Photoassociation Spectroscopy of Laser-Cooled Ytterbium Atoms

We report the photoassociation spectroscopy of laser-cooled ytterbium atoms in an optical trap. We observed more than 90 photoassociation resonances of vibrational levels in the ${}^1\Sigma _u^{+}$ state, including 80 consecutive series, up to 490 GHz detuning with respect to the atomic resonance. From the resonance frequencies we derived the atomic radiative lifetime of the $(6s6p){}^{1}P_1$ state to be $5.464\pm 0.005\mathrm{n}\mathrm{s}$, which is about 2 orders of magnitude improvement over previous results. We also observed line broadening of resonances, which is ascribed to the predissociation to the triplet states, and estimated the transition probability to be 0.2. Furthermore, we observed the decrease of the photoassociation signal intensity, from which the scattering length is estimated to be equal to or less than 3 nm.

Figure 1

Pair potential of ${\mathrm{Y}\mathrm{b}}_2$. The PA light of about $399\mathrm{n}\mathrm{m}$ excites the two free ${}^{1}S_0$ atoms to the molecular excited state ${}^1\Sigma _u^{+}$ which connects asymptotically to the ${}^{1}S_0+{}^{1}P_1$ atomic state in the dissociation limit. The long-range potential of the ${}^1\Sigma _u^{+}$ state is determined by the resonant dipole-dipole interaction $-C_3/R^3$.

Figure 2

Schematic experimental setup. The dense and cold Yb atoms were trapped in the FORT after the MOT with the intercombination transition. The light for the PA that was generated by the frequency-doubled Ti:sapphire laser was applied to the atoms in the FORT. The number of the atoms remaining after the irradiation of the PA beam was measured by the absorption imaging method with the charge-coupled device (CCD).

Figure 3

PA signal of ${}^{174}\mathrm{Y}\mathrm{b}$ atoms. The number of the atoms in the FORT indicated by crosses was plotted against the detuning from the ${}^{1}S_0+{}^{1}P_1$ asymptote. Solid line is the Lorentzian fit. This resonance was assigned to the vibrational quantum number of $v_D^{\prime }-v=177$. The PA light intensity was $30\mathrm{m}\mathrm{W}$.

Figure 4

Resonance frequencies of the consecutive series as a function of the assigned vibrational quantum numbers $v_D^{\prime }-v$ in the range of 103 to 174, in which the line broadening is small enough to determine the PA frequency of the resonances. Both axes are in log scale. The observed detuning with respect to the ${}^{1}S_0+{}^{1}P_1$ asymptote indicated by closed circles was plotted against the assigned vibrational quantum number $v_D^{\prime }-v$. Solid line is the fit with the semiclassical formula of Eq. (2) in the text.