Chemical reactivity of hybrid atom-ion systems

The dynamics of low-temperature chemical reactions  between atoms and ions has recently been a subject  of  many experimental studies. These hybrid systems of atoms and ions were controlled by tuning temperature and density and by  careful state preparation.  Our theoretical study of  the chemical reaction between ultracold Ca atoms and Yb+ ions as well Ba atoms and Li+ ions held in a hybrid trap are focused, first, on the ab initio calculation of the basic electronic properties of the ground and excited molecular potentials  relevant  to the charge-exchange process as well as determined reaction rates with and without external fields.  Furthermore, we explore the effect of a strong electromagnetic field on the charge-exchange reaction between atoms and ions in the realm of cold collisions.

 

We have shown that the charge-exchange reactions can be controlled by linearly-polarized laser radiation of frequency, which is in the range of quasi-molecular electronic energy separation. Using the dressed-state picture or the Floquet Ansatz we construct coupled time-independent Schrodinger equations for the interatomic separation R.

Figure 1 - Schematic of charge exchange  between an atom and an ion.

The mechanism of electromagnetic  field control is based on an interplay between intra-molecular couplings and molecule-field

interactions. We show that laser fields affect the chemical reaction through reversible modification of an effective Hamiltonian via  either non-resonant temporal Stark shifts or resonant ``dipolar'' interactions, leading to both transient- and cw-light-induced nonadia- batic charge transfer. We investigate these processes for various collision energies as well as over a wide range of laser intensities and frequencies.

 

To explore a role of the excited states in the charge-exchange reaction we performed extensive calculation the electronic structure of the BaCa+ molecule,using a relativistic multireference restricted active space configuration-interaction (RMR-RAS-CI) method. Spin-
orbit effects are large in BaCa+ and a relativistic calculation is required. A nonorthogonal basis set is constructed from numerical Dirac-Fock atomic orbitals as well as relativistic Sturmian functions. A symmetric reexpansion of atomic orbitals from one atomic centerto another simplifies the calculation of many-center integrals. At large interatomic sepa-rations the molecular wave function has a pure atomic form that appropriately describesthe molecular dissociation limit. We have tentitatively indicated which potentials correspond toentrance and exit channels in the reactions that occur in UCLA experiment (Phys. Rev. Lett. 109, 223002 (2012)).

 

Sympathetic cooling of molecular ions by collisions with laser-cooled atoms was predicted by Eric Hudson to be efficient at cooling the internal molecular degrees of freedom (Phys. Rev. A 79, 032716 (2009)). Although the prediction relied on proven technolo- gies, it was not previously implemented, possibly owing to the misconception that molecular ions predominantly undergo charge-exchange reactions leading to energetic, neutral molecules. Our group was involved in the first realization of sympathetic cooling of trapped BaCl+ in collisions with Ca atoms co-located in a magneto-optical trap (Nature 495, 490 (2013)). The experiment was per- formed in the group of Dr. E. Hudson at UCLA. Evaluation of how well molecular ions are cooled was crucial. We relied on state-sensitive photodissociative detection of BaCl+ to ionic Ba+ and neutral Cl atoms. This method is applicable to any molecule that can be photodissociated as long as the internal state can be probed within the vibrational relaxation time. The method exploits the fact that, although the photodissociation cross-section is broad, the individual vibrational levels have unique frequency responses for photodissociation. It is, however, necessary to measure the photodissociation cross-section with high precision. We supplied accurate electronic potentials and dipole moments for BaCl+ and created the quantum-mechanical model of the photodissociation cross sections. These simulations determined the cross section of individual vibrational states. Assuming a Maxwell-Boltzmann distribution for the vibrational populations, a fit to the experimental signal gave the internal temperature.

 

Our recent theoretical study of sympathetic cooling through collisional interaction with laser-cooled atoms has established that in cont- rast to the commonly held opinion, there exists a large class of systems that exhibit efficient vibrational cooling and therefore supports a new route to realize the long-sought opportunities offered by molecular structure (to be published in Nature Comm. (2016)).
 

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