Nonadiabatic wave packet dynamics and predissociation resonances in sodium hydride.

Vibrational wave packet dynamics provides an opportunity to explore the energy landscape and the population transfer between nonadiabatically coupled excited electronic states. Here the coupled nonadiabatic dynamics of the C1Σ+ and D1Σ+ states of sodium hydride (NaH) in the gas phase in the adiabatic picture is studied, using a sequence of ultra-fast laser pulses in the femtosecond region. Emergence of different population dynamics and dissociation probabilities is shown by carefully choosing the pulse wavelength, duration and time-shift between the pulses, exciting the molecule from the ground X1Σ+ state via the immediate A1Σ+ state. Quantum dynamics simulations were performed in the adiabatic picture, avoiding the adiabatic to diabatic transformation. Predissociation resonances, i.e. vibrational states with finite lifetimes, arise due to nonadiabatic couplings between bound and continuum states. Here accurate resonance energies and widths are computed providing further insight into the dissociation dynamics.

An adaptive pseudospectral method for wave packet dynamics.

We solve the time-dependent Schrödinger equation for molecular dynamics using a pseudospectral method with global, exponentially decaying, Hagedorn basis functions. The approximation properties of the Hagedorn basis depend strongly on the scaling of the spatial coordinates. Using results from control theory we develop a time-dependent scaling which adaptively matches the basis to the wave packet. The method requires no knowledge of the Hessian of the potential. The viability of the method is demonstrated on a model for the photodissociation of IBr, using a Fourier basis in the bound state and Hagedorn bases in the dissociative states. Using the new approach to adapting the basis we are able to solve the problem with less than half the number of basis functions otherwise necessary. We also present calculations on a two-dimensional model of CO(2) where the new method considerably reduces the required number of basis functions compared to the Fourier pseudospectral method.

Resonant inelastic scattering spectra of free molecules with vibrational resolution.

Inelastic x-ray scattering spectra excited at the 1s(-1)π* resonance of gas phase O2 have been recorded with an overall energy resolution that allows for well-resolved vibrational progressions. The nuclear wave packet dynamics in the intermediate state is reflected in vibrational excitations of the electronic ground state, and by fine-tuning the excitation energy the dissociation dynamics in the predissociative B'(3)Πg final state is controlled.

A perfectly matched layer applied to a reactive scattering problem.

The perfectly matched layer (PML) technique is applied to a reactive scattering problem for accurate domain truncation. A two-dimensional model for dissociative adsorbtion and associative desorption of H(2) from a flat surface is considered, using a finite difference spatial discretization and the Arnoldi method for time-propagation. We compare the performance of the PML to that of a monomial complex absorbing potential, a transmission-free complex absorbing potential, and to exterior complex scaling. In particular, the reflection properties due to the numerical treatment are investigated. We conclude that the PML is accurate and efficient, especially if high accuracy is of significance. Moreover, we demonstrate that the errors from the PML can be controlled at a desired accuracy, enabling efficient numerical simulations.

Dynamical Insights into the Decomposition of 1,2-Dioxetane.

Chemiluminescence in 1,2-dioxetane occurs through a thermally activated decomposition reaction into two formaldehyde molecules. Both ground-state and nonadiabatic dynamics (including singlet excited states) of the decomposition reaction have been simulated, starting from the first O-O bond-breaking transition structure. The ground-state dissociation occurs between t = 30 fs and t = 140 fs. The so-called entropic trap leads to frustrated dissociations, postponing the decomposition reaction. Specific geometrical conditions are necessary for the trajectories to escape from the entropic trap and for dissociation to be possible. The singlet excited states participate as well in the trapping of the molecule: dissociation including the nonadiabatic transitions to singlet excited states now occurs from t = 30 fs to t = 250 fs and later. Specific regions of the seam of the S0/S1 conical intersections that would "retain" the molecule for longer on the excited state have been identified.

How Do Methyl Groups Enhance the Triplet Chemiexcitation Yield of Dioxetane?

