Figure 1

The photoassociation efficiency is determined by the free-bound Franck-Condon overlap between the scattering states and weakly bound excited-state levels (purple line), shown here for photoassociation of ${}^{88}$Sr${}_2$ molecules near an intercombination line transition [11, 12, 19]. For $s$ waves (gray line, scaled by a factor of ${10}^6$ compared to the blue line) the probability density at short internuclear distances is very small, leading to low photoassociation efficiency. A shape resonance (blue line corresponding to a field-free rotational quantum number $l=8$) is a quasibound state trapped behind the rotational barrier. Typically, its energy $E_{\text{SR}}$ is too high for the shape resonance to carry any significant thermal weight in an ultracold cloud. Here we suggest that a strong nonresonant field (red arrow) be employed, which leads to rotational hybridization of the scattering states, effectively moving the position of the shape resonance to lower collision energies $E_{\text{coll}}$ and increasing the thermal weight of the shape resonance.Reuse & Permissions

Figure 2

Photoassociation lines (absorption rate coefficient) for photoassociation into ${\tilde{v}}^{\prime }=-6$ for weak (top) and strong (bottom) nonresonant fields $I$. The intensity of the photoassociation laser is set to $I_{\text{PA}}=1$W/cm${}^2$ and the trap temperature to $T=1\mu$K.Reuse & Permissions

Figure 3

Energy of the $\tilde{l}=4$, 8, 12, and 16 shape resonances vs nonresonant-field intensity.Reuse & Permissions

Figure 4

Contribution of different partial waves $l$ to the resonance wave function for the $\tilde{l}=4,8\text{,}\text{and}12$ shape resonances as a function of the nonresonant intensity. The low-$l$ contributions are shown in the three bottom panels and the high-$l$ contributions are shown in the two top panels. For high nonresonant-field intensities, all resonance wave functions acquire predominantly $s$-wave character.Reuse & Permissions

Figure 5

The ${\tilde{l}}^{\prime }=1$ peak in the presence of the field normalized with respect to the largest field-free peak (with $l^{\prime }=1$) in the rate coefficients for photoassociation into ${\tilde{v}}^{\prime }=-6$ ($T=1\mu$K). Enhancement of the photoassociation rate of about three orders of magnitude is found when the position of a shape resonance comes close to the trap temperature.Reuse & Permissions

Figure 6

(a) Energy levels and (b) projections of the hybridized ${\tilde{v}}^{\prime \prime }=-3,{\tilde{l}}^{\prime \prime }=0$ (red) and ${\tilde{v}}^{\prime \prime }=-4,{\tilde{l}}^{\prime \prime }=0$ (blue) ground-state levels for (a) adiabatic or (b) sudden transfer into their field-free counterparts. For a slow turnoff, nonadiabatic transitions are not expected and all molecules are obtained with $l^{\prime \prime }=0$ (a), while for a sudden turnoff, about 20$\%$ of the molecules have $l^{\prime \prime }=0$ and about 50$\%$ have $l^{\prime \prime }=2$.Reuse & Permissions