Observation of Rydberg-Atom Macrodimers: Micrometer-Sized Diatomic Molecules

Long-range metastable molecules consisting of two cesium atoms in high Rydberg states have been observed in an ultracold gas. A sequential three-photon two-color photoassociation scheme is employed to form these molecules in states, which correlate to $np(n+1)s$ dissociation asymptotes. Spectral signatures of bound molecular states are clearly resolved at the positions of avoided crossings between long-range van der Waals potential curves. The experimental results are in agreement with simulations based on a detailed model of the long-range multipole-multipole interactions of Rydberg-atom pair states. We show that a full model is required to accurately predict the occurrence of bound Rydberg macrodimers. The macrodimers are distinguished from repulsive molecular states by their behavior with respect to spontaneous ionization and possible decay channels are discussed.

Figure 1

(Left panel) Potential-energy curves of two Cs Rydberg atoms in the vicinity of the $43p_{3/2}44s_{1/2}$ dissociation asymptote. Black, red, and blue curves correspond to the molecular states with $\mathrm{\Omega }=0$, 1, and 2, respectively. The intensity of the color codifies the value of the $43p_{3/2}44s_{1/2}$ character (gray corresponding to zero, full color to a more than 0.5% overlap with the asymptotic pair state). Bound macrodimer states are labeled by the letters a–d. (Right panel) The states a (lower curve) and b (upper curve) marked in the left panel are shown on a magnified scale. Energies (dashed lines) and probability densities of selected vibrational eigenstates are indicated.

Figure 2

(a) Energy-level diagram for the two-color three-photon photoassociation of ground-state atoms to a doubly excited molecular Rydberg state $\mathrm{\Psi }(R)$. (b) Sequence of laser excitation and detection by PFI. (c) Sketch of the two-step excitation process, where black and red dots represent ground-state and Rydberg atoms, respectively, and the red circle illustrates the extent of the vibrational wave function of the macrodimer.

Figure 3

Ion signals (black dots) as a function of the detuning from the transition $43p_{3/2}\leftarrow 6s_{1/2}$. (a) Spontaneous-ion signal when no seed atoms are present. (b) PFI signal when $44s_{1/2}$ seed Rydberg atoms are present during excitation. (c) Ratio of spontaneous-ion to PFI signals when $44s_{1/2}$ seed Rydberg atoms are present [the background signal shown in (a) was subtracted]. The red line shows a simulated spectrum for a linewidth of 7 MHz full width at half maximum, as discussed in the text. Labels a and b indicate the frequencies of transitions to the minima of the macrodimer potentials marked by a and b in Fig. 1.

Figure 4

Ratio of spontaneous-ion to PFI signals as function of the detuning of the UV laser ($2\mu \mathrm{s}$ pulse length) from the $44p_{3/2}\leftarrow 6s_{1/2}$ transition when $45s_{1/2}$ seed atoms are present. Ions were extracted after a delay of 0 (the gray points) and $12\mu \mathrm{s}$ (the black points). The spectrum simulated for a linewidth of 5 MHz (vertically scaled and offset) is shown as solid red and gray lines for comparison. (Inset) Measured ratios (circles, squares, triangles, and diamonds) as a function of the extraction delay at the fixed laser detunings marked by the corresponding symbol on the frequency axis. A time-dependent background signal (measured at the frequency indicated by an open circle) was subtracted from all of the curves. Solid curves show the fit of a rate-equation model to the data, the light-blue shaded region indicates the UV-excitation pulse.