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AIP conference proceedings

Control of Atomic and Molecular Processes by Designed External Fields: From Adiabatic to Ultrashort Pulse Strategies

S. Guérin, Theodore E. Simos, George Maroulis

Published Jan 1, 2007• 0 citations
Publisher

Abstract

I present tools and strategies for the control of atomic and molecular processes by external fields, such as laser and cavity fields, with parameters specifically designed to achieve particular goals. Applications to selective population transfer, control of atomic deflection, tunneling effect, alignment of molecules, and quantum information processing are addressed.

DOI
10.1063/1.2836205

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I present tools and strategies for the control of atomic and molecular processes by external fields, such as laser and cavity fields, with parameters specifically designed to achieve particular goals. Applications to selective population transfer, control of atomic deflection, tunneling effect, alignment of molecules, and quantum information processing are addressed.
In the past two decades many schemes to actively control the dynamical evolution of a quantum system have been proposed and established. We focus attention on the use of assisted adiabatic processes to generate selected target states with high efficiency. Specific control protocols that fall in this category are sometimes called shortcuts to adiabaticity. We review two assisted adiabatic population transfer schemes, the counter-diabatic and fast-forward protocols, with the intent of calling attention to their conceptual basis and applications to real systems, specifically vibrational energy transfer in polyatomic molecules and manipulation of Bose-Einstein (BEC) condensates. The particular applications of the counter-diabatic and fast-forward protocols to improving the efficiency of population transfer between vibrational states of a polyatomic molecule that we have studied are (i) selective vibrational energy transfer in the nonrotating thiophosgene (SCCl2) molecule and (ii) enhancing the yield of HNC in the driven rotation free isomerization reaction HCN→CNH. A major component of our considerations concerns reduction of the degradation of population transfer efficiency within a subset of states that is generated by coupling of the subset of states to a manifold of background states. We have examined the influence of a manifold of background vibrational states on population transfer within a subset of embedded vibrational states, and on how that influence can be suppressed. We emphasize that any control protocol must be designed to minimize interference between the background states and excitations within the selected subset of states that are used to define initial and target states. We have also examined the application of the fast-forward protocol to the transport of population between sites of a BEC in an optical lattice. Through the applications we illustrate how the theoretical framework of an assisted adiabatic process depends on the character of the system studied, on the available control parameters, on the constraints imposed by the experimental setting, on the efficiency of population transfer (fidelity) to be achieved, and on the stability of the control process with respect to the variation of the external control fields.
The main goal of this PhD work is an experimental study of coherent excitation of atomic and molecular wavepackets, i.e. superpositions of many quantum eigenstates, by shaped femtosecond pulses. Approaches allowing nearly complete population transfer between quantum eigenstates were well studied in the past within the two level approximation. In this work we focus on adiabatic and non-adiabatic methods of population transfer beyond the two-level approximation. Excitation of multi-level target states is possible due to broad spectrum of an ultrashort pulse which contains frequencies needed for multiple transitions to different states in the final superposition. At the same time, the spectrum of an ultrashort pulse can be modified, or ``shaped'', in order to affect the excitation process and control the amplitudes in the final superposition. Both non-adiabatic and quasi-adiabatic methods were first implemented and studied in electronic wavepackets in alkali atoms. The non-adiabatic approach revealed features linked to the strong-field perturbations of the energy level structure of the quantum system. An adiabatic method was implemented for the first time on a femtosecond time scale, and was thoroughly characterized. The control over complex amplitudes in the target superposition was demonstrated as well as completeness of the population transfer. In the second part of this work, we focused on coherent control of rotational wavepackets in diatomic molecules. Rotational excitation by a periodic train of femtosecond pulses was investigated in the context of ``delta-kicked'' rotor - a paradigm system for studying quantum chaos, and the effect of quantum resonance was demonstrated for the first time in a system of true quantum rotors. Control of uni-directional molecular rotation was proposed and demonstrated with a novel ``chiral pulse train'' - a sequence of femtosecond pulses with polarization rotating from pulse to pulse by a predefined angle. All the developed techniques offer new tools in coherent control of atomic and molecular wavepackets on an ultrashort time scale.
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.
Efficient excitation of an ensemble of atoms and molecules to a desired state is of fundamental importance not only for spectroscopy but also for quantum control of molecular dynamics in chemical and biological processes. The latter usually occurs on a much shorter time scale (in the femtosecond range) giving rise to steadily advanced problem how to excite one of the nearby levels, even when they are well within the broad fs pulse spectrum. We propose a new effective method for selective excitation of atoms based on the atomic coherence and discuss a sensitive method to verify experimentally the proposed mechanism using the technique of quantum beating in pump-probe experiments.