Theoretical Studies Of State-to-state Photodissociation Dynamics Of Water Molecule

Photodissociation is one of the key issue in Chemistry. Quantum state resolved photodissociation dynamics provides us the remarkable understanding for photodissociation reaction mechanism at atomic and molecular level. The photodissociation of water molecule no only has served as a textbook prototype for theoretical and experimental studies, but also plays an important role in atmospheric chemistry, combustion chemistry, and interstellar chemistry and closely associates with the future of hydrogen energy. During the past few years, extensive experimental and theoretical studies have been focused on this system. Experimental studies using HRTOF and LIF have obtained an increasing wealth of data. On the theoretical front, the calculations via quantum mechanical and classical trajectory models have reproduced qulitatively most experimental observations, underscoring an in-depth understanding of the system. Despite extensive theoretical research on photodissociation dynamics in water molecule, however, it is only recently that quantitative agreement with experiment has been achieved for this system based on highly accurate potential energy surfaces and a full-dimensional dynamical model.The substitution of hydrogen by deutrium little changes the electronic character of water, but can change the the photodissociation of water. In order to study the isotope effect in the B band, we use the two-state coupling model including X and B states to investigate the state-to-state quantum dynamics of the photodissociation of D2O in its B band. Quantum dynamical calculations based on the recent developed diabatic potential energy surfaces was carried out using a Chebyshev real wave packet method. The nonadiabatic channel via the DOD conical intersection is facile, direct, and fast, which produces rotationally hot and vibrationally cold OD(X) product. On the other hand, the adiabatic channel on the excited state, leading to the OD(A) product, is dominated by long-lived resonances, which depend sensitively on the potential energy surface. The calculated absorption spectra, product state distributions, branching ratios, and angular distributions are in reasonably good agreement with the latest experimental results.Then, we further consider the effects of the Renner-Teller coupling between A and B states. We extend our theoretical model include all three (B1 A’, A1 A", and X1 A’) electronic states and focus on the competing non-adiabatic pathways leading to the OH(X)+H fragments. Our dynamical results indicate that the RT non-adiabatic pathway plays a relatively minor role in the dissociation. The pathway is shut for K=0, but represents 30-40% of the flux in the OH(X) channel for K>0. Interestingly, the absence of this pathway can be compensated by the other non-adiabatic channel, but its inclusion is vital in order to achieve a quantitative agreement with the experimental results, particularly for rotational state-resolved anisotropy parameter of the OH(X,v=0) fragment.Due to the open-shell nature of the OH molecule, its electronic orbital and spin angular momenta couple. the The photodissociation dynamics of H2O in its first absorption band is investigated on an accurate potential energy surface based on a large number of high-level ab initio points. Several ro-vibrational states of the parent molecule are considered. Different from most previous theoretical studies, the spin-orbit and A-doublet populations of the open-shell OH fragment are reported from full-dimensional wave packet calculations. The populations of the two spin-orbit manifolds are in most cases close to the statistical limit, but the A-doublet is dominated by the A" component, thanks largely to the fast in-plane dissociation of H2O(A1 A"). Comparisons with experimental data and a Franck-Condon model are generally very good, although some discrepancies exist.At last, quantum state distributions of the OH(X/A) fragments generated by the photodissociation of H2O at 121.6 nm are systematically investigated with a full-dimensional quantum dynamical model on accurate coupled PESs. Since all initial and final angular momenta are included in the model, the fine-structure populations of the OH products can be obtained. The calculated rotational state distributions of the two A-doublet levels of OH(X,v=0) exhibit very different characteristics. The A’ states, produced mostly via the Bâ†'X conical intersection pathway, have significantly higher populations than the A" counterparts, which are primarily from the Bâ†'A Renner-Teller pathway. The former features a highly inverted and oscillatory rotational state distribution, while the latter has a smooth distribution with much less rotational excitation. In good agreement with experiment, the calculated total OH(X) rotational state distribution and anisotropy parameters show clear even-odd oscillations, which can be attributed to a quantum mechanical interference between waves emanating from the HOH and HHO conical intersections in the Bâ†'X non-adiabatic pathway. These theoretical studies not only offer valuable insights in to the reaction dynamics and non-adiabatic transitions, but also serve as benchmarks for more approximate models.