Laser Control Of Chemical Reactions Research Articles

Taming Disulfide Bonds with Laser Fields. Nonadiabatic Surface-Hopping Simulations in a Ruthenium Complex.

Laser control of chemical reactions is a challenging field of research. In particular, the theoretical description of coupled electronic and nuclear motion in the presence of laser fields is not a trivial task and simulations are mostly restricted to small systems or molecules treated within reduced dimensionality. Here, we demonstrate how the excited state dynamics of [Ru(S–Sbpy)(bpy)2]2+ can be controlled using explicit laser fields in the context of fewest-switches surface hopping. In particular, the transient properties along the excited state dynamics leading to population of the T1 minimum energy structure are exploited to define simple laser fields capable of slowing and even completely stopping the onset of S–S bond dissociation. The use of a linear vibronic coupling model to parametrize the potential energy surfaces showcases the strength of the surface-hopping methodology to study systems including explicit laser fields using many nuclear degrees of freedom and a large amount of close-lying electronic excited states.

Frequency-sparse optimal quantum control

A new class of cost functionals for optimal control of quantum systems which produces controls which are sparse in frequency and smooth in time is proposed. This is achieved by penalizing a suitable time-frequency representation of the control field, rather than the control field itself, and by employing norms which are of $L^1$ or measure form with respect to frequency but smooth with respect to time. We prove existence of optimal controls for the resulting nonsmooth optimization problem, derive necessary optimality conditions, and rigorously establish the frequency-sparsity of the optimizers. More precisely, we show that the time-frequency representation of the control field, which a priori admits a continuum of frequencies, is supported on only finitely many frequencies. These results cover important systems of physical interest, including (infinite-dimensional) Schrodinger dynamics on multiple potential energy surfaces as arising in laser control of chemical reactions. Numerical simulations confirm that the optimal controls, unlike those obtained with the usual $L^2$ costs, concentrate on just a few frequencies, even in the infinite-dimensional case of laser-controlled chemical reactions.

Laser Control of Chemical Reactions by Phase Space Structures

Abstract The question of what initial conditions or what kinds of laser fields can effectively carry a system to the desired products is one of the most intriguing subjects in chemistry. In this paper, a scheme for designing a laser field to guide a chemical reaction system into the product is presented on the basis of the phase space structure of the reaction with an illustrative simple example. The method exploits recent findings [Kawai et al., J. Chem. Phys.2011, 134, 024317] that have revealed the existence of a rigorous reactivity boundary separating the reactive and nonreactive trajectories in the phase space (more precisely, the semiclassical phase space representation of the quantum system). Referring to the time propagation of the system, the method designs an electric field that actively shifts this reactivity boundary so that it catches more the system in the reactant and releases the system in the product. The success of this scheme for designing of the field gives a further support to the interpretation for the laser control of chemical reactions.

Laser control of physicochemical processes; experiments and applications

This review outlines experimental advances that have been made in laser control of physicochemical processes, with an emphasis on the 2004–2006 period. After a brief introduction, an overview of the technology available for delivering ultrashort shaped femtosecond pulses is presented. Special attention is given to recent progress on laser control of chemical reactions and the application of this concept to molecular identification. We also cover control of simpler systems such as atoms and diatomic molecules. Laser control of large molecules in solution is also reviewed from the point of view of selective spectroscopic excitation with applications in microscopy and control of nanoparticles. We conclude with an outlook that takes into account the physical limitations that will dictate the best strategies to achieve robust laser control of physicochemical processes.

Quantum control by ultrafast dressed states tailoring

Strong field quantum control using shaped intense femtosecond laser pulses is investigated. The physical mechanism relies on selective population of dressed states (SPODS). With time resolved photoelectron spectroscopy the ultrafast dressed state population dynamics during the ionization of potassium atoms is directly observed. High selectivity of the dressed state population and tunability of the dressed state energies is demonstrated experimentally. Wave packet simulations on a diatomic model molecule confirm that SPODS is also applicable to molecules. We conclude that SPODS with simple ultrashort optimal pulse shapes is suitable for robust laser control of chemical reactions.

Analytic pulse design for selective population transfer in many-level quantum systems: Maximizing the amplitude of population oscillations

State-selective preparation and manipulation of discrete-level quantum systems such as atoms, molecules, or quantum dots is the ultimate tool for many diverse fields such as laser control of chemical reactions, atom optics, high-precision metrology, and quantum computing. Rabi oscillations are one of the simplest, yet potentially quite useful mechanisms for achieving such manipulation. Rabi theory establishes that in the two-level systems resonant drive leads to the periodic and complete population oscillations between the two system levels. In this paper an analytic optimization algorithm for producing Rabi-like oscillations in the general discrete many-level quantum system is presented.

