Separation of enantiomers by ultraviolet laser pulses in H2POSH: π pulses versus adiabatic transitions

Effective discrimination of chiral molecules in a cavity

We present a scheme to realize precise discrimination of chiral molecules in a cavity. Assisted by additional laser pulses, cavity fields can evolve into different coherence states with contrary-sign displacements according to the handedness of molecules. Consequently, the handedness of molecules can be read out with homodyne measurement on the cavity, and the successful probability is nearly unity without very strong cavity fields. Numerical results show that the scheme is insensitive to errors, noise, and decoherence. Therefore, the scheme may provide helpful perspectives for accurate discrimination of chiral molecules.

Highly Efficient Detection and Separation of Chiral Molecules through Shortcuts to Adiabaticity

A highly efficient method for optical or microwave detection and separation of left- and right-handed chiral molecules is proposed. The method utilizes a closed-loop three-state system in which the population dynamics depends on the phases of the three couplings. Because of the different signs of the coupling between two of the states for the opposite chiralities the population dynamics is chirality dependent. By using the "shortcuts to adiabaticity" concept applied to the stimulated Raman adiabatic passage technique, one can achieve 100% contrast between the two enantiomers in the population of a particular state. It can be probed by light-induced fluorescence for large ensembles or through resonantly enhanced multiphoton ionization for single molecules.

Chiral distinction by ultrashort laser pulses: electron wavepacket dynamics incorporating magnetic interactions

The qualitative and quantitative distinction of enantiomers is one of the key issues in chemical analysis. In the last years, circular dichroism (CD) has been combined with laser ionization mass spectrometry (LIMS), applying resonance enhanced multiphoton ionization (REMPI) with ultrashort laser pulses. We present theoretical investigations on the CD in the populations of the first electronic excited state of the REMPI process, caused by the interaction of 3-methyl-cyclopentanone with either left or right circular polarized fs-laser pulses. For this we performed multistate laser driven many electron dynamics based on ab initio electronic structure calculations, namely, TD-CIS(D)/6-311++(2d,2p). For a theoretical description of these experiments, a complete description of the field-dipole correlation is mandatory, including both electric field-electric dipole and magnetic field-magnetic dipole interactions. The effect of various pulse parameters on the CD are analyzed and compared with experimental results to gain further understanding of the key elements for an optimal distinction of enantiomers.

OPTIMALLY CONTROLLED VIBRATIONAL POPULATION TRANSFER IN A DIATOMIC QUANTUM SYSTEM

A time-dependent formulation of quantum control is employed to investigate optimally controlled vibrational population transfer in a diatomic quantum system. The problem of finding the optimal laser field needed to achieve a specific quantum transition from an initial state to the desired target goal is formulated using an iterative method and the conjugate gradient method (CGM). The time-dependent Schrödinger equation is solved with interaction of laser radiation with matter included within a dipole approximation in the Hamiltonian. Appropriate boundary conditions are chosen for the evolution problem. The control objective is chosen as the value of transition probability from an initial state to a target state. A comparison is made between the results obtained using the iterative method and the CGM for optimization. Finally, quantum bits are encoded using the vibrational states of the diatomic in the regime of low-vibrational excitation.

Stereoselective isomerization of an ensemble of adsorbed molecules with multiple orientations: stochastic laser pulse optimization for selective switching between achiral and chiral atropisomers

We present quantum dynamical simulations for the laser driven isomerization of an ensemble of surface mounted stereoisomers with multiple orientations. The model system 1-(2-cis-fluoroethenyl)-2-fluorobenzene supports two chiral and one achiral atropisomers upon torsion around the C-C single bond connecting phenyl ring and ethylene group. An infrared picosecond pulse is used to excite the internal rotation around the chiral axis, thereby controlling the chirality of the molecule. In order to selectively switch the molecules--independent of their orientation on a surface--from their achiral to either their left- or right-handed form, a stochastic pulse optimization algorithm is applied. The stochastic pulse optimization is performed for different sets of defined orientations of adsorbates corresponding to the rotational symmetry of the surface. The obtained nonlinearly polarized laser pulses are highly enantioselective for each orientation.

