Metastable Feshbach Molecules in High Rotational States

We experimentally demonstrate ${\mathrm{Cs}}_2$ Feshbach molecules well above the dissociation threshold, which are stable against spontaneous decay on the time scale of 1 s. An optically trapped sample of ultracold dimers is prepared in a high rotational state and magnetically tuned into a region with a negative binding energy. The metastable character of these molecules arises from the large centrifugal barrier in combination with negligible coupling to states with low rotational angular momentum. A sharp onset of dissociation with increasing magnetic field is mediated by a crossing with a lower rotational dimer state and facilitates dissociation on demand with a well-defined energy.

Figure 1

Illustration of the basic idea of a long-lived metastable Feshbach molecule in a high rotational state. The solid lines illustrate two molecular potential curves, with high rotational angular momentum and with zero rotation. Direct dissociation through the large centrifugal barrier is strongly suppressed. Indirect dissociation by coupling at short distances to a partial wave with low angular momentum is negligible because of the large difference in the rotational quantum numbers.

Figure 2

Energy curves of the relevant ${\mathrm{Cs}}_2$ dimer states as function of the magnetic field [16, 17]. The $s$-wave threshold ($E=0$) and the height of the $d$-wave centrifugal barrier ($E=h\times 3.7\mathrm{MHz}$) are indicated. The quantum state relevant for this work is labeled with $6l(4)$. The open squares represent spectroscopic data on this state obtained from measuring its magnetic moment, and the arrows indicate the path for preparation of the dimers (for details see text).

Figure 3

Loss measurements of the $6l(4)$ molecular sample for different magnetic fields. One set of data is taken below the dissociation threshold ($B=21.6\mathrm{G}$); three further sets (24.7, 27.0, 27.1 G) refer to situations above threshold. The lines are fits to the measurements including two-body and one-body decay. The inset shows the resulting two-body loss coefficient $L_2$, where the shaded region indicates the magnetic field region above threshold. The horizontal line indicates the average $L_2$ below threshold.

Figure 4

Dissociation rate $\alpha$ for the $6l(4)$ state as function of magnetic field obtained from an unconstrained fit (closed squares) and a constrained fit, fixing $L_2$ to its value below threshold (open squares). The inset shows the corresponding one-body lifetime as function of energy above threshold.

Figure 5

The number of dissociated molecules as function of the magnetic field. The curve is a fit, based on modeling the amount of $g$-wave character around the crossing between the $6l(4)$ state and the $6g(5)$ state. The inset shows an absorption image of rapid dissociated $6l(4)$ dimers at a magnetic field of 27.8 G (averaged over 16 shots).