Alexey Kondorskiy

Semiclassical theory of electronically nonadiabatic chemical dynamics: Incorporation of the Zhu–Nakamura theory into the frozen Gaussian propagation method

The title theory is developed by combining the Herman-Kluk semiclassical theory for adiabatic propagation on single potential-energy surface and the semiclassical Zhu-Nakamura theory for nonadiabatic transition. The formulation with use of natural mathematical principles leads to a quite simple expression for the propagator based on classical trajectories and simple formulas are derived for overall adiabatic and nonadiabatic processes. The theory is applied to electronically nonadiabatic photodissociation processes: a one-dimensional problem of H2+ in a cw (continuous wave) laser field and a two-dimensional model problem of H2O in a cw laser field. The theory is found to work well for the propagation duration of several molecular vibrational periods and wide energy range. Although the formulation is made for the case of laser induced nonadiabatic processes, it is straightforwardly applicable to ordinary electronically nonadiabatic chemical dynamics.

Laser control of electronic transitions of wave packet by using quadratically chirped pulses

An effective scheme is proposed for the laser control of wave packet dynamics. It is demonstrated that by using specially designed quadratically chirped pulses, fast and nearly complete excitation of wave packet can be achieved without significant distortion of its shape. The parameters of the laser pulse can be estimated analytically from the Zhu-Nakamura theory of nonadiabatic transition. If the wave packet is not too narrow or not too broad, then the scheme is expected to be utilizable for multidimensional systems. The scheme is applicable to various processes such as simple electronic excitation, pump-dump, and selective bond breaking, and it is actually numerically demonstrated to work well by taking diatomic and triatomic molecules (LiH, NaK, H(2)O) as examples.

Control of chemical dynamics by lasers: theoretical considerations.

Theoretical ideas are proposed for laser control of chemical dynamics. There are the following three elementary processes in chemical dynamics: (i) motion of the wave packet on a single adiabatic potential energy surface, (ii) excitation/de-excitation or pump/dump of wave packet, and (iii) nonadiabatic transitions at conical intersections of potential energy surfaces. A variety of chemical dynamics can be controlled, if we can control these three elementary processes as we desire. For (i) we have formulated the semiclassical guided optimal control theory, which can be applied to multidimensional real systems. The quadratic or periodic frequency chirping method can achieve process (ii) with high efficiency close to 100%. Concerning process (iii) mentioned above, the directed momentum method, in which a predetermined momentum vector is given to the initial wave packet, makes it possible to enhance the desired transitions at conical intersections. In addition to these three processes, the intriguing phenomenon of complete reflection in the nonadiabatic-tunneling-type of potential curve crossing can also be used to control a certain class of chemical dynamics. The basic ideas and theoretical formulations are provided for the above-mentioned processes. To demonstrate the effectiveness of these controlling methods, numerical examples are shown by taking the following processes: (a) vibrational photoisomerization of HCN, (b) selective and complete excitation of the fine structure levels of K and Cs atoms, (c) photoconversion of cyclohexadiene to hexatriene, and (d) photodissociation of OHCl to O + HCl.

Semiclassical Formulation of Optimal Control Theory

In the present paper, semiclassical formulation of optimal control theory is made by combining the conjugate gradient search method with new approximate semiclassical expressions for correlation function. Two expressions for correlation function are derived. The simpler one requires calculations of coordinates and momenta of classical trajectories only. The second one requires extra calculation of common semiclassical quantities; as a result additional quantum effects can be taken into account. The efficiency of the method is demonstrated by controlling nuclear wave packet motion in a two-dimensional model system.

SEMICLASSICAL FROZEN GAUSSIAN PROPAGATION METHOD FOR ELECTRONICALLY NONADIABATIC CHEMICAL DYNAMICS: MØLLER OPERATOR FORMULATION AND INCORPORATION OF THE ZHU-NAKAMURA THEORY

The title theory is developed by combining the Herman–Kluk semiclassical theory for adiabatic propagation on single potential energy surface and the Zhu–Nakamura theory for nonadiabatic transition. A fairly simple expression for the propagator based on classical trajectories is derived using the Moller operator formulation of scattering theory. The theory takes into account almost all quantum effects that occur during nonadiabatic transition, especially at low energies and is expected to be applicable to general chemical dynamics of high dimensions. Application to a two-dimensional model system shows that the theory works well for the propagation duration of several molecular vibrational periods and wide wave packet energy range.

Effects of Plasmon–Exciton Interaction in the Spectra of Light Absorption by Hybrid Systems Consisting of Two- and Three-Layer Organometallic Nanoparticles

The effects of plasmon-exciton interaction on the spectra of light absorption by hybrid systems of two- and three-layer nanoparticles that consist of a metallic nucleus, the outer shells of ordered molecular dye J-aggregates, and an intermediate passive organic spacer between them is analyzed theoretically. It is established that the type of the absorption spectra and the efficiency of a near-field electromagnetic coupling between the particles in the system depends largely on the distance between the centers of concentric spheres and the direction of light polarization.

Particle Shape Effects in the Extinction Spectra of Gold and Silver Nanoparticles

Particle shape effects in the absorption and scattering spectra of gold and silver nanoparticles are studied for optical-range radiation. Numerical calculations of extinction cross section are performed using the FDTD approach for particles of three shapes: disks, triangular prisms, and triradial-base prisms. It is established that the spectra of nanoparticles of different shapes differ qualitatively even if they are of the same characteristic dimensions. The obtained results can be used to develop a spectroscopic procedure for determining the shape of metallic nanoparticles and producing quantitative estimates of their characteristic dimensions.

The Nonadiabatic Trajectory

In this chapter, theoretical approaches are reviewed on the concept of nonadiabatic trajectories as guides for the traveling localized wave packets, which could be used to treat electronically nonadiabatic molecular dynamics. The main difficulty to design nonadiabatic trajectories comes from the fact that coupled Hamiltonian has no well-established classical counterpart;that is, it is necessary to treat purely quantum effect of nonadiabatic coupling in terms of classical and statistical mechanics right at the stage of the nonadiabatic trajectory propagation. The several approaches have shown the powerful performance on this issue.

Laser Control of Chemical Dynamics. II. Control of Wavepacket Motion

An efficient semiclassical optimal control theory for controlling wavepacket dynamics on a single adiabatic potential energy surface applicable to systems with many degrees of freedom is discussed in detail. The approach combines the advantages of various formulations of the optimal control theory: quantum and classical on the one hand and global and local on the other. The efficiency and reliability of the method are demonstrated, using systems with two and four dimensions as examples.

Selective Excitation of Metastable Atomic States by Femto- and Attosecond Laser Pulses

The possibility of achieving highly selective excitation of low metastable states of hydrogen and helium atoms by using short laser pulses with reasonable parameters is demonstrated theoretically. Interactions of atoms with the laser field are studied by solving the close-coupling equations without discretization. The parameters of laser pulses are calculated using different kinds of optimization procedures. For the excitation durations of hundreds of femtoseconds direct optimization of the parameters of one and two laser pulses with Gaussian envelopes is used to introduce a number of simple schemes of selective excitation. To treat the case of shorter excitation durations, optimal control theory is used and the calculated optimal fields are approximated by sequences of pulses with reasonable shapes. A way to achieve selective excitation of metastable atomic states by using sequences of attosecond pulses is introduced.