The long-range multipole interactions between ultra-cold Rydberg atoms form adiabatic potentials, one of which shows a binding potential that can be used to bind Rydberg-Rydberg molecules. Rydberg-atom molecule, known as macrodimer due to its larger size (~μm), has the properties of the abundant vibrational energy levels and large electric dipole moment and so on. Compared with Rydberg atom, the Rydberg molecule, including Rydberg-ground molecule and Rydberg-Rydberg molecule, is susceptible to manipulate by an external field and possesses potential applications in the weak-signal detection, the quantum gas correlation measurement and the vacuum fluctuation and so on.
In this paper, we investigate a (60D_5/2)₂ Rydberg macrodimer theoretically and experimentally. In the calculation, we take into account the multipole interaction of a Rydberg-atom pair, including dipole-dipole, dipole-quadrupole, dipole-octupole and quadrupole-quadrupole interaction and so on. The adiabatic potential of 60D_5/2 Rydberg-atom pair is obtained by diagonalizing the interaction Hamiltonian on a grid of internuclear separations, R. The potential depth and binding length of the Rydberg molecular potential well are obtained. In experiment, we prepare the ultra-cold Cs (60D_5/2)₂ Rydberg molecules by a two-color photoassociation method in a cesium ultracold atom trap. The first-color (pulse-A) resonantly excites a seed Rydberg atom A, and the second color (pulse-B) is detuned and resonantly excites the second Rydberg atom B near to the atom A. Both pulse-A and pulse-B are two-photon excitations (852 nm + 510 nm), between which their 852-nm lasers have the same frequency, whereas the 510-nm laser frequency of the pulse-A is set to be resonant with the atomic transition and the frequency of the pulse-B is detuned by using a double-passed acousto-optic modulator. When the pulse-B is detuned to the molecular binding energy, atom-A and-B are bonded, forming an ultra-cold Cs (60D_5/2)₂ Rydberg molecule. The two-color photoassociation spectra of Rydberg-Rydberg molecules are detected by the field ionization of Rydberg atoms and molecules with a ramped electric field. Molecular spectra are compared with calculated adiabatic molecular potentials, which yields the binding energy and equilibrium internuclear distance. The two-color photoassociation method used in this work has a doubly resonant character that results in the enhanced excitation rate.