Direct photoassociative formation of ultracold KRb molecules in the lowest vibrational levels of the electronic ground state

We report continuous direct photoassociative formation of ultracold KRb molecules in the lowest vibrational levels $(v^{\prime \prime }=0-10)$ of the electronic ground state $\left(X{}^1{\Sigma }^{+}\right)$, starting from ${}^{39}$K and ${}^{85}$Rb atoms in a magneto-optical trap. The process exploits a new-found resonant coupling between the $2\left(1\right),v^{\prime }=165$ and $4\left(1\right),v^{\prime }=61$ levels, which exhibit an almost equal admixture of the uncoupled eigenstates. The production rate of the $X^1{\Sigma }^{+}$ $(v^{\prime \prime }=0)$ level is estimated to be $5\times {10}^3$ molecules/s.

Figure 1

Schematic for formation and detection of ground-state KRb. The PA laser is detuned below the K$\left(4s\right)$ + Rb$\left(5p_{1/2}\right)$ asymptote. The intermediate states used for detection of the lowest vibrational levels of the ground-state KRb molecules are $2{}^3{\Sigma }^{+}$, $3{}^1{\Sigma }^{+}$, and $1{}^1\Pi$. At long range, the $1{}^1\Pi$ state correlates to the 4(1) state and dissociates to the K$\left(4s\right)$ + Rb$\left(5p_{3/2}\right)$ asymptote while the $2{}^3{\Sigma }^{+}$ state diabatically correlates to the 2(1) and $2\left(0^{-}\right)$ states and dissociates to the K$\left(4s\right)$ + Rb$\left(5p_{1/2}\right)$ asymptote.

Figure 2

Photoassociation spectra of KRb with the detection laser tuned to transitions of (a) $X{}^1{\Sigma }^{+}(v^{\prime \prime }=84)$ to $4{}^1{\Sigma }^{+}(v^{\prime }=47)$ at 16 635.8 cm${}^{-1}$ and (b) $X{}^1{\Sigma }^{+}(v^{\prime \prime }=0)$ to $2{}^3{\Sigma }^{+}(v^{\prime }=38)$ at 15 116.14 cm${}^{-1}$, showing the $2\left(1\right),v^{\prime }=165$ and the $4\left(1\right),v^{\prime }=61$ states. The lines marked by asterisks are $``$hyperfine ghost” artifacts arising due to a small population in the bright hyperfine states ($F=2$ for ${}^{39}$K and $F=3$ for ${}^{85}$Rb) of ${}^{39}$K (4S) and ${}^{85}$Rb (5S) in our dark spot MOT. Hyperfine ghosts are present in spectrum (b) also, but are not labeled to avoid congestion.

Figure 3

Plot of $E_J$ vs $J(J+1)$, where J is the rotational quantum number and $E_J=J(J+1)B_v$. $B_v$ is the rotational constant. The theoretical values of $B_v$ are calculated using level [21] with single-channel potentials calculated by Rousseau et al. [18].

Figure 4

Comparison of hyperfine structure of the 2(1), $v^{\prime }=165$ (black, thicker width) and 4(1), $v^{\prime }=61$ (red, narrower width) states for $J^{\prime }=1,2,$ and 3. The detection laser frequency for all the PA scans is 16 635.8 cm${}^{-1}$. Also, as the scan speeds and laser powers were not closely matched, we do not consider the differences in the linewidths to be significant.

Figure 5

Near-identical REMPI spectra of KRb with the PA laser at (a) 12 547.6 cm${}^{-1}$ and (b) 12 547.9 cm${}^{-1}$. The line marked by an asterisk represents the detection frequency of 16 635.8 cm${}^{-1}$ used for the PA scans in Figs. 2a and 4. Note that the intensity scales in (a) and (b) are the same.

Figure 6

REMPI detection spectra of KRb molecules, at lower laser frequencies. The vertical lines indicate assignments from $X{}^1{\Sigma }^{+},v^{\prime \prime }=0$ to several levels of the intermediate states $1{}^1\Pi ,2{}^3{\Sigma }^{+}$, and $3{}^1{\Sigma }^{+}$. The black circles ($•$) indicate ${}^{85}$Rb two photon transitions that leak into the molecular channel. The PA laser was set to the 4(1), $v^{\prime }=61,J^{\prime }=1$ level for this scan.

Figure 7

An expanded view of a selected region of the detection spectrum (Fig. 6) of KRb molecules. The red vertical lines (bottom) indicate ground-state molecules detected via the $2{}^3{\Sigma }^{+}$ state and the blue vertical lines (top) indicate those detected via the $3{}^1{\Sigma }^{+}$ state. The $v^{\prime }$ corresponds to the intermediate state ($2{}^3{\Sigma }^{+}$ or $3{}^1{\Sigma }^{+}$ in the present case) vibrational levels and $v^{\prime \prime }$ corresponds to the ground $X{}^1{\Sigma }^{+}$ state. The green vertical lines (middle) correspond to the detection of molecules in $a{}^3{\Sigma }^{+}$ via the $3{}^3{\Sigma }^{+}$, $v^{\prime }=11$ state. In the region of the spectrum shown in this figure nine lines (above the 4.5 vertical threshold) are assigned, while three are still unassigned.