Small Molecular Reaction Dynamics And Their Laser Control

Molecule reaction dynamics is a subject of studying molecular collision and scatting in microscopic level, and thus reveling the basic mechanism of chemical reaction, so as to deepen understanding of the nature of the chemical reaction. It is established based on modern molecular and atomic physics, laser theory, crossed molecular beam method, spectroscopy and computer technology. Under the sup-port of modern theory and technology, it can investigate the dynamic structure and reaction process in different molecular systems. In experimental side, crossed molecular beams, laser induced fluorescence detection and chemiluminiscence tech-nologies have been successfully applied on the investigation of molecular scatting. In theoretical side, it usually has two steps. First, we need to construct the potential energy surfaces of the reacting system. Then, by resolving the dynamical equations, we will obtain several dynamical information such as reaction probabilities, integral cross sections and rate constants,.etc.Femtosecond laser pulse, due to its short time duration, is originally used to observe the movement of atoms and electrons. Afterwards, with the development of modern technology the laser intensity increases sharply, and it allows researches to study the molecular reaction dynamics from the stage of ’observation’ to ’control-ling’. The control mechanism is that, if the laser intensity is strong enough it will distort the inner coulomb potential, and then the reaction pathway will be changed. Thus, we can control the reaction dynamics through adjusting the laser frequency, intensity and envelope. For the molecular scatting, we can select the initial state with appropriate laser pulse, and thus the reaction process will be controlled. For the photodissociation dynamics, the probabilities of excited states can be controlled by pulsed laser fields. Finally, the product distributions will be changed. Moreover, in the diadiabatic dynamics, the intense field can make the coupling between each corresponding states being weakened or strengthened which will change the product channels.In theoretical treatment, the strictest and most accurate way of studying the molecular reaction dynamics is quantum mechanical calculation. It is based on the principles of quantum mechanics and the dynamical information can be obtained by solving the system’s Schrodinger equations. In this article, the method we used and developed is called time-dependent wave packet (TDWP) method. The initial pack-et is constructed in grid space, after propagated with time, finally, the dynamical information such as, reaction probabilities, cross sections will be obtained. For the molecular reaction dynamics, TDWP method is an effective method and the calcu-lation results were usually consistent well with the experimental data. In addition to the efficiency in numerical side, its physical significance is also more intuitive. Some quantum phenomena such as, tunneling effect, zero point energy.etc. can be well explained by TDWP method. At this point, the classical method is incompa-rable. In this article, we will study the triatomic-molecular reaction dynamics and their laser controlling.Compared with diatomic molecules, the triatomic molecules have more com-plex spatial structure and configuration of the electronic states, so the study of their reaction dynamics and laser controlling is still a challenging problem. Without the influence of the external field, the Hamiltonian of triatomic-molecule can be written in three parts:the kinetic energy term, the rotational kinetic energy term, and the potential energy term. In spatial coordinate, the potential term is diagonal, and thus it can multiply with wave packet directly. The Fourier transformation method is used for dealing with kinetic energy term. The kinetic energy term is a second order partial differential operator in spatial coordinate, but it is diagonal in momentum space. Thus, the mechanism is, we transform the wave packet from spatial coordi-nate to momentum coordinate using Fourier transformation firstly, then the kinetic energy terms can multiply with wave packet directly, and finally we transform its results back to spatial coordinate using inverse Fourier transformation, the action is finished. This scheme has some advantages:high precision, little time consuming and easy to convergence. It will be more complicate when dealing with rotational kinetic energy term. For triatomic molecule, it not only has three space rotational freedoms, but also includes the relative rotation between atom A and molecule BC. Because the angular operator is diagonal in finite basis representation (FBR), we need to transform the wave packet from discrete variable representation (DVR) to FBR using orthogonal transformation. The Wigner-D matrix is used for represent-ing the three rotational angular freedom. Since the multiplication of D matrix can be represent by 3-j symbols, so we can use rotational numbers instead for describing the spatial rotations. It can be achieved with a single angle bases set to describe all of the rotation which greatly reduces the computation redundancy. For reactive scattering, since the total angular momentum (J) of the system is conserved, so the corresponding Schrodinger equation can be taken separately. Finally, the total cross section and the rate constant can be obtained by assembling the reaction