measured geometric mean temperature of the MOT at two laser powers. Error bars represent 1-$\sigma$ uncertainties.