Kunihito Hoki

Selective preparation of enantiomers by laser pulses: quantum model simulation for H2POSH

Abstract This Letter presents the first quantum model simulation of the selective preparation of enantiomers by means of optimal, elliptically polarized, infrared picosecond laser pulses. The laser-driven molecular dynamics is demonstrated by the time evolution of the representative wavepacket, from the initial state which corresponds to a 50:50% racemate of two equivalent enantiomers with opposite chiralities towards the nearly 100:0% preparation of a single enantiomer. The wavepacket dynamics is based on the quantum ab initio potential energy surface and dipole functions for the torsional vibration of the hydrogen atom around the P–S molecular axis of the model system H 2 POSH.

Selective preparation of enantiomers from a racemate by laser pulses: model simulation for oriented atropisomers with coupled rotations and torsions

Abstract We design a laser pulse which drives a racemate of oriented atropisomers at low temperature to a preferential target enantiomer. The overall laser pulse consists of a series of individual circularly polarized laser pulses which induce corresponding selective transitions between coupled rotational and torsional states. The underlying theory is derived in detail for a model system. It consists of two fragments which may carry out torsional and rotational motions around a molecular bond which is oriented along the direction of the laser pulses. Exemplarily, results are demonstrated for the model system H 2 POSH in the electronic ground state, based on a quantum chemical ab initio potential and on the components of the dipole functions describing the laser–dipole interaction. The series of laser pulses for the preparation of the pure enantiomers for this demanding system is based on analogous results for simpler scenarios, originally starting from local control.

Locally designed pulse shaping for selective preparation of enantiomers from their racemate

We present a method for the design of laser fields to control a selective preparation of enantiomers from their racemate. An expression for two components of the laser pulses [EX(t) and EY(t)] propagating along the Z axis is derived using a locally optimized control theory in the density operator formalism. This expression was applied to a selective preparation of (R-, L-) enantiomers from preoriented phosphinotioic acid (H2POSH) at low temperatures. The target operator was set for the populations to be localized in one side of the double-well potential. First, a simple one-dimensional model was treated. Then, a two-dimensional model in which a free rotation around the preoriented torsional axis is included was briefly considered. In the one-dimensional model, almost complete preparation of the enantiomers was obtained. The optimal electric field consists of a sequence of two linearly polarized pulses with the same phases but with different magnitudes. This means that the resultant electric field is linea...

Quantum control of molecular handedness in a randomly oriented racemic mixture using three polarization components of electric fields

A new laser control scenario is presented for obtaining substantial amounts of enantiomeric enrichment from a randomly oriented racemic mixture. This is carried out by using three polarization components of electric fields; one is used for orientation, the other two for controlling the chirality. The effectiveness is demonstrated by numerical simulations on the enantiomeric enrichment of the axial chiral H 2 POSH molecule.

Quantum optimal control of electron ring currents in chiral aromatic molecules

We report the results of optimal control simulations of pi-electron rotation (ring current) in a six-membered chiral aromatic molecule, 2,5-dichloro[n](3,6)pyrazinophane (DCP), attached at a surface and excited by a linearly polarized UV laser. DCP has a pair of optically allowed, quasidegenerate pi-electronic excited states. The laser pulse to generate an approximate angular momentum eigenstate consisting of the quasidegenerate states was designed using the global optimal control theory. For both counterclockwise and clockwise pi-electron rotations, the calculated objective functional and target yield as a function of the angle of the photon polarization vector show two maxima and two minima. The origin of the two minima is coherent excitation to only one of the quasidegenerate states. The two maxima arise from creation of a superposition of the quasidegenerate states. The optimal control pulse at the maxima is a two-color laser field resonant with the quasidegenerate states. The electric field of the optimal control pulse consists of two parts: a slowly oscillating part with phase phi(env) and a rapidly oscillating one. The phase phi(env) is a crucial parameter for determination of the rotation direction of pi electrons at the end of control. The results of the optimal control simulations suggest that pi-electron rotation can be controlled by applying a two-color laser field with adjusted phases.