proba-bilities for various Js. However, when the molecule is exposed to external field, it will not be a conservative system anymore, so J will not be a constant. To order to describe the interaction between molecule and laser fields, the wave functions are expanded in terms of molecular rotational basis, and dipole approximation is used in interaction terms. In addition, according to the difference of electronic states’ configuration the transition mechanism can be divided in vertical transition and parallel transition. Finally, after time propagation, the dynamical information of laser interacting with triatomic molecule will be obtained.Based on that described above, our works can be listed as follows:1. Quantum reaction dynamics of the C+H2(D2)→CH(D)+H(D) on a new potential energy surface. Reactions between excited atomic carbon and molecular hydrogen play an important role in combustion, atmosphere and astrochemistry. Significant experimental and theoretical efforts have been devoted in the past to the understanding of both kinetic and dynamic aspects of these reactions. In the interstellar medium, the detection of CH radical can provide the basis in search of carbon-bearing macromolecules, which is an essential element of life. The initial state-specified integral cross section and the rate constant are obtained using the Chebyshev real wave packet method. To assess the quality of the new CH2 PES used in our dynamic calculations, we calculate the low-lying energy levels using Lanczos algorithm. The vibrational energy levels agree well with the experimental data. The reaction probabilities display oscillatory structure due to the numerous long-lived resonances supported by the deep potential well. The rate constants show nearly temperature independence at the range of 100 K-350 K. At 300 K, the value of calculated kinetic isotopic effect is 0.79, which is a little higher than experimental result (0.7).2. Laser-Driven Isomerization of HCN →HNC. The HCN molecule has re-ceived much attention as a prototypical case of a triatomic molecule in studying of optical control of molecular dynamics. It consists of three components:a light atom H and two heavy atoms C, N. H atom is easy to combine with C and N respectively which can form different molecular structure HCN and CNH. The ground state PES of HCN has a double well structure which is much suited for the investigation of laser control molecular dynamics. Thus, we employed time-dependent wave packet method to interpret the isomerization dynamics of HCN molecules induced by an intense picosecond infrared laser field. Considering the molecular rotational degrees of the freedom, the wave functions are expanded in terms of molecular rotational bases. Our full-dimensional quantum model includes the full Coriolis coupling in the molecular kinetic energy Hamiltonian and dipole approximation in interaction terms. The numerical results show that the field-induced molecule rotational ex-citation plays an important role in the isomerization dynamical process. Some phenomena appear such as two-step two-photon absorption and highly oscillatory structure in rotational state distributions. The centrifugal sudden (CS) approxima-tion calculation is also carried out and compared in this work; it is shown that the Coriolis couplings may lead to a significant decrease in the isomerization rate but highly enhances molecular rotational excitation.3. Laser-induced dissociation dynamics of triatomic molecule. We present a detailed theoretical approach to investigate the laser-induced dissociation dynam- ics of a triatomic molecule in full dimensional case. In this method, The short-time Chebyshev propagation method is used for time propagation. Because the propagator is expanded with the complex Chebyshev polynomial, the Hamiltonian can action the wave function directly without the diagonalization of Hamiltonian matrices. As an example of the application of this formalism, the dissociation dy-namics H3+→H2++H induced by ultrashort UV laser pulses are investigated on new Born-Oppenheimer potential energy surfaces. Our numerical results show that the signals of dissociation products will be easier to observe as the increasing of field strength. Driven by a 266 nm laser beam, the calculated central value of kinetic-energy-release is 2.04 eV which shows excellent agreement with the experimental estimation of 2.1 eV. When the H3+ ion is rotationally excited, the spatial distribu-tion of product fragments will become well converged.This thesis includes six chapters. We make an introduction in chapter 1 and briefly present the recent development of molecular reaction dynamics and some new phenomena when the molecule is exposed to intense external fields. In chapter 2, we give a detailed introduction of the time-independent wave packet method. In chapter 3, we theoretically calculate the reaction dynamic of C+H2 and test the accuracy of new potential energy surface which is deepen the insight of this complex insertion mechanism. In chapter 4, we investigate the laser-driven isomerization of HCN→HNC in full dimensional case. In Chapter 5, we present a quantum mech-anism treatment to investigate the full dimensional photodissociation dynamics of triatomic molecular and the benchmark triatomic molecular system of H3+ ion was taken as an example to interpret the dissociation dynamics induced by a pulsed laser field based on the new PESs. Chapter 5 gives conclusions and outlooks.