Chemiluminescence is the emission of light as a result of a nonadiabatic chemical reaction. The present work is concerned with understanding the yield of chemiluminescence, in particular how it dramatically increases upon methylation of 1,2-dioxetane. Both ground-state and nonadiabatic dynamics (including singlet excited states) of the decomposition reaction of various methyl-substituted dioxetanes have been simulated. Methyl-substitution leads to a significant increase in the dissociation time scale. The rotation around the O-C-C-O dihedral angle is slowed; thus, the molecular system stays longer in the "entropic trap" region. A simple kinetic model is proposed to explain how this leads to a higher chemiluminescence yield. These results have important implications for the design of efficient chemiluminescent systems in medical, environmental, and industrial applications.

Accurate time propagation for the Schrodinger equation with an explicitly time-dependent Hamiltonian.

Several different numerical propagation techniques for explicitly time-dependent Hamiltonians are discussed and compared, with the focus on models of pump-probe experiments. The quality of the rotating wave approximation is analyzed analytically, and we point out under which circumstances the modeling becomes inaccurate. For calculations with the fully time-dependent Hamiltonian, we show that for multistate systems, with either time or space dependence in the interstate coupling, the fourth order truncated Magnus expansion can be reformulated so that no commutators appear. Our results show that the split-operator method should only be used when low accuracy is acceptable. For accurate and efficient time stepping, the Magnus-Lanczos approach appears to be the best choice.

Experimental and theoretical study of the photodissociation of bromo-3-fluorobenzene.

The UV photodissociation of bromo-3-fluorobenzene under collisionless conditions has been studied as a function of the excitation wavelength between 255 and 265 nm. The experiments were performed using ultrafast pump-probe laser spectroscopy. To aid in the interpretation of the results, it was necessary to extend the theoretical framework substantially compared to previous studies, to also include quantum dynamical simulations employing a two-dimensional nuclear Hamiltonian. The nonadiabatic potential energy surfaces (PES) were parameterized against high-level MS-CASTP2 quantum chemical calculations, using both the C-Br distance and the out-of-plane bending of the bromine as nuclear parameters. We show that the wavelength dependence of the photodissociation via the S0-->1pipi*-->1pisigma* channel, accessible with a approximately 260 nm pulse, is captured in this model. We thereby present the first correlation between experiments and theory within the quantitative regime.

Reduced-order modeling, error estimation, and the role of the start-vector: the recursive residue generation method revisited.

The recursive residue generation method (RRGM) [Wyatt, R. E. Adv. Chem. Phys. 1989, 73, 231] is re-derived using the formalism of reduced-order modeling [Bai, Z. Appl. Numerical Math. 2002, 43, 9]. A stopping criteria for the RRGM recursions is proposed, on the basis of an expression for an upper bound to the absolute error [Bai, Z.; Ye, Q. Electron. Trans. Numerical Anal. 1998, 7, 1]. It is further pointed out that, in general, the start-vector has a negligible effect on the convergence of the RRGM.

Calculation of highly excited vibrational states using a Richardson-Leja-Davidson scheme.

An efficient computational scheme for calculating highly excited vibrational eigenstates is proposed, combining a Richardson-Leja spectral filter with a novel version of the Davidson method [J. Comput. Phys. 17, 87 (1975)]. Highly excited eigenstates of the Rb2 and H2O molecules are computed to test and verify the method. On the average less than 2.5 outer recursions per eigenstate are needed. For each outer Davidson recursion, less than 20 inner filter recursions per eigenstate are needed on the average.

Photodissociation of bromobenzene, dibromobenzene, and 1,3,5-tribromobenzene.

Quantum chemical calculations have been performed on the ground state and several low-lying excited states of bromobenzene, ortho-, meta-, and para-dibromobenzene, and 1,3,5-tribromobenzene using high-level ab initio and hybrid density-functional methods. Experimental observations of ultrafast predissociation in these molecules are clarified from extensive theoretical information about all low-energy potential-energy curves together with symmetry arguments. The intriguing observation that o- and m-dibromobenzene have two ultrafast predissociation channels while bromobenzene, p-dibromobenzene, and 1,3,5-tribromobenzene only have one such channel is explained from the calculated potential-energy curves. These show that the lowering of point-group symmetry from C2v to Cs along the main photodissociation reaction coordinate, which only occurs in o- and m-dibromobenzene, opens up a new predissociation channel. Dynamical quantum simulations based on the calculated potential-energy curves are used to estimate the coupling strength at the intersystem crossing point in bromobenzene.