Control of molecular handedness using pump-dump laser pulses

A theoretical method for controlling handedness of preoriented enantiomers starting from an equal mixture of right(R)-handed and left(L)-handed molecules using linearly polarized femtosecond laser pulses is presented. The essence of the method lies in the fact that the molecular handedness of oriented enantiomers is reflected in the direction of the electronic transition moment vector. A pump-dump control scheme via an electronic excited state is considered for controlling molecular handedness in a femtosecond time scale. The direction of the polarization vector of the pump pulse and that of the dump pulse are determined in such a way that there is the largest interaction between the laser and the L-(R-) handed molecules, while the interaction with R-(L-) handed ones vanishes. In the case in which both the pump and dump pulses are independent of each other with no overlap between them, an analytical expression for the yield of molecular handedness is derived by solving the equation of motion of the densit...

Consultation algorithm for computer shogi: move decisions by majority

A new algorithm that runs on a computer with interconnected processors has been designed for Shogi. The algorithm adopts consultation between many individual players. A method that can create multiple players from one program is presented. Applying a simple rule to select a decision on a single move, the consultation algorithm improves the performance of computer Shogi engines. It is also demonstrated that a council system consisting of three well-known Shogi programs: YSS, GPS, and BONANZA plays better games than any of the three programs individually.

Theory of stimulated Raman adiabatic passage in a degenerated reaction system: Application to control of molecular handedness

We have developed a new type of stimulated Raman adiabatic passage (STIRAP) that is applicable to a degenerated reaction system. The direction of the photon polarization vector is the adiabatic parameter in the STIRAP. The molecular handedness of H2POSH, a preoriented phosphinotioic acid that has two stable configurations, L and R enantiomers, is used as a model system. The control of molecular handedness in both pure and mixed state cases are considered. In the case of a pure state, a STIRAP with a linearly polarized single laser allows an almost complete transfer from an L (R) enantiomer to the other by adiabatically changing its polarization direction. The adiabatic criterion for changing the polarization direction is clarified. In the case of a mixed state, a STIRAP with two linearly polarized laser pulses allows a selective preparation of pure enantiomers from its racemic mixture. In the low temperature limit, a five-level model reduces a three-level model by setting the direction of the polarization...

Molecular motors driven by laser pulses: Role of molecular chirality and photon helicity

The results of a theoretical study on molecular motors driven by laser pulses are presented. The roles of molecular chirality and photon helicity in determination of their unidirectional rotation are clarified. An expression for an instantaneous angular momentum of motors driven by lasers in the density matrix formalism was derived. Assuming randomly oriented molecular motors, the initial distribution-averaged instantaneous angular momentum in the dipole approximation was obtained. Taking into account parity inversion symmetry of molecular motors in the averaged instantaneous angular momentum, it is shown that the directions of the averaged instantaneous angular momentum of (R)- and (S)-chiral molecular motors are opposite, but that the magnitudes are the same. This is independent of polarization of laser fields. That is, the chiral motors driven by a linearly polarized optical field creates a unidirectional motion in a molecular fixed frame. On the other hand, the direction of rotation in the laboratory fixed frame is decided by a circularly polarized laser regardless of its molecular chirality. A simple example of real chiral molecular motors is used to demonstrate the interplay of molecular chirality and photon helicity in determination of their unidirectional rotation. The internal rotation of the CHO group plays the role of the engine of the motor. The time evolution of the rotational wave packets of the molecular motors driven by linearly or circularly polarized laser pulses was numerically evaluated and the dynamical behaviors were analyzed. Effects of temperature on the instantaneous angular momentum of the molecular motors are presented as well.

Selective preparation of enantiomers by laser pulses: From optimal control to specific pump and dump transitions

Starting from optimal control, various series of infrared, ultrashort laser pulses with analytical shapes are designed in order to drive a preoriented molecule from its ground torsional state, which represents the coherent superposition of left and right atropisomers, towards a single enantiomer. Close analysis of the population dynamics, together with the underlying symmetry selection rules for the laser induced transitions, yields the mechanism. Namely, the molecule is driven from its ground vibrational state towards the coherent superposition of the lowest doublet of states via a doublet of excited torsional states with opposite symmetries. This pump-and-dump mechanism can be achieved by simpler series of analytical laser pulses. This decomposition of the optimal pulse into analytical subpulses allows us to design different scenarios for the selective preparation of left or right enantiomers. Exemplary this is demonstrated by quantum simulations of representative wave packets for the torsional motions ...