Photoassociation and optical stabilization of ultracold field-linked tetratomic molecules

Trapped samples of ultracold molecules provide powerful platforms for quantum information processing, precision measurements, studies in cold-controlled chemistry, and simulation of quantum many-body systems. However, they are often short-lived because close collisions between them result in trap loss. To mitigate such loss, shielding methods have been extensively developed and have recently been successfully implemented. Shielding is achieved by external static electric fields or near-resonant microwaves. The external field responsible for shielding also allows creation of weakly bound ultracold tetratomic molecules (“tetramers”). Recently, such tetramers have been realized from pairs of ultracold alkali-metal diatoms using an external microwave field. These tetramers are termed field-linked (FL) molecules as an external field is necessary to create them. The motivation of this work is to develop a methodology to create deeper bound tetramers starting from the loosely bound FL tetramers. Our methodology is inspired by photoassociation of ultracold atoms to produce molecules in the excited electronic states and stimulated Raman adiabatic passage (STIRAP) transfer of weakly bound diatoms to deeply bound molecules in the ground vibronic state via an excited intermediate state. We envisage similar routes of creating deeply bound tetramers starting from weakly bound states or a pair of colliding diatoms. We consider static-electric field shielded alkali diatomic molecules initially in their ground electronic X¹Σ⁺+X¹Σ⁺ pair state. We identify the excited electronic manifold X¹Σ⁺+b³Π₀ for photoassociation and an intermediate state for STIRAP transfer to deeply bound states in the X+X manifold. For this, we develop shielding methods for X+b and predict Frank-Condon factors (FCFs) between FL states of X+b and X+X. We also predict photoassociation spectra for shielded molecules to form FL tetramers in X+b manifold. We obtain favourable FCFs between ground and excited tetramer states and promising photoassociation spectra. Our theoretical results should guide future experiments for stabilizing weakly bound ultracold tetramers.