Giant Helium Dimers Produced by Photoassociation of Ultracold Metastable Atoms

We produce giant, purely long-range helium dimers by photoassociation of metastable helium atoms in a magnetically trapped, ultracold cloud. The photoassociation laser is detuned close to the atomic $2^3{S}_1-2^3{P}_0$ line and produces strong heating of the sample when resonant with molecular bound states. The temperature of the cloud serves as an indicator of the molecular spectrum. We report good agreement between our spectroscopic measurements and our calculations of the five bound states belonging to a $0_u^{+}$ purely long-range potential well. These previously unobserved states have classical inner turning points of about $150a_0$ and outer turning points as large as $1150a_0$.

Figure 1

Molecular potentials involved in the electronic excitation of two $2^3{S}_1$ spin-polarized ${}^4\mathrm{H}\mathrm{e}$ atoms. The ${}^5\Sigma _g^{+}$ potential is taken from 10. Note the change in energy and length scales between the ${}^5\Sigma _g^{+}$ and the purely long-range $0_u^{+}$ potential wells. The excited potentials are calculated as described in the text and are the only experimentally accessible potentials linked to the $2^3{S}_1+2^3{P}_0$ asymptote. Five bound states ($v=0$ to 4) are observed in the $0_u^{+}$ well. Their inner turning points are around $150a_0$, their outer turning points range from $250a_0$ ($v=0$) to $1150a_0$ ($v=4$).

Figure 2

Experimental setup. A detailed description has been published in 11. (1) Metastable helium atoms are trapped and evaporated down to a few $\mu \mathrm{K}$ and a peak density of the order of ${10}^{13}{\mathrm{c}\mathrm{m}}^{-3}$. (2) The cloud is illuminated inside the magnetic trap by the PA laser for a few ms. (3) The cloud is released. After ballistic expansion, it is imaged on a CCD camera with a magnification of 1.

Figure 3

Detection of the line $v=4$ in the $0_u^{+}$ potential well. (a) Peak optical density, (b) atom number, and (c) temperature in $\mu \mathrm{K}$ versus PA laser frequency. Each point represents a new evaporated cloud after PA pulse illumination and ballistic expansion. The curves in graphs (a) and (b) indicate the averaging of data over five adjacent points. The curve in graph (c) is a Lorentzian fit to the data.