Ultracold $${ }^{88}\mathrm {Sr}_2$$ Molecules in the Absolute Ground State

Jun Rui

Patrick Zabawa

Nathan Gilfoy, Stephan Falke, Colin Bruzewicz +2 more

Timur V. Tscherbul, Yu. V. Suleimanov, Vincenz̊o Aquilanti +1 more

We present an accurate quantum mechanical study of molecule–molecule collisions in the presence of a magnetic field. The work focuses on the analysis of elastic scattering and spin relaxation in collisions of O 2 ( 3 Σ g – ) molecules at cold (∼0.1 K) and ultracold (∼10 –6 K) temperatures. Our calculations show that magnetic spin relaxation in molecule–molecule collisions is extremely efficient except at magnetic fields below 1 mT. The rate constant for spin relaxation at T =0.1 K and a magnetic field of 0.1 T is found to be as large as 6.1 × 10 –11 cm –3 s –1 . The magnetic field dependence of elastic and inelastic scattering cross sections at ultracold temperatures is dominated by a manifold of Feshbach resonances with the density of ∼100 resonances per Tesla for collisions of molecules in the absolute ground state. This suggests that the scattering length of ultracold molecules in the absolute ground state can be effectively tuned in a very wide range of magnetic fields. Our calculations demonstrate that the number and properties of the magnetic Feshbach resonances are dramatically different for molecules in the absolute ground and excited spin states. The density of Feshbach resonances for molecule–molecule scattering in the low-field-seeking Zeeman state is reduced by a factor of 10.

Kon H. Leung

Jayita Banerjee

Ultracold heteronuclear polar KRb molecules have enjoyed tremendous attention for many years now. The presence of a large molecule fixed dipole moment in the absolute rovibrational ground state (v''=0, J'' =0) makes external control very convenient, which opens up the prospect of studying various physical phenomenon in many areas of physics and chemistry. The methods used so far to produce the absolute ground state of KRb have involved very complicated techniques. Molecules formed in weakly bound levels were transferred to the absolute ground state through the multiple steps of transfer processes involving several lasers with extremely narrow linewidths. My thesis research demonstrates an alternate simpler method of formation of the lowest vibrational levels of the ground state utilizing PA combined with radiative decay, which has been so far useful to form ultracold molecules in the higher vibrational levels of the ground state. This thesis presents two main topics of research. The first part of the thesis consists of results describing the scheme which provides a single-step, continuous, and irreversible method of forming KRb molecules in their absolute ground state. The second topic involves spectroscopy on the two excited electronic states of KRb which may provide alternative pathways for the formation of the absolute ground state (X 1Σ+, (v''=0, J'' =0)) and suitable pathways for efficient formation of the lowest metastable state (a 3Σ+, (v''=0, N'' =0)). I believe this research will lay the foundation for many new exciting experiments in the future.