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Physical Review A

Dynamics of an ultracold weakly bound heteronuclear diatomic molecule in an ultrashort laser pulse

Ziang Li, Xuhui Bai, Yide Yin, Gao‐Ren Wang, Shu‐Lin Cong

Published Mar 18, 2024• 0 citations
Publisher

Abstract

We propose a theoretical model used for dynamically manipulating an ultracold weakly bound heteronuclear alkali-metal diatomic molecule by a nonresonant single-cycle THz laser pulse. Taking the $^{6}\mathrm{Li}^{23}\mathrm{Na}$ molecule as an example, we first calculate the variation of the weakly bound state and quasibound state as a function of external electric field, and then reveal abundant interference patterns during and after the laser pulse due to the superpositions between states involving weakly bound state, scattering state, and quasibound state. Moreover, one can map out the nodes of the probability density of the weakly bound molecules by measuring the wavepacket dynamics.

DOI
10.1103/physreva.109.033311

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We carry out the first quantum dynamics calculations on ultracold atom-diatom collisions in isotopic mixtures. The systems studied are spin-polarized 7Li+6Li7Li^{7}\mathrm{Li}+^{6}\mathrm{Li}^{7}\mathrm{Li}7Li+6Li7Li, 7Li+6Li2^{7}\mathrm{Li}+^{6}\mathrm{Li}_{2}7Li+6Li2​, 6Li+6Li7Li^{6}\mathrm{Li}+^{6}\mathrm{Li}^{7}\mathrm{Li}6Li+6Li7Li, and 6Li+7Li2^{6}\mathrm{Li}+^{7}\mathrm{Li}_{2}6Li+7Li2​. Reactive scattering can occur for the first two systems even when the molecules are in their ground rovibrational states, but is slower than vibrational relaxation in homonuclear systems. Implications for sympathetic cooling of heteronuclear molecules are discussed.
In this article we address the general approach for calculating dynamical dipole polarizabilities of small quantum systems, based on a sum-over-states formula involving in principle the entire energy spectrum of the system. We complement this method by a few-parameter model involving a limited number of effective transitions, allowing for a compact and accurate representation of both the isotropic and anisotropic components of the polarizability. We apply the method to the series of ten heteronuclear molecules composed of two of (Li,Na,K,Rb,Cs) alkali-metal atoms. We rely on both up-to-date spectroscopically-determined potential energy curves for the lowest electronic states, and on our systematic studies of these systems performed during the last decade for higher excited states and for permanent and transition dipole moments. Such a compilation is timely for the continuously growing researches on ultracold polar molecules. Indeed the knowledge of the dynamic dipole polarizabilities is crucial to model the optical response of molecules when trapped in optical lattices, and to determine optimal lattice frequencies ensuring optimal transfer to the absolute ground state of initially weakly-bound molecules. When they exist, we determine the so-called ‘magic frequencies’ where the ac-Stark shift and thus the viewed trap depth, is the same for both weakly-bound and ground-state molecules.
Recent advances in the ultrashort laser technology have spawned a series of new directions of research in atomic and molecular physics. This work compounds two studies on the control and probing of atomic matter by ultrafast laser pulses. The first study is concerned with the control of quantum wavepackets focusing by chirped, ultrashort laser pulses. The ability to control wavepacket focusing is important for the achievement of specific excitation pathways, for controlling chemical reactivity or even molecular synthesis. In this work, we present a general method for wavepacket control in the weak field limit. Numerical simulations involving the ICN molecule confirm our theoretical framework. The second study refers to the ionization of sodium Rydberg states via half-cycle pulses (HCPs). For this process, our full quantum-mechanical calculations demonstrate an asymmetry in both ionization rates and ionization spectrum between the downhill (Stark red-shifted) and uphill (Stark blue- shifted) states. The asymmetry is explained by appealing to a semi-classical model of the interfering ionization pathways.
Photoassociation is a possible route for the formation of chemical bonds. In this process, the binding of colliding atoms can be induced by means of a laser field. Photoassociation has been studied in the ultracold regime and also with temperatures well above millikelvins in the thermal energy domain, which is a situation commonly encountered in the laboratory. A photoassociation mechanism can be envisioned based on the use of infrared pulses to drive a transition from free colliding atoms on the electronic ground state to form a molecule directly on that state. This work takes a step in this direction, investigating the laser-pulse-driven formation of heteronuclear diatomic molecules in a thermal gas of atoms including rotational effects. Based on the assumption of full system controllability, the maximum possible photoassociation yield is deduced. The photoassociation probability is calculated as a function of the laser parameters for different temperatures. Additionally, the photoassociation yield induced by subpicosecond pulses of a priori fixed shape is compared to the maximum possible yield.