Helium-Droplet-Assisted Preparation of Cold RbSr Molecules

We present a combined experimental and theoretical study of the RbSr molecule. The experimental approach is based on the formation of RbSr molecules on helium nanodroplets. Utilizing two-photon ionization spectroscopy, an excitation spectrum ranging from 11 600 up to $23000{\mathrm{cm}}^{-1}$ was recorded. High level ab initio calculations of potential energy curves and transition dipole moments accompany the experiment and facilitate an assignment of transitions. We show that RbSr molecules desorb from the helium droplets upon excitation, which enables dispersed fluorescence spectroscopy of free RbSr. These spectra elucidate $X{}^2{\mathrm{\Sigma }}^{+}$ ground and excited state properties. Emission spectra originating from states corresponding to the $\mathrm{Rb}(5s{}^{2}S)+\mathrm{Sr}(5s5p{}^{3}P)$ asymptote were recorded; spin-orbit coupling was included for the simulation. The results should provide a good basis for achieving the formation of this molecule in cold collisions, thus offering intriguing prospects for ultracold molecular physics.

Figure 1

Selection of calculated doublet state potential energy curves of the RbSr molecule obtained with a MRCI approach and the CASSCF method. The inset shows relativistic potential energy curves relevant for DF spectroscopy, calculated by applying a Breit-Pauli Hamiltonian to MRCI wave functions.

Figure 2

Excitation spectrum of RbSr molecules on helium nanodroplets recorded by R2PI spectroscopy. The stick diagram represents the calculated Frank-Condon factors ($\mathrm{FCF}*{\mathrm{TDM}}^2$) from the MRCI potential energy curves, transitions into ${}^2{\mathrm{\Sigma }}^{+}$ and ${}^2\mathrm{\Pi }$ states are shown as red and blue sticks, respectively. Note that beyond $16000{\mathrm{cm}}^{-1}$ calculations become increasingly challenging because of the proximity of atomic asymptotes and the difference between the stick spectra and experimental spectra increases, e.g., the shift (calc-measured) of $4{}^2\mathrm{\Pi }\leftarrow X{}^2{\mathrm{\Sigma }}^{+}$ amounts to $\sim 1250{\mathrm{cm}}^{-1}$.

Figure 3

Dispersed fluorescence spectrum of RbSr formed on helium nanodroplets upon excitation of the $4{}^2{\mathrm{\Sigma }}^{+}({\nu }^{\prime }=0)\leftarrow X{}^2{\mathrm{\Sigma }}^{+}({\nu }^{\prime \prime }=0)$ transition. The right panel shows the region in the vicinity of the excitation laser, the (${\nu }^{\prime }\text{−}{\nu }^{\prime \prime }$) 0-0 and 0-1 transitions are marked. The left panel shows fluorescence light originating from lower states, which are populated during the relaxation of the molecule. The simulated spectrum is shown as a blue line above the experimental spectrum (red).