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Two-pulse selective photochemistry that exploits population transfer via adiabatic passage is considered for the case that there are degenerate product states with different lifetimes. As an example, a four-level model system with a complex symmetric Hamiltonian is constructed. Analytical and numerical studies of this model system demonstrate that extensive control over the product branching ratio can be achieved by detuning either the pump pulse or the Stokes pulse while maintaining negligible population in the intermediate state. This control approach represents a significant simplification of both the Kobrak–Rice extended stimulated Raman adiabatic passage scheme and the Chen–Shapiro–Brumer strong-field control scheme. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The classical theory of the rate of unimolecular isomerization developed by Gray and Rice is extended in two ways. First, an improved state dependent approximation to the system separatrix is introduced. Second, a systematic approximation for the rate of intramolecular energy transfer in one isomer is introduced. These new approximations considerably improve the accuracy of the prediction of the rate of isomerization. Indeed, using the analysis described in this paper, the calculated rates of isomerization as a function of energy for the model system studied by Gray and Rice are in excellent agreement with the exact rates obtained from trajectory calculations. [ABSTRACT FROM AUTHOR]

We describe a classical theory of unimolecular reaction rate which is derived from the analysis of Davis and Gray by use of simplifying approximations. These approximations concern the calculation of the locations of, and the fluxes of phase points across, the bottlenecks to fragmentation and to intramolecular energy transfer. The bottleneck to fragment separation is represented as a vibration–rotation state dependent separatrix, which approximation is similar to but extends and improves the approximations for the separatrix introduced by Gray, Rice, and Davis and by Zhao and Rice. The novel feature in our analysis is the representation of the bottlenecks to intramolecular energy transfer as dividing surfaces in phase space; the locations of these dividing surfaces are determined by the same conditions as locate the remnants of robust tori with frequency ratios related to the golden mean (in a two degree of freedom system these are the cantori). The flux of phase points across each dividing surface is calculated with an analytic representation instead of a stroboscopic mapping. The rate of unimolecular reaction is identified with the net rate at which phase points escape from the region of quasiperiodic bounded motion to the region of free fragment motion by consecutively crossing the dividing surfaces for intramolecular energy exchange and the separatrix. This new theory generates predictions of the rates of predissociation of the van der Waals molecules HeI2, NeI2 and ArI2 which are in very good agreement with available experimental data. [ABSTRACT FROM AUTHOR]

Examines the nature of the photoselective adiabatic passage approach proposed for product formation control in chemical reactions. Insight into chemical reaction control in strong fields; Prediction of sodium dimer physical realistic simulation results; Use of intense laser fields to control chemical reactivity.

A time-dependent formulation of two-photon spectroscopy is employed to show that selectivity of reactivity can be achieved via coherent two-photon processes. The problem of finding the optimum waveform (i.e., coherent pulse sequence) that will maximize the formation of a desired chemical species is formulated as a problem in the calculus of variations, and solved for two different cases. [ABSTRACT FROM AUTHOR]

We present a novel approach to the control of selectivity of reaction products. The central idea is that in a two-photon or multiphoton process that is resonant with an excited electronic state, the resonant excited state potential energy surface can be used to assist chemistry on the ground state potential energy surface. By controlling the delay between a pair of ultrashort (femtosecond) laser pulses, it is possible to control the propagation time on the excited state potential energy surface. Different propagation times, in turn, can be used to generate different chemical products. There are many cases for which selectivity of product formation should be possible using this scheme. We illustrate the methodology with numerical application to a variety of model two degree of freedom systems with two inequivalent exit channels. Branching ratios obtained using a swarm of classical trajectories are in good qualitative agreement with full quantum mechanical calculations. [ABSTRACT FROM AUTHOR]

A previously reported method for shaping electromagnetic field pulses to achieve chemical selectivity is extended and applied to a simple multiple level model system. The pulse shaping approach is based on optimal control theory, where both the time-dependent Schrödinger equation and the constant pulse energy are used as constraints on the variational scheme. A conjugate gradient direction method is used to direct the convergence of the iterative process used to calculate the optimum pulse shape. The method is applied to a five-level system interacting with an optical (laser) field. Results demonstrating selectivity and stability are compared to those of other recent related investigations. [ABSTRACT FROM AUTHOR]

A brief overview of studies of optical control of dynamical processes in solution is presented, with a focus on control of selection of product in a chemical reaction. It is argued that there are circumstances under which adiabatic population transfer in the liquid phase is possible, and that variations of such transfer processes can be used to control dynamical processes in solution. [ABSTRACT FROM AUTHOR]

Abstract: This article is a brief tutorial dealing with the conceptual aspects of (i) excitation transfer in molecular assemblies, such as occurs in the early steps of photosynthesis, (ii) radiationless transitions, which are ubiquitous participants in the light induced isomerization reactions that support vision and (iii) active control of product selection in a chemical reaction. All of these processes exhibit explicit quantum effects that arise from competition between coherent and incoherent evolution of the initial state of the system expressed in the atom and/or electron dynamics. Topic (iii) is associated with how a molecular assembly can be designed to optimize the use of coherent processes to improve efficiency of conversion of the initial excitation to product under the constraint that there are fluctuations in the surrounding medium. [Copyright &y& Elsevier]

Discusses efforts by chemists to alter chemical reactions. Repercussions on the dynamics of molecular evolution; Use of the science known as quantum interference control; Limitations; Outlook. INSET: Multiple-path intense-field interference control.

`Examines the identification of the diffraction patterns, generated using X-ray pulses of only picosecond duration. Revealation of the motion of individual atoms as iodine molecules dissociate; Determination of chemical reactions, as atoms change position to transform one molecule into another; Observation of biological function of the molecule.

Comments on the use of optical control in molecular dynamics. Use of femtosecond laser pulses to monitor reactions in real time; Use of a probe pulse to observe changes in the bonding of sodium atoms; Increasing use of active molecular dynamics control to steer the results of a chemical reaction toward a desired output; Fundamental questions remaining to be addressed about the procedure.