Laser-induced control of (multichannel) intracluster reactions

An experimental and theoretical study on the excited Ba ··· FCH3(A) photodissociation yield as a function of the excitation laser fluence is reported. Experimentally, it was found that the two- photodissociation channel yields, i.e. the reactive BaF and non-reactive Ba ∗ products, increased exhibiting a similar behaviour, as the laser fluence changed from 0.2 up to ca. 4 mJ/cm 2 . Beyond this value the BaF yield levels off and the Ba ∗ decreases over the 4-7 mJ/cm 2 range. The theoretical simulation of the excited state electron-ion dynamics within the time-dependent density functional theory revealed that the reactive channel dominated the photofragmentation dynamics as it occurs within a femtosecond time scale and became accelerated as the photodissociation laser fluence increased. By contrast, the non-reactive channel only manifested for low laser fluences at the nano/picosecond time regime resulted inactive as the laser fluence increased. A simple scheme to control the dynamics of the intracluster multichannel reaction is suggested in which the slowest the channel the easiest to close it as the excitation laser power increases. PACS. 36.40.Jn Reactivity of clusters - 36.40.Qv Stability and fragmentation of clusters - 34.50.Rk Laser-modified scattering and reactions Laser control of chemical reactions is currently attracting a considerable attention from both theoretical and exper- imental point of view (1-3). Among several schemes to implement such an objective one of the most promising is based on quantum interference. For unimolecular reac- tions, in which a single reactant molecule is transformed to several product molecules, two types of control have been applied insofar. Namely, the "two-beam interference" (4-6) and the "two-pulse time delay" control (7,8). While these two control schemes are based on a limited number of op- timisation parameters (i.e., the phase difference between the two lasers and the time delay between the pump and dump laser pulses, respectively) a new powerful method has been developed (9) based on the adaptative shaping of femtosecond laser pulses which works satisfactorily even for complex systems. Although, in many cases, the elec- tron/ion dynamics of the system is not fully understood, there are examples in which the dynamics of a unimolec- ular reaction induced by an optimal femtosecond pulse, generated from adaptative learning algorithms, has been deciphered (10).

CONTROL OF QUANTUM SYSTEMS

A quantum system subject to external fields is said to be controllable if these fields can be adjusted to guide the state vector to a desired destination in the state space of the system. Fundamental results on controllability are reviewed against the background of recent ideas and advances in two seemingly disparate endeavours: (i) laser control of chemical reactions and (ii) quantum computation. Using Lie-algebraic methods, sufficient conditions have been derived for global controllability on a finite-dimensional manifold of an infinite-dimensional Hilbert space, in the case that the Hamiltonian and control operators, possibly unbounded, possess a common dense domain of analytic vectors. Some simple examples are presented. A synergism between quantum control and quantum computation is creating a host of exciting new opportunities for both activities. The impact of these developments on computational many-body theory could be profound.

Mechanisms for laser control of chemical reactions

During the past several years, a new technique for realistic simulations of the interaction of light with matter has been developed and employed. Recent simulations of laser pulses interacting with molecules clearly demonstrate the potential for control of chemical reactions through various mechanisms, which include the following: (i) excitation of electrons to states that have different bonding properties; (ii) control of electron populations through a coherent pump-pulse, control-pulse sequence; and (iii) control of molecular vibrations through a pump-control sequence. Significant chemical insights are gained when one can watch a realistic animation of species interacting and reacting. One can monitor the time evolution of electronic states and their occupancy, as well as the motion of the atoms. One can also observe the evolution from reactants to products through transition states. Finally, one can determine how this evolution is affected by the various properties of the laser pulses, including intensity, duration, phase, and the interval between pulses.

Laser control of chemical reactions.

Chemical reactions are at the heart of chemistry and the dream of controlling the outcome of these reactions is an old one. Thus, with given reactants, a solvent and perhaps assisted by a catalyst, we would like to 'steer' the reactants into a particular desired product. This review focuses on how to control the dynamics of chemical reactions, beyond traditional temperature control, with the emphasis on unimolecular reactions. The electromagnetic radiation of lasers can induce so-called coherent dynamics. The recent theoretical and experimental results on this coherent control are explained and illustrated with computational and experimental examples.

Control theory applied to quantum chemistry: some tracks

We present a short introduction to the models in use for the simulation of the evolution of a molecular system in the context of Quantum Chemistry. We explain how these models can provide experts at control theory with interesting topics of investigation, and how control theory can in turn be of great usefulness in the modelling of the laser control of chemical reactions. Some tracks for mathematical investigations are indicated.

Spiers Memorial Lecture Stereochemistry and control in molecular reaction dynamics

The evolving understanding of the stereodynamics of atomic and molecular reactions has led, quite naturally, to the evolution of strategies for their control. The excitement of designing reaction control machinery has captured the imagination of many theoreticians, who aspire to becoming molecular quantum ‘mechanics’ and, at the same time, the challenge of implementing their ideas is also exciting many reaction dynamical ‘engineers". A key tool is provided by the laser, and the Discussion which this lecture introduces, could equally well have been titled ‘Laser control of chemical reactions’. The Lecture provides a survey of the current status of stereodynamics and control, principally of bimolecular reactions. It includes a review of strategies for the passive control of chemical reactions evolving “under their own steam’', and of their stereodynamics, and of strategies for their active control under the influence of continuous wave or pulsed optical fields, where reactions are “made to dance according to the theoreticians' and increasingly, the experimenters' tune’'.

Laser control of chemical reactions

Experiments show how product pathways can be controlled by irradiation with one or more laser beams during individual bimolecular collisions or during unimolecular decompositions. For bimolecular collisions, control has been achieved by selective excitation of reagent vibrational modes, by control of reagent approach geometry, and by control of orbital alignment. For unimolecular reactions, control has been achieved by quantum interference between different reaction pathways connecting the same initial and final states and by adjusting the temporal shape and spectral content of ultrashort, chirped pulses of radiation. These collision-control experiments deeply enrich the understanding of how chemical reactions occur.

Alternate schemes for the coherent laser control of chemical reactions

Several schemes are presented for the coherent laser control of chemical reactions. They are based on the principle of interference between quantum mechanical excitation pathways first proposed by Brumer and Shapiro [Acc. Chem. Res. 22, 407 (1989)]. The conclusion that these schemes may by useful for coherent laser control is based on the fact that, for each of the schemes, the quantum mechanical interference effect has been observed previously in nonlinear optical experiments; however, no attempt to control the interference was attempted in these experiments.