From stochastic pulse optimization to a stereoselective laser pulse sequence: simulation of a chiroptical molecular switch mounted on adamantane

Quantum dynamical simulations for the laser-controlled isomerization of 1-(2-cis-fluoroethenyl)-2-fluorobenzene mounted on adamantane are reported based on a one-dimensional electronic ground-state potential and dipole moment calculated by density functional theory. The model system 1-(2-cis-fluoroethenyl)-2-fluorobenzene supports two chiral and one achiral atropisomers upon torsion around the C-C single bond connecting the phenyl ring and ethylene group. The molecule itself is bound to an adamantyl frame which serves as a model for a linker or a surface. Due to the C3 symmetry of the adamantane molecule, the molecular switch can have three equivalent orientations. An infrared picosecond pulse is used to excite the internal rotation around the chiral axis, thereby controlling the chirality of the molecule. In order to selectively switch the molecules--independent of their orientations-- from their achiral to either their left- or right-handed form, a stochastic pulse optimization algorithm is applied. A subsequent detailed analysis of the optimal pulse allows for the design of a stereoselective laser pulse sequence of analytical form. The developed control scheme of elliptically polarized laser pulses is enantioselective and orientation-selective.

Laser-operated chiral molecular switch: quantum simulations for the controlled transformation between achiral and chiral atropisomers

We report quantum dynamical simulations for the laser controlled isomerization of 1-(2-cis-fluoroethenyl)-2-fluorobenzene based on one-dimensional electronic ground and excited state potentials obtained from (TD)DFT calculations. 1-(2-cis-fluoroethenyl)-2-fluorobenzene supports two chiral and one achiral atropisomers, the latter being the most stable isomer at room temperature. Using a linearly polarized IR laser pulse the molecule is excited to an internal rotation around its chiral axis, i.e. around the C-C single bond between phenyl ring and ethenyl group, changing the molecular chirality. A second linearly polarized laser pulse stops the torsion to prepare the desired enantiomeric form of the molecule. This laser control allows the selective switching between the achiral and either the left- or right-handed form of the molecule. Once the chirality is "switched on" linearly polarized UV laser pulses allow the selective change of the chirality using the electronic excited state as intermediate state.

Theory of “laser distillation” of enantiomers: Purification of a racemic mixture of randomly oriented dimethylallene in a collisional environment

Enantiomeric control of 1,3 dimethylallene in a collisional environment is examined. Specifically, our previous "laser distillation" scenario wherein three perpendicular linearly polarized light fields are applied to excite a set of vib-rotational eigenstates of a randomly oriented sample is considered. The addition of internal conversion, dissociation, decoherence, and collisional relaxation mimics experimental conditions and molecular decay processes. Of greatest relevance is internal conversion which, in the case of dimethylallene, is followed by molecular dissociation. For various rates of internal conversion, enantiomeric control is maintained in this scenario by a delicate balance between collisional relaxation of excited dimethylallene that enhances control and collisional dephasing, which diminishes control.

Optical resolution of oriented enantiomers via photodissociation: quantum model simulations for H2POSD

We demonstrate quantum mechanically how to resolve enantiomers from an oriented racemic mixture taking advantage of photodissociation. Our approach employs a femtosecond ultraviolet (UV) laser pulse with specific linear polarization achieving selective photodissociation of one enantiomer from a mixture of L and R enantiomers. As a result, the selected enantiomer is destroyed in the electronically excited state while the opposite enantiomer is left intact in the ground state. As an example we use H2POSD which presents axial chirality. A UV pulse excites the lowest singlet excited state which has nsigma* character and is, therefore, strongly repulsive along the P-S bond. The model simulations are performed using wavepackets which propagate on two dimensional potential energy surfaces, calculated along the chirality and dissociation reaction coordinates using the CASSCF level of theory.

Quantum phase control of entanglement

A method of phase control of entanglement in two-qubit systems is proposed. We show that by changing a relative phase of the pulses that drive the transitions in a two-qubit system with closed-loop couplings, one can control entanglement at will. The method relies on adiabatic dynamics via time-delayed pulse sequences and can be implemented with both resonant and nonresonant transitions.

