Optical excitation and stabilization of ultracold field-linked tetratomic molecules

Abstract

We propose a coherent optical population transfer of weakly bound field-linked (FL) tetratomic molecules (tetramers) to deeper FL bound states using stimulated Raman adiabatic passage. We consider static-electric-field shielded polar alkali-metal diatomic molecules and corresponding FL tetramers in their $\textrm{X}^1Σ^+$+$\textrm{X}^1Σ^+$ ground electronic state. We show that the excited metastable $\textrm{X}^1Σ^+$+$\textrm{b}^3Π$ electronic manifold supports FL tetramers in a broader range of electric fields with collisional shielding extended to zero field. We calculate the Franck-Condon factors between the ground and excited FL tetramers and show that they are highly tunable with the electric field. We also predict photoassociation of ground-state shielded molecules to the excited FL states in free-bound optical transitions. We propose proof-of-principle experiments to implement stimulated Raman adiabatic passage and photoassociation using FL tetramers, paving the way for the formation of deeply bound ultracold polyatomic molecules.

Figures (4)

• Figure 1: (a) Schematic showing the envisaged STIRAP using FL tetramers (solid horizontal lines). Molecules with more than one FL state in $X$+$X$ may be used for demonstrating STIRAP via FL states of $X$+$b$. Additionally, the PA of shielded dimers colliding in $X$+$X$ (red dashed line) to FL states in $X$+$b$ may be envisaged. The natural linewidth $\gamma_\textrm{e}$ of the $b$ state determines the lifetime of the FL states in $X$+$b$. A representative vertical green dashed line separates the short (SR) and long (LR) ranges. Panels (b) and (c): Energies $E_\textrm{rot}$ of the rotor pair levels of LiCs for thresholds (b) $X$+$b$ and (c) $X$+$X$ as a function of $F$. Panel (c) is universal for any polar diatomic molecule in state $X$. However, energies in panel (b) depend on the differences in $\mu$ and $b_\textrm{rot}$ for states $X$ and $b$, so they are molecule-dependent.
• Figure 2: Panels (a) and (b): Adiabats for LiCs correlating to partial waves $L=0,2,4,6$ for the initial threshold (a) ($X, 1, 0)$+$(b,2,0)$, and (b) $(X,1,0)$+$(X,1,0)$ at $F=3.57 b_{\textrm{rot},X}/\mu_X$. The adiabats are shown relative to their initial thresholds. The incoming s-wave ($L=0$) channel is shown in red. Insets show expanded views of the long-range potential wells that support FL tetramer states. Panels (c)-(e): Rate coefficients $k$ for elastic scattering (red) and total two-body loss (black) for collisions in $X$+$b$ [solid curves in (c) and (d)] and $X$+$X$ [dashed curves in (e)].
• Figure 3: Upper panels show binding energies $E_n$ of FL tetramer states for (a) NaCs and (b) LiCs in their $X$+$X$ (dashed black) and $X$+$b$ (solid colored) thresholds as functions of $F$. The FCFs are shown in the lower panels. Their dash type and colors correlate to the tetramer states of $X$+$X$ and $X$+$b$, respectively, from the upper panel.
• Figure 4: Photoassociation rates $k_\textrm{PA}$ for colliding dimers in $X$+$X$ with $E_\textrm{coll}=500$ nK$\times k_\textrm{B}$ as functions of the binding energy of $X$+$b$ tetramer for (a) RbCs ($F=2.82$ kV/cm, $\lambda_0=1190$ nm) and (b) LiCs ($F=7.30$ kV/cm, $\lambda_0=1160$ nm) for different laser intensities with $I_0=5$$\mu$W/cm$^2$.