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Bulletin of the American Physical Society

Laser assisted charge transfer in the realm of cold collisions

Alexander Petrov, Constantinos Makrides, Svetlana Kotochigova

Published Jun 9, 2015• 0 citations
OpenAlex

Abstract

This reaction 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. The mechanism of elec- tromagnetic field control is based on an interplay between intra-molecular couplings and molecule-field interactions. We show that laser field affects 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 non-adiabatic charge transfer. We investigate these processes for various collision energies as well as over a wide range of laser intensities and frequencies.

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This reaction 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. The mechanism of elec- tromagnetic field control is based on an interplay between intra-molecular couplings and molecule-field interactions. We show that laser field affects 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 non-adiabatic charge transfer. We investigate these processes for various collision energies as well as over a wide range of laser intensities and frequencies.
The presence of coherent radiation fields can modify various atomic, ionic, and molecular processes. We examine cases where the changes can be effected without actual absorption or emission of photons. The radiation fields cause optical Stark shifts of the energies and dress the nonadiabatic couplings between states. Rates of processes such as dissociation, predissociation, atomic and molecular collisions can be radiatively controlled. Formation of new avoided crossings due to radiative interaction gives rise to new phenomena. We give both exact numerical results from which the influences of the radiation field on arbitrary configurations can be calculated, and simple but accurate analytic results, from which these effects can be conveniently assessed. It is shown that the inelastic transition probability is decreased at a true crossing and is increased at an avoided crossing as a result of the presence of the nonresonant radiation field. Furthermore, for two parallel levels with constant nonadiabatic coupling, it is shown that the amplitudes of the nonadiabatic transition are invariant, but its flopping frequency is reduced by the field. Depending on final measurements with the field on or off, general behaviors of the inelastic transition probability as a function of the field parameters and the charge-system parameters are predicted and the possibility of inversion (from a field-free value of less than \textonehalf{} to a value greater than \textonehalf{}) is demonstrated. Experimental investigation with iodine molecules is suggested. Comparison of our nonperturbative results with those of stationary perturbation theory shows that the latter is inadequate in an important parameter region where the field modification of processes is greater than 1%.
We explore the effects of microwave laser fields on spin-changing transitions in collisions of polar molecules in a $^{2}\ensuremath{\Sigma}$ electronic state with atoms in the presence of a static magnetic field. We use the dressed-state formalism of quantum optics to describe the interaction of the radiation field with the collision complex and perform rigorous scattering calculations to study spin-changing transitions in collisions of CaH molecules with He atoms at temperatures below 1 K. Our calculations demonstrate that collision-induced spin relaxation of $^{2}\ensuremath{\Sigma}$ molecules is enhanced near avoided crossings between different Zeeman levels. We show that the positions of the avoided crossings can be modified by varying the frequency and the intensity of the microwave laser field and discuss the mechanisms of microwave field control of spin-dependent interactions in cold open-shell molecules.
The authors present a detailed study of low-temperature collisions between CaD molecules and He atoms in superimposed electric and magnetic fields with arbitrary orientations. Electric fields do not interact with the electron spin of the molecules directly but modify their rotational structure and, consequently, the spin-rotation interactions. The authors examine molecular Stark and Zeeman energy levels as functions of the angle between the fields and show that rotating fields may induce and shift avoided crossings between the Zeeman levels of the rotationally ground and rotationally excited states of the molecule. The dynamics of molecular collisions are extremely sensitive to external fields near these avoided crossings and it is shown that molecular collisions may be controlled by varying both the strength and the relative orientation of the fields. The effects observed in this study are due to interactions of the isolated molecules with external fields so the conclusions should be relevant for collisions of molecules with other atoms or collisions of molecules with each other. This study demonstrates that electric fields may be used to enhance or suppress spin-rotation interactions in molecules. The spin-rotation interactions induce nonadiabatic couplings between states of different total spins in systems of two open-shell species and it is suggested that electric fields might be used for controlling nonadiabatic spin transitions and spin-forbidden chemical reactions of cold molecules in a magnetic trap.
We present a theoretical formalism to treat the ultracold dynamics of a pair of colliding polar molecules submitted to two laser fields. We express the dressed Hamiltonian including the dipole-dipole interaction of the colliding molecular pair, both in their ground and electronic excited states, as well as their interaction with the two laser fields. We apply adiabatic elimination of the electronic excited state to reduce the size of the dressed-state basis in which the dressed Hamiltonian is expressed. In an application, we investigate the feasibility of two-photon collisional shielding between two 23Na39K^{23}\mathrm{Na}^{39}\mathrm{K}23Na39K molecules, which could be favored by the Raman resonance condition suppressing unwanted spontaneous emission and photon scattering. We demonstrate the influence of laser Rabi frequencies on the dynamics through the computation of elastic, inelastic, and reactive collision rates, which is still insufficient to ensure noticeable shielding.
Merging two subfields, collisions and spectroscopy, this book presents the latest research proposals, innovative experiments and fresh ideas. Topics considered include radiative collision induced electron continuum-continuum scattering; models of laser-induced chemical reactions; experimental studies of laser-induced ionizing collisions; laser photodissociation of Na/sub 2/ + ions produced by laser resonant ionization of Na vapor; steady-state electron-atom collision in a laser field; conditions for the observation of the predicted splitting of the laser assisted electron impact of the differential cross section of atoms; transitions between resonances; some proposals for experiments; above-threshold multiphoton ionization; observability of optical collisions in the frequency and the time domain; laser switched collisions - a diagrammatical approach; polarization of atomic fluorescence excited by molecular photodissociation; multiphoton selective excitation; on the role of chaotic dynamics with many basins of attraction; predissociation in a strong electromagnetic field; theory of double resonance; and coherent excitation of polyatomic molecules up to highly excited vibrational states by laser light.
We investigate QED field-dependent effects in radiative shifts of atomic levels driven by a strong laser field. Our focus is on indirect laser-induced corrections to the Lamb shift of an atomic reference state due to a strong laser field which near-resonantly couples two other states, but not the reference state. Our aim is to show how to generalize previous treatments of related scenarios to the case of an induced indirect coupling. We also derive an expression of self-energy corrections in the Floquet picture and analyse various coupling schemes with respect to the shift of Floquet quasi-energies. The semi-classical Floquet picture and the fully quantized dressed-state approach are contrasted.