Theory of the two step enantiomeric purification of 1,3 dimethylallene

An application of a recently proposed [P. Kral et al., Phys. Rev. Lett. 90, 033001 (2003)] two step optical control scenario to the purification of a racemic mixture of 1,3 dimethylallene is presented. Both steps combine adiabatic and diabatic passage phenomena. In the first step, three laser pulses of mutually perpendicular linear polarizations, applied in a "cyclic adiabatic passage" scheme, are shown to be able to distinguish between the L and D enantiomers due to their difference in matter-radiation phase. In the second step, which immediately follows the first, a sequence of pulses is used to convert one enantiomer to its mirror-imaged form. This scenario, which only negligibly populates the first excited electronic state, proves extremely useful for systems such as dimethylallene, which can suffer losses from dissociation and internal conversion upon electronic excitation. We computationally observe conversion of a racemic mixture of dimethylallene to a sample containing approximately 95% of the enantiomer of choice.

Implementing quantum gates on oriented optical isomers

Optical enantiomers are proposed to encode molecular two-qubit information processing. Using sequences of pairs of nonresonant optimally polarized pulses, different schemes to implement quantum gates, and to prepare entangled states, are described. We discuss the role of the entanglement phase and the robustness of the pulse sequences which depend on the area theorem. Finally, possible scenarios to generalize the schemes to n-qubit systems are suggested.

Quantum control of gas-phase and liquid-phase femtochemistry

Active control of chemical reactions on a microscopic (molecular) level, that is, the selective breaking or making of chemical bonds, is an old dream. However, conventional control agents used in chemical synthesis are macroscopic variables such as temperature, pressure or concentration, which gives no direct access to the quantum-mechanical reaction pathway. In quantum control, by contrast, molecular dynamics are guided with specifically designed light fields. Thus it is possible to efficiently and selectively reach user-defined reaction channels. In the last years, experimental techniques were developed by which many breakthroughs in this field were achieved. Femtosecond laser pulses are manipulated in so-called pulse shapers to generate electric field profiles which are specifically adapted to a given quantum system and control objective. The search for optimal fields is guided by an automated learning loop, which employs direct feedback from experimental output. Thereby quantum control over gas-phase as well as liquid-phase femtochemical processes has become possible. In this review, we first discuss the theoretical and experimental background for many of the recent experiments treated in the literature. Examples from our own research are then used to illustrate several fundamental and practical aspects in gas-phase as well as liquid-phase quantum control. Some additional technological applications and developments are also described, such as the automated optimization of the output from commercial femtosecond laser systems, or the control over the polarization state of light on an ultrashort timescale. The increasing number of successful implementations of adaptive learning techniques points at the great versatility of computer-guided optimization methods. The general approach to active control of light-matter interaction has also applications in many other areas of modern physics and related disciplines.

Quantum control of molecular chirality: optical isomerization of difluorobenzo[c]phenanthrene

The results of a theoretical study are presented on quantum control of a chiral exchange reaction of a polyatomic molecule by using infrared laser pulses. Difluorobenzo[c]phenanthrene was chosen to be the simplest model for its helical chirality exchange reaction. This molecule has two stable configurations: M and P forms. From the viewpoint of chemical reaction dynamics, isomerization is regarded as the movement of one of the two representative points that initially correspond to the two forms to the position of the other representative point, while the other representative point remains in its initial position. The ground-state potential energy surface and dipole moment functions required to control this reaction were evaluated at the MP2/6-31+G(d,p) and MP2/TZV+(d,p) levels of molecular orbital (MO) theory. An effective potential energy surface (PES) that is a function of twisting motion of the benzene rings and wagging motion of the CF(2) group was constructed on the basis of the MO results. An analytical expression for the effective PES and that for the dipole moment functions were prepared to make the isomerization control tractable. A quantum control method in a classical way was applied to the isomerization of preoriented difluorobenzo[c]phenanthrene in low temperature limits. The time evolution of the representative point of the M form and that of the P form are separately evaluated to determine the optimal laser fields. This means that the laser control produces pure helical enantiomers from a racemic mixture. Representative points are replaced by the corresponding nuclear wave packets in this treatment. The derived control laser field consists of two linearly polarized E(x)() and E(z)() components that are perpendicular to each other. These components are pi-phase-shifted when the representative point is in the transition-state regions. Under the irradiation of this laser pulse, one of the two representative points of the isomerization is transferred to the target position along the intrinsic reaction path between the enantiomers, while the other representative point remains in its initial potential well. This results in one-way isomerization control, that is, the M(P) to P(M) form. The isomerization is completed with yields of ca. 70% within a few picoseconds. Temporal behaviors of the nuclear wave packet whose center corresponds to the representative point are drawn to see how the desired chiral exchange reaction proceeds in the presence of the control field, while its reverse process is suppressed.