Photoassociative Spectroscopy at Long Range in Ultracold Strontium

We report photoassociative spectroscopy of ${{}^{88}\mathrm{S}\mathrm{r}}_2$ in a magneto-optical trap operating on the ${}^{1}S_0\to {}^{3}P_1$ intercombination line at 689 nm. Photoassociative transitions are driven with a laser red detuned by 600–2400 MHz from the ${}^{1}S_0\to {}^{1}P_1$ atomic resonance at 461 nm. Photoassociation takes place at extremely large internuclear separation, and the photoassociative spectrum is strongly affected by relativistic retardation. A fit of the transition frequencies determines the ${}^{1}P_1$ atomic lifetime ($\tau =5.22\pm 0.03\mathrm{n}\mathrm{s}$) and resolves a discrepancy between experiment and recent theoretical calculations.

Figure 1

Molecular levels and photoassociative transitions. Resonant excitation occurs near the outer molecular turning point of states of the ${}^1\Sigma _u^{+}$ potential, and it can lead to trap loss through two processes. The molecule can radiatively decay at smaller internuclear separation to two ground state atoms with kinetic energies exceeding the trap depth. This is known as radiative escape (RE). In a state-changing collision (SC), at small internuclear separation the molecular state changes to one corresponding at long range to a lower-lying electronic configuration of free atoms. The atoms exit the collision with greatly increased kinetic energy and escape the trap.

Figure 2

(a) Spectrum for photoassociation to the $v^{\prime }=60$ molecular state. Detuning is with respect to the atomic ${}^{1}S_0-{}^{1}P_1$ resonance. The PAS laser intensity is $1.4\mathrm{m}\mathrm{W}/{\mathrm{c}\mathrm{m}}^2$. The Lorentzian fit yields a FWHM linewidth of $70\pm 7\mathrm{M}\mathrm{H}\mathrm{z}$. (b) Representative spectra for all transitions observed in this study. Lines connect the data points to guide the eye. Conditions vary for the individual scans that make up the full spectrum, so amplitudes and linewidths should not be quantitatively compared. The baseline has been adjusted to match the expected curve for overlapping Lorentzians. (c) Differences between experimental and calculated positions of the 14 measured levels.

Figure 3

Comparison of experimental (exp.) and theoretical (th.) values for the lifetime of the ${}^{1}P_1$ level.

Figure 4

(a) Number of atoms as a function of time for atoms in the intercombination line MOT. The PAS laser is tuned near the $v^{\prime }=59$ molecular resonance. In one data set the PAS laser is on resonance, and in the other it is tuned to the blue of resonance by 42 MHz, and loss due to photoassociation is small. (b) $\beta$ as a function of PAS laser intensity for several molecular resonances. Error bars denote statistical uncertainties.